Principles of Plant Genetics and Breeding. George Acquaah
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varies among species, being extreme in species such as alfalfa in which inbreeding produces homozygous plants that fail to survive. Further, the effect of inbreeding is most significant in the first five to eight generations, and negligible after the eighth generation in many cases.
3.9.2 Applications
Inbreeding is desirable in some breeding programs. Inbred cultivars of self‐pollinated species retain their genotype through years of production. In cross‐pollinated species, inbred lines are deliberately developed for use as parents in hybrid seed production. Similarly, partially inbred lines are used as parents in the breeding of synthetic cultivars and vegetatively propagated species by reducing the genetic load. Another advantage of inbreeding is that it increases the genetic diversity among individuals in a population, thereby facilitating the selection process in a breeding program.
3.9.3 Mating systems that promote inbreeding
Mating is a way by which plant breeders impact the gene frequencies in a population. Four mating systems are commonly used to effect inbreeding: self‐fertilization, full‐sib mating, half‐sib mating, and backcrossing. Self‐fertilization is the union of male and female gametes; full‐sib mating involves the crossing of pairs of plants from a population. In half‐sib mating, the pollen source is random from the population, but the female plants are identifiable. In a backcross, the F1 is repeatedly crossed to one of the parents. Self‐fertilization and backcrossing are the most extreme forms of inbreeding attaining a coefficient of inbreeding (F) of 15/16 after four generations of mating. Autopolyploids have multiple alleles and hence can accumulate more deleterious alleles that remain masked. Inbreeding depression is usually more severe in autopolyploids than diploid species. However, the progression to homozygosity is much slower in autopolyploids than in diploids.
3.10 Concept of population improvement
The general goal of improving open or cross‐pollinated species is to change the gene frequencies in the population toward fixation of favorable alleles while maintaining a high degree of heterozygosity. Unlike self‐pollinated species in which individuals are the focus and homozygosity and homogeneity are desired outcomes of breeding, population improvement focuses on the whole group, not individual plants. Consequently, open‐pollinated populations are not homogeneous.
3.11 Types
The population can be changed by one of two general strategies (i.e. there are two basic types of open pollinated populations in plant breeding) – by population improvement and by development of synthetic cultivars. To develop cultivars by population improvement entails changing the population en masse by implementing a specific selection tactic. A cultivar developed this way is sustainable in a sense, maintaining its identity indefinitely through random mating within itself in isolation. The terminology “synthetic” is used to denote an open pollinated cultivar developed from combining inbred or clonal parental lines. However, the cultivar is not sustainable and must be reconstituted from parental stock. Other usage of the term occurs in the literature.
3.11.1 Methods of population improvement
Some form of evaluation precedes selection. A breeding material is selected after evaluating the variability available. Similarly, advancing plants from one generation to the next is preceded by an evaluation to determine individuals to select. In self‐pollinated species, individuals are homozygous and when used in a cross their genotype is precisely reproduced in their progeny. A progeny test is hence adequate for evaluating an individual's performance. However, open‐pollinated species are heterozygous plants and are further pollinated by other heterozygous plants growing with them in the field. Progeny testing is hence not adequately evaluative of the performance of individual plants of such species. A more accurate evaluation of performance may be achieved by using pollen (preferably from a homozygous source – inbred line) to pollinate the plants. As previously described, the method of evaluating the performance of different mother plants in a comparative way using a common pollen source (tester line) is called a test cross. The objective of such a test is to evaluate the performance of a parent in a cross, a concept called combining ability.
The methods used by plant breeders in population improvement may be categorized into two groups, based on the process for evaluating performance. One group of methods is based solely on phenotypic selection and the other on progeny testing (genotypic selection). The specific methods include mass selection, half‐sib, full‐sib, recurrent selection, and synthetics.
Key references and suggested reading
1 Anderson, J.B. and Kohn, L.M. (1998). Genotyping, gene genealogies and genomics bring fungal population genetics above ground. Trends in Ecology and Evolution 13 (11): 444–449.
2 Ayala, F.J. and Campbell, C.A. (1974). Frequency‐dependent selection. Annual Review of Ecology, Evolution, and Systematics 5: 115–138.
3 Brown, J.K.M. (1996). The choice of molecular marker methods for population genetic studies of plant pathogens. New Phytologist 133: 183–195.
4 Cornelius, P.L. and Dudley, J.W. (1974). Effects of inbreeding by selfing and full‐sib mating in a maize population. Crop Science 14: 815–819.
5 Crow, J.F. and Kimura, M. (1970). An Introduction to Population Genetics Theory. New York: Harper and Row.
6 Falconer, D.S. (1981). Introduction to Quantitative Genetics, 2e. NY: Longman.
7 Hartl, D.L. and Clark, A.G. (1997). Principles of Population Genetics, 3e. Sunderland, MA: Sinauer Associates Inc.
8 Hayward, M.D. and Breese, E.L. (1993). Population structure and variability. In: Plant Breeding: Principles and Practices (eds. M.D. Hayward, N.O. Bosemark and I. Ramagosa), 16–29. London: Chapman and Hall.
9 Hedrick, P.W. (1985). Genetics of Populations. Boston: Jones & Bartlett.
10 Li, C.C. (1976). A First Course in Population Genetics. Pacific Grove, CA: Boxwood.
11 McDonald, B.A. and Linde, C. (2002). Pathogen population genetics, evolutionary potential, and durable resistance. Annual Review of Phytopathology 40: 349–379.
12 McDonald, B.A. and McDermott, J.M. (1993). Population genetics of plant pathogenic fungi. Bioscience 43: 311–319.
13 Milgroom, M.G. and Fry, W.E. (1997). Contributions of population genetics to plant disease epidemiology and management. Advances in Botanical Research 24: 1–30.
Internet resources
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/Hardy_Weinberg.html – Excellent discussion of population genetics.
Outcomes assessment
Part A
Please answer these questions true or false:
1 Inbreeding promotes heterozygosity.
2 Naturally cross‐breeding species are more susceptible to inbreeding than naturally self‐pollinated species.
3 In Hardy‐Weinberg equilibrium gene frequencies add up to unity.
4 Open‐pollinat ed species can be improved by mass selection.
Part B
Please answer the following questions:
1 Define the terms (a) population and (b) gene pool.
2 Give three major factors that influence the genetic structure of a population during the processes of transmission of genes from one generation to another.
3 Explain