An Introduction to Plant Breeding E-Book

Jack Brown

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 612

Veröffentlichungsjahr: 2011

Beschreibung

Plants have been successfully selectively bred for thousands of years, culminating in incredible yields, quality, resistance and so on that we see in our modern day crops and ornamental plants. In recent years the techniques used have been rapidly advanced and refined to include molecular, cell and genetic techniques. An Introduction to Plant Breeding provides comprehensive coverage of the whole area of plant breeding. Covering modes of reproduction in plants, breeding objectives and schemes, genetics, predictions, selection, alternative techniques and practical considerations. Each chapter is carefully laid out in a student friendly way and includes questions for the reader. The book is essential reading for all those studying, teaching and researching plant breeding.

Leseprobe

Contents

1 INTRODUCTION

REQUIREMENTS OF PLANT BREEDERS

EVOLUTION OF CROP SPECIES

NATURAL AND HUMAN SELECTION

CONTRIBUTION OF MODERN PLANT BREEDERS

THINK QUESTIONS

2 MODES OF REPRODUCTION AND TYPES OF CULTIVAR

INTRODUCTION

MODES OF REPRODUCTION

TYPES OF CULTIVAR

ANNUALS AND PERENNIALS

REPRODUCTIVE STERILITY

THINK QUESTIONS

3 BREEDING OBJECTIVES

INTRODUCTION

PEOPLE, POLITICAL AND ECONOMIC CRITERIA

INCREASING GROWER PROFITABILITY

INCREASING END-USE QUALITY

INCREASING PEST AND DISEASE RESISTANCE

CONCLUSIONS

THINK QUESTIONS

4 BREEDING SCHEMES

INTRODUCTION

DEVELOPMENT OF SELF-POLLINATING CULTIVARS

DEVELOPING MULTILINE CULTIVARS

DEVELOPMENT OF OUTBREEDING CULTIVARS

DEVELOPING SYNTHETIC CULTIVARS

DEVELOPING HYBRID CULTIVARS

DEVELOPMENT OF CLONAL CULTIVARS

DEVELOPING APOMICTIC CULTIVARS

SUMMARY

THINK QUESTIONS

5 GENETICS AND PLANT BREEDING

INTRODUCTION

QUALITATIVE GENETICS

QUANTITATIVE GENETICS

TESTING THE MODELS

QUANTITATIVE TRIAL LOCI

THINK QUESTIONS

6 PREDICTIONS

INTRODUCTION

HERITABILITY

DIALLEL CROSSING DESIGNS

CROSS PREDICTION

THINK QUESTIONS

7 SELECTION

INTRODUCTION

WHAT TO SELECT AND WHEN TO SELECT

RESPONSE TO SELECTION

APPLIED SELECTION

CROSS PREDICTION

PARENTAL SELECTION

THINK QUESTIONS

8 ALTERNATIVE TECHNIQUES IN PLANT BREEDING

INTRODUCTION

INDUCED MUTATION

INTERSPECIFIC AND INTERGENERIC HYBRIDIZATION

TISSUE CULTURE

PLANT TRANSFORMATION

MOLECULAR MARKERS IN PLANT BREEDING

THINK QUESTIONS

9 SOME PRACTICAL CONSIDERATIONS

INTRODUCTION

EXPERIMENTAL DESIGN

GREENHOUSE MANAGEMENT

FIELD PLOT TECHNIQUES

USE OF COMPUTERS IN PLANT BREEDING

RELEASE OF NEW CULTIVARS

GENETICALLY MODIFIED CROP PLANTS

THINK QUESTIONS

Index

© 2008 by Jack Brown and Peter D.S. Caligari

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First published 2008 by Blackwell Publishing Ltd

ISBN: 978-1-4051-3344-9

Library of Congress Cataloging-in-Publication DataBrown, Jack Houghton.An introduction to plant breeding/Jack Brown and Peter D.S. Caligari.p. cm.Includes bibliographical references and index.ISBN: 978-1-4051-3344-9 (pbk.: alk. paper)1. Plant breeding. I. Caligari, P. D. S. (Peter D. S.) II. Title.SB123.B699 2006 631.5′2–dc222006019915

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1

INTRODUCTION

REQUIREMENTS OF PLANT BREEDERS

The aim of plant breeding is to develop superior cultivars, which are adapted to specific environmental conditions and suitable for economic production in a commercial cropping system.

The basic concept of varietal development is very simple and involves three distinct operations:

produce or identify genetically variable germplasm;carry out selection procedures on genotypes from within this germplasm to identify superior genotypes with specified characteristics;stabilize and multiply these superior genotypes and release cultivars for commercial production.

The general philosophy underlying any breeding scheme is to maximize the probability of creating, and identifying, superior genotypes which will make successful new cultivars. In other words they will contain all the desirable characteristics/traits necessary for use in a production system.

Plant breeders can be categorized into two types. One group of plant breeders is employed within private companies, while the other group works in the public sector (e.g. government funded research institutes or universities). Private sector and public sector breeders have different approaches to the breeding process. Many of the differences that exist between public and private breeding programmes are related to the time available for variety release, types of cultivar developed and priorities for characters in the selection process. For example, breeders within the public sector are likely to have a number of responsibilities, related to academic activities or extension services, as well as producing new varieties. Private sector plant breeders tend to have a more clearly defined goal, developing new cultivars and doing it as quickly as possible. In addition, many private breeding organizations are, or are associated with, agrochemical companies. As a result varietal development may be designed to produce cultivars suitable for integration with a specific production system.

Despite the apparent simple description of the breeding process given above, in reality plant breeding involves a multidisciplinary approach. Irrespective of whether a breeding scheme is publicly or privately managed, a successful plant breeder will require knowledge in many (if not all) of the following subjects:

Evolution It is necessary to have knowledge of past progress in adapting crop species if additional advances are to continue into the future. When dealing with a crop species, a plant breeder benefits from knowledge of the time scale of events that have modelled the given crop. For example, the time of domestication, geographic area of origin and prior improvements are all important and will help in setting feasible future objectives.

Botany The raw material of any breeding scheme is the available germplasm (lines, genotypes, accessions, etc.) from which variation can be generated. The biological relationship, which exists within a species and with other species, will be a determining factor indicating germplasm variability and availability.

Biology Knowledge of plant biology is essential to create genetic variation and formulate a suitable breeding and selection scheme. Of particular interest are modes of reproduction, types of cultivar and breeding systems.

Genetics The creation of new cultivars requires manipulation of genotypes. The understanding of genetic procedures is therefore essential for success in plant breeding. Genetics is an ever developing subject but knowledge and understanding that is particularly useful will include single gene inheritance, population genetics, the likely frequencies of genotypes under selection and the prediction of quantitative genetic parameters – all of which will underlie decisions on what strategy of selection will be most effective.

Pathology A major goal of plant breeding is to increase productivity and quality by selecting superior genotypes. A limiting factor in economic production is the impact of pests and diseases. Therefore developing cultivars, which are resistant to detrimental pathogens, has been a major contributor to most cost effective production with reduced agrochemical inputs. Similarly, nematodes, insect pests and viruses can all have detrimental effects on yield and/or quality. Therefore plant breeders must also have knowledge of nematology, entomology and virology.

Weed Science The response of a genotype to competition from weed populations will have an effect on the success of a new cultivar. Cultivars that have poor plant establishment, or lack subsequent competitive ability, are unlikely to be successful, particularly in systems where reduced, or no, herbicide applications are desirable, or their use is restricted. Similarly, in many cases genotypes respond differently, even to selective herbicides. Herbicide tolerance in new crops is looked upon favourably by many breeding groups, although cultivar tolerance to broad-spectrum herbicides can cause management difficulties in crop rotations.

Food Science Increasing end-use quality is being identified as one of the major objectives of all crop breeding schemes. As most crop species are grown for either human or animal consumption, knowledge of food nutrition and other related subjects is important.

Biometry Managing a plant breeding scheme has aspects that are no different from organizing a series of large experiments over many locations and years. To maximize the probability of success it is necessary to use an appropriate experimental approach at all stages of the breeding scheme. Plant breeding is continually described as ‘a numbers game’. In many cases this is true, and successful breeding will result in vast data sets on which selection decisions are to be made. These decisions often have to be made during short periods, for instance between harvesting one crop and planting another. Therefore, plant breeders are required to be good data managers.

Agronomy It is the aim of crop breeders to predict how newly identified genotypes will perform over a wide range of environments. This will require research into agronomic features that may relate to stress tolerance, such as heat, drought, moisture, salinity, and fertility. These experiments are essential in order that farmers (the primary customer) are provided with the optimal agronomic husbandry parameters, which will maximize genetic potential of the new variety.

Molecular Biology Advances in molecular biological techniques are having an increasing role in modern plant breeding. Molecular markers are increasingly used by plant breeders to help select (indirectly and directly) for characters that are difficult to evaluate in the laboratory, or are time consuming, or expensive to determine accurately on a small plot scale. Genetic engineering and other tissue culture operations are becoming standard in many plant breeding schemes and it is likely that further advances will be made in the future. Knowledge of all these techniques and continued awareness of ongoing research will be necessary so that new procedures can be integrated into the breeding scheme where appropriate.

Production The contributions that farmers, and other growers, have made to varietal development should never be underestimated. It also should be noted that growers are the first customers for plant breeding products. The probability of a new cultivar being successful will be maximized (or at least the probability of complete failure reduced) if growers and production systems are considered as major factors when designing breeding systems.

Management There is a need to manage people, time and money. It has already been stated that plant breeding is a multidisciplinary science and this means being able to integrate and optimize people’s effort to effectively use breeders’ time. The length of most breeding programmes means that small proportional savings in time can be valuable and it hardly needs emphasizing that breeding needs to be cost effective and therefore the cost of the programme is always going to be important.

Communication Most varietal development programmes consist of inputs from more than one scientist and so it is necessary that plant breeders are good communicators. Verbal and written communication of results and test reports will be a feature in all breeding schemes. Research publications and grant proposals are of major importance, particularly to public breeders, if credibility and funding is to be forthcoming. Finally, at least some plant breeders must be good at passing on the essential information about the subject to future plant breeders!

Information Technology The science underlying plant breeding is continually advancing, the agronomic practices are continually being upgraded, the end-users’ choices change and the political context continually affects agriculture. This means that it is vital that breeders talk to these different groups of people but also use what ever technology is available to keep up to date on developments as they occur – or better before they occur!

Psychic The success of most plant breeding schemes is not realized before many years of breeding and trialling have been completed. It may be twelve or more years between the initial crosses and varietal release. Plant breeders therefore must be crystal ball gazers and try to predict what the general public and farming community will need in the future, what diseases will persist twelve years ahead and what quality characters will command the highest premiums.

In summary, therefore, successful plant breeders need to be familiar with a range of scientific disciplines and management areas. It is not, however, necessary to be an expert or indeed an authority in all of these. However, greater knowledge of the basic science underlying the techniques employed, and of the plant species concerned, in terms of the biology, genetics, history and pathology will increase the chances of a breeder succeeding in developing the type of cultivars most suitable for future exploitation.

EVOLUTION OF CROP SPECIES

Plant breeding consists of the creation and manipulation of genetic variation within a crop species, and selection of desirable recombinants from within that variation. The process is therefore an intensification of a natural process, which has been ongoing since plants first appeared on earth. As soon as humans started carrying out settled agriculture they effectively started plant breeding. In this section the main features of crop plant evolution will be covered briefly. The study of evolution is a vast and detailed subject in itself, and it will not be possible to cover more than an introduction to it in this book. Emphasis will be on the areas that are most important from a plant breeding standpoint.

Knowledge of the evolution of a plant species can be invaluable in breeding new cultivars. Studies of evolution can provide knowledge of the past changes in the genetic structure of the plant, an indication of what advances have already been achieved or might be made in the future, and help to identify relatives of the domesticated plant which could be used in interspecific or intergeneric hybridization to increase genetic diversity or introduce desirable characters not available within existing crops.

Why did hunter gathers become farmers?

It is difficult to arrive at a firm understanding as to why humans became a race of farmers. Early humans are believed to have been foragers and later hunters. Why then did they become crop producers? Farming is believed to have started shortly after the last ice age. At that time there may have been a shortage of large animals for hunters to hunt due to extinctions. Indeed, little is known about the order of agricultural developments. Did man domesticate animals and then domesticate crops to feed these beasts, or were crops first domesticated, and from this the early farmers found that they could benefit from specifically growing sufficient food to feed livestock? The earliest farmers may also have been fishermen who tended not to travel continually and were more settled in one region. In this latter case, perhaps the first farmers were women who took care of the farming operation while the males fished and hunted locally. It may simply have been that some ancient people became tired of nomadic travel in search of food, became bored with living in tents and opted for a quiet life on the farm! The answers are not known, although it can often be interesting to postulate why this change occurred. One misconception about the switch from hunting-gathering to farming is that farming was easier. It has been shown that gathering food requires considerably less energy than cultivating and growing crops. In addition, skeletal remains show that the initial farmers were smaller framed and more sickly than their hunter-gatherer counterparts.

Irrespective of the reason which caused mankind to cultivate crops, few would question that the beginning of farming aligned with the beginning of what most of us would consider civilization. Farming created communities, community structure and economies, group activities, enhanced trade and monetary systems to name but a few. There is also little doubt that the total genetic change achieved by early farmers in moulding our modern crops has been far greater than that achieved by the scientific approaches that have been applied to plant breeding over the past century. Given that these early farmers were indeed cultivating crops, it is not surprising that they would propagate the most productive phenotypes, avoid the individuals with off-taste, and choose not to harvest those plants which were spiny. Even today among peasant farmers there is a general trend to select the best plants for re-sowing the next year’s crops. Early farmers may have used relatively sophisticated plant breeding techniques as there is evidence that some native Americans have a long established understanding of maintaining pure line cultivars of maize by growing seed crops in isolation from their production fields.

What crops were involved? And when did this occur?

Today’s world food production is dominated by small grain cereal crops, with world production of maize (Zea Mays), rice (Oryza sativa) and wheat (Triticum spp.); each being just under 600 million metric tones annually (Figure 1.1). Major root crops include potato (Solanum tuberosum), cassava (Manihot esculenta), and sweet potato (Ipimiea batatas). Oilseed crops are soybean oil (Elaeis guineensis), coconut palms (Cocos nucifera), and rapeseed (Brassica napus). World production of fruit and vegetables are similar where tomato (Lycopersico esculentum), cabbage (Brassica oleracea) and onion (Alliums spp.) are leading vegetable crops, whilst orange (Citrus sinesis), apple (Malus spp.), grape (Vitaceae spp.) and banana (Musa aceminata and M. balbisiana) predominate amongst the fruits. Many of these modern day crops were amongst the first propagated in agriculture.

Figure 1.1 World production of major crops.

Many studies have been made to determine the date when man first cultivated particular crops. The accuracy of dating early plant tissue has improved over the past half century with the use of radio-carbon methods. It should, however, be noted that archaeological material which remains well preserved has not proved easy to find. Many of the most significant findings have been from areas of arid environments (e.g. the eastern Mediterranean and Near East, New Mexico and Peru). These arid regions favour the preservation of plant tissue over time, and not surprisingly, are the areas where most archaeological excavations have taken place. Conversely, there is a lower probability of finding well preserved plant remains in regions with wetter, and more humid, climates. Therefore, archaeological information may provide an interesting, but surely incomplete, picture.

A summary of the approximate time of domestication and centre of origin of the world’s major crop species, and a few recent crop additions, is presented in Table 1.1. It should be reiterated that many crop species have more than one region of origin, and that archaeo-logical information is continually being updated. This table is therefore very much an over-simplification of a vast and complex picture.

Table 1.1 Estimated time of domestication and centre of origin of major crop species. It should be noted that many crop species have had more than one suggested origin and that archaeological information is continually being updated. This table is therefore a simplification of an incompletely understood and complex picture.

CropTime of domestication (years)Possible region of originCerealsMaize, Zea Mays7000Mexico, Central AmericaRice, Oryza sativa4500Thailand, Southern ChinaWheat, Triticum spp.8500Syria, Jordan, Israel, IraqBarley, Hordeum vulgare9000Syria, Jordan, Israel, IraqSorghum, Sorgum bicolour8000Equatorial AfricaOilseedsSoybean, Glycine max2000Northern ChinaOil palm, Elaeis guineensis9000Central AfricaCoconut palm, Cocos nucifera100Southern AsiaRapeseed, Brassica napus500Mediterranean EuropeSunflower, Helianthus3000Western United StatesPulsesBeans, Phaseolus spp.7000Central America, MexicoLentil, Lens culinaris7000Syria, Jordan, Israel, IraqPeas, Pisum sativum9000Syria, Jordan, Israel, IraqRoot cropsPotato, Solanum tuberosum7000PeruCassava, Manihot esculenta5000Brazil, MexicoSweet potato, Ipimiea batatas6000South Central AmericaSugar beet, Beta vulgaris300Mediterranean EuropeVegetablesTomato, Lycopersico esculentum3000Western South AmericaCabbage, Brassica oleracea3000Mediterranean EuropeOnion, Allium4500Iran, Afghanistan, PakistanFruitOrange, Citrus sinesis9000South-eastern AsiaApple, Malus spp.3000Asia Minor, Central AsiaGrape, Vitaceae spp.7000Eastern AsiaBanana, Musa aceminata, M. balbisiana4500South-east AsiaOthersCotton, Gossypium4500Central America, BrazilCoffee, Coffea spp.500West EthiopiaRubber, Hevea brasiliensis200Brazil, Bolivia, ParaguayAlfalfa, Medicago sativa4000Iran, Northern Pakistan

Some of the earliest recorded information which shows human domestication of plants, comes from the region in the Near East known as the ‘Fertile Crescent’ (including the countries of Turkey, Syria, Israel, Iran and Iraq). Domestication of crops in this region surrounding the Tigris River, began before 6000–7000 BC. Two of the worlds leading cereal crops, einkorn and emmer wheat, and barley have their centre of origin in this region. In addition, archeological remains of onion, peas, and lentil, dating back to over 7000 years have all been found within the Fertile Crescent. In the Americas, similar or slightly later, dates of cultivation have been shown for beans and maize in central Mexico and Peru, and potato, cassava, and sweet potato in Peru and western South America. Sunflower (Helianthus) is the only major crop species with a centre of origin in North America and indeed most other crops grown in the USA and Canada evolved from other continents. Rice, soybean, sugarcane (Gramineae andropogoneae), and the major fruit species (orange, apple and banana) were all first domesticated in China and the Asian continent a few millennia BC. Examination of archeological remains show that the dates of crop domestication in Africa were later; yet sorghum, oil palm, and coffee are major world crops that have their centre of origin in this continent. Similarly, cabbage and a few other vegetable crops have their centre of origin in Europe. Given more research, it may be found that many more of today’s crops were domesticated at earlier periods.

Several crops of importance have been domesticated relatively recently. Sugar beet was not grown commercially in Europe until the 18th century, while rubber, date palm and coconut palms were not domesticated until the end of the 19th century. The forage grasses, clovers, and oilseed rape (Brassica napus L. or B. rapa L.) also are recently domesticated crops, although some researchers would argue that these crops have yet to make the transition necessary to be classified as truly domesticated. New crops are still being recognized today. The advent of bioenergy crops has identified the oilseed crop camelina (Camelina spp.), and the biomass crop switchgrass (Panicum virgatum) as potential new crops species which have yet to be grown in large scale commercialization.

A high proportion of today’s major crops come from a very small sub-sample of possible plant species (Figure 1.1). It has been estimated that all the crop species grown today come from 38 families and 91 genera. Therefore, the source of our present day crops are more diverse than we have shown, although they still only represent a fraction of the total families and genera which have been estimated to exist within the angiosperms as a whole. Also, it should be noted that the sources of origin of these crops are spread over Europe, the Near East, Asia, Africa and America.

At some time in the past, each of our present day crop species must have originated in one, or more, specific regions of the world. Originally it was thought that there were only 12 major centres of origin including the Near East, Mediterranean, Afghanistan, the Pacific Rim, China, Peru, Chile, Brazil/Paraguay and the USA. More recent research has altered this original view and it is now apparent that:

Crops evolved in all regions of the world where farming was practiced.The centre of origin of any specific crop is not usually a clearly defined geographic region. Today’s major crops are more likely to have evolved over large areas.Early farmers and nomadic travellers would have been responsible for widening the region where early crops have been found and added confusion concerning the true centres of origin.Regions of greatest crop productivity are rarely related to the crop’s centre of origin.

Overall therefore, domesticated crops have originated from at least four of the six world continents (America, Europe, Africa and Asia). Australian aborigines remained hunter-gatherers and did not become farmers, and indeed farming in Australia is a relatively new activity started after western settlers arrived there. No surprisingly therefore few of today’s major agricultural crops originated in Australia; however, a recently domesticated crop (Macadamia nuts) does have its origin in this continent.

NATURAL AND HUMAN SELECTION

All domesticated crops have been developed from wild, “weedy” ancestors. Early farmers modified weed species into modern-day crops through a process of genetic manipulation and selection. As a result these crop species have been sufficiently altered such that they can be considered to be domesticated. A definition of domestication has been given by Professor N.W. Simmonds as follows: “a plant population has been domesticated when it has been substantially altered from the wild state and certainly when it has been so altered to be unable to survive in the wild”. The first part of this definition can certainly be readily accepted for almost all modern-day agricultural crops, although we still propagate many crops (e.g. date palm) where the crop species are modified only slightly from ancient ancestors. It is not always possible to relate domestication with a lack of potential to survive in non-cultivated situations since many commercially grown plants survive as volunteer weeds, or “escapes”, in either the same, or different, regions to those in which they are most commonly grown commercially.

In the evolution of crop species we can often distinguish between natural and human selection. Natural selection tends to favour the predominance of the most adapted plant types, which manage to reproduce and disperse their progeny, while tolerating the stress factors that prevail in a particular environment. Therefore the natural selection favours plant phenotypes which have the greatest chance of survival, reproduction, and distribution of progeny. For example, wild cereal plants tend to have many small seeds at maturity and disperse their seed by shattering. These seeds also are likely to be attached to a strong awn to aid dispersal. Similarly, wild potato species produce many small tubers, have their tubers develop at the end of very long stolons (so that daughter plants do not have to occupy ground too close to the parent), and many have tubers with high levels of toxin, which discourage animals from eating them.

Human selection is the result of conscious decisions by a farmer or plant breeder to keep the progeny of a particular parent and discard others. Human selection is not usually directed to better survival in the wild (and indeed is often detrimental to survival outside cultivation). As an example, breeders have developed cereal cultivars which have fewer, but larger seeds, that do not shatter their seeds at maturity and that have a non-persistent awn. Similarly potato breeders have selected plants with fewer, but larger tubers, shorter stolons and with reduced levels of toxins in the tuber. Human selection also has produced crops that are more uniform in the expression of many of their characteristics. For example, they have selected seeds that all mature at the same time, with uniform germination, and fruits with uniform fruit size and shape. In more recent times plant breeders’ selection has tended to result in shorter plants, greater harvest index, and increased ease of harvest. A large number of our crop species that used to require harvest by hand can now be harvested by machine, mainly as a consequence of their small stature and uniform ripening.

There is of course a range of characteristics that would have been positively selected both by natural evolution and early plant breeders. These might include aspects of yield potential, tolerance to stress factors and resistance to pests and diseases.

CONTRIBUTION OF MODERN PLANT BREEDERS

Around the turn of the 20th century the foundation of modern plant breeding was laid. Darwin’s ideas on the differential survival of better adapted types were combined with those of Mendel on the genetic basis for the inheritance of plant characters. These two theories, combined with the research of scientists such as Weissman on the continuity of germplasm, and the analyses of Johannsen resulting in the idea of genotype/phenotype relationships, provided the scientific foundation of modern plant breeding.

There is little doubt that mankind has had a tremendous influence in moulding the morphology, plant types, end uses, and productivity of most crop species. Early farmers have taken wild, weedy plants and developed them into commercially viable agricultural crops. The contribution of modern plant breeding efforts is not always clearly defined nor can their achievements be easily measured.

Over the past century the world’s human population has risen dramatically (Figure 1.2). World human population first exceeded one billion in 1804. It took a further 118 year of population expansion to double the world population. The human generation born after World War II (1945 to 1955) are often referred to as ‘baby boomers’. Interestingly this is the first generation to witness the world’s population double, from 3 billion to 6 billion individuals. It has further been estimated that within the next 20 years another 2 billion people will inhabit this earth.

Figure 1.2 World population increase.

Population explosion, combined with mass urbanization, and proportionally fewer farmers lead to fears from world population specialists of world-wide hunger and famine. However, since the start of the ‘baby boom’ era, the yield of almost all our major agricultural crops has increased as dramatically as human population. Cereal crop and oilseed crop production increased by over 120% and 130%, respectively (Figure 1.3). Similar increases in vegetable production of 80%, fruit production of 43%, pulses by 40% and root crops by 36% have taken place in a 50 year time span. When world agricultural production is adjusted according to population increase (Figure 1.4), cereal production per capita has increased by 17%, and fruit, vegetable, and oilseed production per capita has increased by 28%, 62% and 16%, respectively, while production of root crops and pulses has reduced per head of capita in the world.

Figure 1.3 Total world crop production 1955 and 2005.

Figure 1.4 Total world crop production per capita 1955 and 2005.

These yield increases have been brought about by a combination of higher soil fertility (mainly due to additions of inorganic nitrogen fertilizers), improved chemical control of diseases and pests, better weed control through improved agronomic practices and herbicides, and better crop agronomic practices (e.g. correct plant densities) as well as by growing genetically improved cultivars.

So, how much of the improved yield can be attributed to the plant breeder (i.e. genetic change) and how much to better farming practices (i.e. environmental change)?

Yield increases of more than 100% have been found between single cross maize cultivars over the traditional homozygous varieties. Many researchers have attributed this increase to the heterotic advantage of single crosses over homozygous inbred lines, and therefore conclude that the contribution of plant breeding must be very high. However, a complication arises when comparing single cross hybrids, where selection has been aimed at maximum hybrid productivity, against inbred lines which have been chosen for their combining ability rather than their own performance per se.

It might be suggested that the question could only be answered properly by growing a range of old and new varieties under identical agricultural conditions. Since most modern cultivars are dependent on high levels of soil fertility and the application of herbicides, insecticides and fungicides, these would have to be used in the comparison trial. However, older cultivars were not grown under these conditions. Certainly, older cereal varieties tend to be taller than newer ones and are therefore more prone to lodging (flattening by wind or rain) when grown under conditions of high soil fertility. These considerations also show that cultivars are bred to best utilize the conditions under which they are to be cultivated. Nevertheless, several attempts to compare old and new cultivars have been undertaken in an attempt to determine the contribution of modern plant breeding to recent yield increases.

In one comparison carried out in the United Kingdom, winter wheats ranging in introduction date from 1908 to 1980 were simultaneously evaluated in field trials. In a similar experiment, spring barley cultivars ranging in introduction dates from 1880 to 1980 were compared. From the wheat cultivars available in the mid-1940s the grain yield from this study was about 5.7 t ha−1 but from the most recently introduced cultivars from 1980, yields were about 50% higher. There was a similar improvement in barley yield over the same period of about 30%. Therefore, considering these studies, breeding contributed about half to the more than doubled cereal yield between 1946 and 1980.

In contrast, a study carried out in potatoes, with cultivars with dates of introduction from 1900 to 1982, (Figure 1.5) found that modern plant breeding had been responsible for a very small contribution to the more than doubled potato yield in the United Kingdom. This study in potato may, in part, explain why ‘Russet Burbank’, introduced before 1900, still dominates potato production in the USA; while the cultivar ‘Bintji’, introduced in 1910, remains a leading potato cultivar in the Netherlands.

Figure 1.5 Saleable yield of tubers from potato cultivars grown over a three year period. Yield is related to the year that each cultivar was introduced into agriculture.

In conclusion, modern day crops have shown significant yield increases over the past century. It would be wrong to suggest that the major contributor to this increase has simply been a direct result of plant breeding. Increases have rather resulted from a combination of plant breeding and improvements in crop husbandry. For example, the increased use of inorganic nitrogen fertilizer has greatly increased wheat (and other cereal) yield. However, this was allied with the introduction of semi-dwarf and dwarf wheat cultivars that allowed high nitrogen fertilizer application without detrimental crop lodging. Without the addition of nitrogen fertilizers would the dwarf wheat cultivars have been beneficial? Perhaps not. However, would high nitrogen fertilizer application have been possible without the introduction of dwarf wheat cultivars? It is difficult to know. The overall increase achieved to date has resulted in both genetic and non-genetic changes in agriculture.

In the future the same is likely to be true: that the next leap in crop productivity will result from a marked change in agronomic practice, plus the introduction of plant types that can best utilize this husbandry change. What changes will these be? It is impossible to know with any certainty. Recent moves to reduced tillage systems may be one option that could be considered and that would require specific cultivars to maximize performance under these situations.

Similarly, advances in recombinant DNA techniques may result in the development of crops with markedly different performances and adaptations to those available today. Introduction of these crop types may necessitate a major (or minor) change in crop husbandry to utilize the potential of these genetically modified crops.

THINK QUESTIONS

(1) ‘The yield of many crops species has risen dramatically over the last 50 years. This has been the direct result of plant breeding during this period and hence the trend is likely to continue over the next 50 years’. Briefly discuss this statement.

(2) Different crop species originated in different regions of the world. List the centre of origin of the following ten crop species: Onion (Allium), Alfalfa (Medicago sativa), Rice (Oryza sativa), Potato (Solanum tuberosum), Soybean (Glycine max), Millet (Eleusine coracana), Cotton (Gossypium spp.), Sunflower (Helianuthus spp.), Wheat (Triticum spp.), and Apple (Malus spp.).

(3) ‘The place of origin of crops, their history and evolution are events from the past and therefore have no relevance to modern plant breeding’ True or False? Discuss your answer.

(4) Many believe that civilization (of man) started with the beginnings of agriculture. Basically there are two forms of agriculture: (1) rearing animals for meat, milk etc. and (2) raising crops for human or animal feed. No one knows which form of agriculture evolved first (or maybe both types started together). Explain why (in your opinion) one form came before the other or both forms evolved at the same time.

(5) A combination of natural selection, and selection directed by plant breeders (early and modern) has influenced the crops we now grow. List five charactereristics that mankind has selected which would not have been selected by a natural evolution process

(6) Have modern plant breeders improved the genetic fitness of our agricultural crop species, or have they simply selected plant types that are more suited to modern agricultural systems?

2

MODES OF REPRODUCTION AND TYPES OF CULTIVAR

INTRODUCTION

The most appropriate type of cultivar which can be developed to best fit the needs of a production situation will be determined, in part, by the breeding system and mode of reproduction of the species involved.

A cultivar (or variety) is defined as a group of one or more genotypes which have a combination of characters giving it distinctness, uniformity and stability (DUS).

Distinctness When a cultivar is ‘released’ for production it has to be proven that it is indeed new and that it is distinct from other already available cultivars. Distinctness is often defined on the basis of morphological characters that are known not to be greatly influenced by the environment. But other features such as physiology, disease or virus reaction, insect resistance and chemical quality may be used as well as, increasingly, molecular characterization in some countries (i.e. DNA markers). In the Guide to UK National Listings distinctness is described as follows: ‘The variety, whatever the origin, artificial or natural, of the initial variation from which it has resulted shall be clearly distinguishable, by one or more important characteristics, from any other plant variety’. This requirement is in part to ensure that new cultivars have not previously been registered by other breeding organizations. Information used to determine distinctness, also can be used later to identify and protect proprietary ownership of that cultivar so that other organizations cannot register the same cultivar or trade without permission, (within a set period) in that cultivar.

Uniformity Uniformity is related to the level and type of variation which is exhibited (usually phenotypic) between different plants within the cultivar. Any such variation should be predictable and capable of being described by the breeder. The variation should also be commercially acceptable and occur with no greater a frequency than that defined for that type of cultivar (as we will see below). The amount of variation that is permitted to exist in released cultivars varies according to the country of release. For example, in the United States, provided the degree and type of variation is clearly stated when the cultivar is released, the breeders can decide exactly how much heterogeneity exists for any character. In European countries, regulations regarding uniformity are more clearly defined and requirements for these to be adhered to. In the United Kingdom the guidelines read ‘The plant variety shall be such that the plants of which it is composed are, apart from a very few aberrations, and account being taken of the distinctive features of the reproductive system of the plants, similar or genetically identical as regards the characteristics, taken as a whole, which are considered by the Ministers for the purpose of determining whether the variety is uniform or not’.

Stability Stability of a cultivar means that it must remain true to its description when it is reproduced or propagated. Again the requirements for this differ between countries – in Europe it is generally by statute while in the United States and Canada it is usually considered to be the responsibility of the breeder to ensure stability. Again the UK guidelines give a description of what they mean by stability and it is ‘The plant variety shall continue to exhibit its essential characteristics after successive reproductions or, where the breeder has defined a particular cycle of reproduction, at the end of each cycle of reproduction’.

The requirement of distinctness is needed to protect proprietary cultivars and ensure that different organizations are not trying to claim the same cultivar and identify such cultivars as to their breeder. The requirements of uniformity and stability are there to protect the growers and to ensure that they are being sold something that will grow and exhibit the characteristics described by the breeder.

The further requirement of any new cultivar, is perhaps obvious, but nevertheless is a statutory requirement in many countries and referred to as value for cultivation and use (VCU). VCU can be determined by two primary methods and there will always be debate regarding which system is better. In the United Kingdom, which organizes statutory trials, VCU is described as follows: ‘The quality of the plant variety shall in comparison with the qualities of other plant varieties in a national list, constitute either generally or as far as production in a specific area is concerned, a clear improvement either as regards crop farming or the use made of harvested crops or of products produced from these crops. The Qualities of the plant variety shall for this purpose be taken as a whole, and inferiority in respect of certain characteristics may be offset by other favourable characteristics’.

In a few countries (including the United States) any plant breeder can sell seed from a cultivar developed and registered, irrespective of how well adapted it is to a given region or how productive the cultivar is likely to be. The choice of which cultivar to grow is left entirely to farmers and producers. It is common that farmers will allot a small proportion of the farm to plant a new cultivar, and if acceptable, will increase hectarage with time. Obviously, unadapted cultivars or those, which have inferior end-use quality, are unlikely to gain in acreage in this way. Similarly, companies (seed or breeding) and organizations rely on their reputation to sell their products and reputations can easily be tarnished by releasing and selling inferior products.

However, it is more common that countries have statutorily organized trialling schemes to determine VCU of cultivars that are to be released. This testing is usually conducted over two or three years, in a range of environments that the cultivars are likely to be grown. If breeding lines perform better than cultivars already available in that country, then government authorities will place that cultivar on the National List. Only cultivars that are included on the National List are eligible for propagation in that country. In some countries, newly listed cultivars also are entered into further statutory trials for one to two additional years. Based on performance in these extra years’ trials, cultivars may be added to a Recommended Varieties List. This effectively means that the government authority or testing agency is recommending that is would be a suitable new cultivar for farmers to consider. The theoretical advantage of statutory VCU testing is that it only allows ‘the very best cultivars’ to be grown and prevents unadapted cultivars from being sold to farmers. The major drawbacks of the Regulatory Trialling schemes are:

Mistakes are inevitably made (although it is difficult to estimate at what rate) in that potentially desirable adapted new cultivars simply do not do well in the test conditions, for whatever reason. In this situation the cultivar is removed from further screening and all the time and effort expended by the breeder on that genotype’s development would have been wasted.Authorities (or their agents), who organize these trials are often limited by resources and cannot always evaluate the number of test entries that may be submitted as thoroughly as might be desirable. In these situations there is often a ‘lottery system’ introduced where: companies are allowed to enter a certain number of test entries; not all entries are grown in all trial sites; and a non-comprehensive set of control cultivars included.Statutory trials suffer the same deficiencies as all small plot evaluation tests: they do not always reproduce or mimic the conditions or situations that occur on a realistic scale of production.They delay the period from a cultivar being developed to when it is released for commercial production. In some crop species (e.g. potato) this is not a great problem as the rate of increase of seed tubers in potato is low and it would normally take several years to increase sufficient tuber seeds to be planted on a commercial scale (a rate of increase of approximately 10:1). However, in other seed crops, for example rapeseed (oilseed rape), the rate of seed increase (particularly if off-season increase is possible) can be considerable, around 1000:1, and a three to four year delay in release can be costly to breeding companies.

The criteria for judging both DUS and VCU will be strongly determined by the type of species, particularly its mode of reproduction and multiplication for production.

MODES OF REPRODUCTION

It is essential to have an understanding of the mode(s) of reproduction prior to the onset of a plant breeding programme. The type of reproduction of the species (at least in commerce) will determine the way that breeding and selection processes can be maximized to best effect. There are two general types of plant reproduction sexual and asexual.

Sexual reproduction

Sexual reproduction involves fusion of male and female gametes that are derived either from two different parents or from a single parent. Sexual reproduction is, of course, reliant on the process of meiosis. This involves megaspores within the ovule of the pistil and the male microspores within the stamen. In a typical diploid species, meiosis involves reductional division by meiosis of the 2n female cell to form four haploid megaspores, by the process of megasporogenesis. This process in male cells, to form microspores, is called microsporogenesis. Fertilization of the haploid female cell by a haploid male pollen cell results in the formation of a diploid 2n embryo. The endosperm tissue of the seed can result from the union of two haploid nuclei from the female with another from the pollen, and hence ends up as being 3n.

Asexual reproduction (mitosis) is the multiplication of plant parts or by the production of seeds (apomixis) that do not involve the union of male and female gametes. The process of mitosis will result in two cells that are identical in genetic make-up and of the same composition as the parental cell.

Seeds are effectively classified according to the source of pollen that is responsible for the fertilization. In the case of self-pollination the seeds are a result of fertilization of female egg cells by pollen from the same plant. Cross-pollination occurs when female egg cells are fertilized by pollen from a different plant, usually one that is genetically different. As a result plant species are usually classified into self-pollinating and cross-pollinating species. This is of course a gross generalization. There are species which are effectively 100% self-pollinating, those that are 100% cross-pollinating but there exists a whole range of species that cross-pollinate or self-pollinate to varying degrees. From the top 122 crop plants grown worldwide, 32 are mainly self-pollinating species, 70 are predominantly cross-pollinating, and the remaining 20 are cross-pollinating but do show a degree of tolerance to successive rounds of inbreeding.

The method of pollination will be an important factor in determining the type of cultivar that can, or will, be most adapted to cultivation. For example, most species that can be readily used in hybrid production are generally cross-pollinating but need to be tolerant of inbreeding by selfing. This is because the hybrids are effectively the cross-pollinated progeny between two inbred genotypes.

Self-pollinating species are tolerant of inbreeding and consequently deleterious recessive genes are not common. They tend to have flower structures and behaviour that promote selfing. Individual lines of descent tend to approach homozygosity, shows little heterotic advantage when out-crossed and, individually, tend to have a narrower range of adaptation.

Cross-pollinating species tend to be intolerant to inbreeding, principally because they carry many deleterious recessive genes (these exist in the populations since they can be tolerated in heterozygous form). Generally, cross-pollinating species:

Have a crossing mechanism that promotes out-crossingShow greater heterotic effectAre more widely adaptable to many different environmentsHave individual plants that are highly heterozygous at many loci

Particularly important are the outcrossing mechanisms. Cross-pollinating species often have distance barriers, time barriers or other mechanisms, which limit or prohibit self-pollination. Plants may be monoecious, where separate male and females flowers are located on different parts of the plant (e.g. maize) or indeed dioecious, where male and female flowers occur on different plants. Cross-pollination is also favoured in many cases where male pollen is shed at a time when the female stigma on the same plant is not receptive.

Another, more clearly defined sets of mechanisms are those termed as self-incompatibility. Self-incompatibility occurs when a plant, which has fully functional male and female parts, will not produce mature seed by self-pollination. There is a set of mechanisms that have naturally evolved to increase cross-pollination within plant species and hence promote heterozygosity. Adaptation to environmental conditions is greater if wider ranges of genotypes are produced in a progeny (i.e. the progeny shows greater genetic variation). Thus the chances of survival of at least some of the progeny will be enhanced, or at least the chance of extinction will be reduced.

There are a number of mechanisms than can determine self-incompatibility in higher plants:

Pollen may fail to germinate on the stigma of the same plant flowerPollen tubes fail to develop down the style and hence do not reach the ovaryPollen tube growth is not directed towards the ovule and hence pollen tubes fail to enter the ovaryA male gamete that enters the embryo sac may fail to unite with the egg cellFertilized embryos resulting from self-pollination but these do not produce mature seeds

In several species (e.g. Brassica spp.) self-incompatibility can be overcome by bud-pollination, where pollen is applied to receptive stigmas of plants before the flowers open, as the self-incompatibility mechanism is not functional at this reproductive stage. Self-incompatibility is rarely complete and usually a small proportion of selfed seed can be produced under certain circumstances. For example, it has been found that environmental stress factors (particularly caused by applying salt solution to developing flowers) tends to increase the proportion of self-seed produced.

Asexual reproduction

Asexual reproduction in plants produces offspring that are genetically identical to the mother plant, and plants that are produced this way are called clones. Asexual reproduction can occur by two mechanisms: reproduction through plant parts that are not true botanical seeds and reproduction through apomixis.

Reproduction through plant parts

A number of different plant parts can be responsible for asexual reproduction. For example, the following are some of the possible organs that are reproductive propagules of plants:

A bulb is a modified shoot consisting of a very much shortened stem enclosed by fleshy leaves (e.g. a tulip or an onion);A corm is a swollen stem base bearing buds in the axils of scale-like remains of leaves from the previous years growth (e.g. gladiolus);A cutting is an artificially detached part of a plant used as a means of vegetative propagation.A rhizome is an underground stem with buds in the axils of reduced leaves (e.g. mint or couch grass);A stolon is a horizontally growing stem that roots at nodes (e.g. strawberry runners);A tuber is a swollen stem that grows beneath the soil surface bearing buds (e.g. potato).

Reproduction by apomixis

Asexual propagation of plant seeds can occur in obligate and facultative apomicts. In obligate apomicts, the seed that is formed is asexually produced while in facultative apomicts, most seeds are asexually produced although sexual reproduction can occur.

Apomixis can arise by a number of mechanisms that differ according to which plant cells are responsible for producing an embryo (i.e. androgenesis from the sperm nucleus of a pollen grain; apospory, from somatic ovary cells; diplospory, from 2n megaspore mother cells; parthenogenesis from an egg cell without fertilization; and semigamy from sperm and egg cells independently without fusion). Apomixis can occur spontaneously, although in many cases pollination must occur (pseudogamy) if viable apomictic seeds are to be formed. Although the role of pseudogamy is not understood in most cases, pollination appears to stimulate embryo or endosperm development.

TYPES OF CULTIVAR

It may seem obvious that modes of reproduction determine the type of cultivar that is produced for exploitation. Cultivar types include pure-lines, hybrids, clones, open-pollinated populations, composite-crosses, synthetics and multilines. Obviously it would be difficult, if not impossible to develop a pure-line cultivar of a crop species like potato (Solanum tuberosum) as it is mainly reproduced vegetatively, and has many deleterious (or lethal) recessive alleles. Similarly, pea (Pisum sativum) is almost an obligate self-pollinator and so it would be difficult to develop hybrid pea, if nothing else seed production is likely to be expensive. A brief description of the different types of cultivar is presented below.

Pure-line cultivars

Pure-line cultivars are homozygous, or near-homozygous, lines. Pure-line cultivars can be produced most readily in naturally self-pollinating species (e.g. wheat, barley, pea, soybeans). But they can also be produced from species that we tend to consider as cross-pollinating ones (e.g. pure-line maize, gynoecious cucumber and onion). There is no universally agreed definition of what constitutes a pure-line cultivar, but it is generally accepted that it is normally one in which the line is homozygous for the vast majority of its loci (usually 90% or more).

The most common method used to develop pure-line cultivars from inbreeding species is to artificially hybridize two chosen (usually) homozygous parental lines, self the heterozygous first filial generation (F1) to obtain F2 seed, and continue selfing future generations, upto a point where the line is considered to be ‘commercially true breeding’ maybe the F6 or F7. At the same time it has been common to carry out recurrent phenotypic selection on the segregating population over each generation.

Open-pollinated cultivars

Open-pollinated cultivars are heterogeneous populations comprised of different plants, which are genetically non-identical. The component plants tend to have a high degree of heterozygosity. Open-pollinated cultivars are almost exclusively from cross-pollinating species. Plants within these populations have been selected to a standard that allows for variation in many traits but which shows ‘sufficient’ stability of expression in the characters of interest. Stability of these traits can be used to pass the DUS requirements necessary for cultivar release. Examples of open pollinated cultivars would include onions, rye and non-hybrid sweet corn.

In developing outbreeding cultivars the initial hybridization (the point at which the genetic diversity and variation is exposed) is usually between two outbreeding populations. In this case segregation is apparent at the F1 generation. Desirable populations are identified and improved by increasing the frequency of desirable phenotypes within them.

Hybrid cultivars

Hybrid cultivars (single cross, three-way cross, and double cross hybrids) are very homogeneous and highly heterozygous. A true F1 hybrid cannot be reproduced from seed of the hybrid generation because the progeny would segregate and result in a very nonuniform crop (although sometimes F2 hybrid cultivars are sown).

Hybrid breeding is perhaps the most complex of the breeding methods. The process of cultivar development involves at least two stages. The first stage is to select desirable inbred lines from chosen out pollinated populations. These inbred selections are then used in test crosses to allow their comparison and assessment in relation to their general or specific combining ability. Superior parents are selected and these are then hybridized to produce seed of the hybrid cultivar. The parent lines are then maintained and used to continually reproduce the F1 hybrids.

Clonal cultivars

Clonal cultivars are genetically uniform but tend to be highly heterozygous. Uniformity of plant types is maintained through vegetative rather than sexual reproduction. Cultivars are vegetatively propagated by asexual reproduction (clones) including cuttings, tubers, bulbs, rhizomes and grafts (e.g. potato, peaches, apples, chrysanthemums). A cultivar can also be classified as a clone if it is propagated through obligate apomixis (e.g. buffelgrass).

Clonal varietal development begins by either sexual hybridization of two parents (often clones) or the selfing of one of them to generate genetic variability through the normal process of sexual reproduction. Most of the parental lines will be highly heterozygous and segregation will begin at the F1 stage. Desirable recombinants are selected from amongst the clonal propagules. Breeding lines are maintained and multiplied through vegetative reproduction and hence the genetic constitution of each selection remains ‘fixed’.

Synthetic cultivars

Inter-crossing a set number of seed lines generates a synthetic variety. In the simplest sense a ‘first generation two-parent synthetic’ is very similar to an F1 hybrid. Synthetic lines can be derived from cross-pollinated lines or self-pollinating lines, although the latter instance is not common. Synthetic cultivars have a series of categories (Syn.1, Syn.2,…, Syn.n) according to the number of open pollinated generations that have been grown since the synthetic line was generated. For example the first generation synthetics are classified as Syn.1, if this population is then selfed or out pollinated the next generation is classified as Syn.2 etc.

The use of synthetic cultivars has been most successful in cases where crop species show partial self-incompatibility (e.g. alfalfa). Examples of other crops where synthetic varieties have been released include rapeseed (B. rapa cultivar types), rye, pearl millet, broom grass and orchard grass.

Multiline cultivars

Multiline cultivars are mixtures or blends of a number of different cultivars or breeding lines. Each genotype in the mixture will be represented by at least 5% of the total seed lot. Many multilines are the result of developing near isogenic lines and using these to initiate the mix. These cultivars are usually self-pollinating species. A multiline is therefore not the same as a synthetic where the aim is to maintain heterozygosity by inter-crossing between the parent lines. Multilines became popular with the aim of increasing disease resistance by reducing the pressure for a pathogen to evolve/mutate to overcome the biological resistance. For example, near isogenic lines of barley, which differ in that each line has a different qualitative disease resistance, could be mixed to make a multiline. The main thought is to make the epidemiology of the pathogen such that it would be less likely to evolve virulence to all resistance genes in the mixture.

Composite-cross cultivars

Composite-cross populations are cultivars derived by inter-crossing two or more cultivars or breeding lines. These cultivar types have all tended to be inbreeding species (e.g. barley or lima beans). After the initial hybridizations have been carried out the composite-cross population is multiplied in a chosen environment such that the most adapted segregants will predominate and those less adapted to these conditions will occur at lower frequencies. A composite-cross population cultivar is therefore continually changing and can be considered (in a very loose sense) similar to old land races. Breeders’ seed can never be maintained as the cultivar was originally released.

ANNUALS AND PERENNIALS

Plant species are categorized into annuals and perennials. World crop plants are fairly evenly distributed between annuals (approximately 70 species) and perennials (approximately 50 species). All major self-pollinating crop species are annuals while the greatest majority of cross-pollinating crops are perennials. Perennials pose greater difficulty in breeding than most annuals. Most perennials do not become reproductive within the first years of growth from seed. Most perennials are clonal cultivars and this can cause additional difficulties in maintaining disease free parental lines and breeding material. Winter annuals require vernalization of chill treatment before moving from vegetative to reproductive and can increase the time necessary for developing cultivars.

REPRODUCTIVE STERILITY

Female and male sterility has been identified in many crop species. Genetic and cytoplasmic male sterility have been identified in several plant species. Plant breeders in designing breeding programmes can utilize sterility and breeding schemes can be designed specifically to accommodate sterility, particularly in developing hybrid cultivars. Sterility also can pose problems and limits to the choice of parental cross combinations that are possible.

THINK QUESTIONS

(1) Complete the following table by assigning a YES or NO to each of the 16 cells.

(2) Inbreeding and outbreeding species tend to have different characteristics. Explain factors that would determine if a given species should be classified as inbreeding or outbreeding.