PBG 650 Advanced Plant Breeding Module 3: Changes in gene frequency due to selection

Systematic changes in gene frequency Falconer & Mackay, Chapter 2 Predictable in both magnitude and direction –Migration –Mutation –Selection Quantities of interest –Δq = q 1 – q 0 –q t –q at equilibrium (when Δq = 0) Multiple forces at work (e.g., mutation and selection) Important in nature (maybe less so in breeding populations)

Selection Individuals differ in their contribution of genes to the next generation  fitness = adaptive value = selective value Types of selection Viability - the probability of survival through each reproductive stage –Darwin’s natural selection Fertility – number of offspring produced –may work in opposition to survival –Darwin’s sexual selection

Components of fitness Zygote Adult Gamete Zygote viability mating success fertility (capability to have offspring) fecundity (#gametes) viability (survival) Artificial selection fitness is defined in the context of the environment in which individuals live, mate, and reproduce

Relative fitness Absolute fitness refers to the number of gametes transmitted by a zygote, but this is seldom observed directly We generally make observations on zygotes, so we measure relative fitness in comparison to either the average for the population or the best genotype s = coefficient of selection

Selection To quantify fitness, we have to consider the degree of dominance with respect to fitness –may not be the same as the dominance with respect to the expression of the primary trait controlled by the gene Relative fitness of A 1 A 2 s is the selection coefficient h is the degree of dominance When h = 0 W 12 = W 11 (A 1 is completely dominant) When h = 1 W 12 = W 22 (A 2 is completely dominant)

Degree of Dominance for Fitness no dominance partial dominance complete dominance overdominance A1A1A1A1 A2A2A2A2 A1A2A1A2 A1A1A1A1 A2A2A2A2 A1A2A1A2 1-s 1-(1/2)s 1 1-s 1-hs 1 A1A1A1A1 A2A2A2A2 A1A2A1A2 1-s 1 A1A1A1A1 A2A2A2A2 A1A2A1A2 1-s 2 1-s 1 1 fitness

Selection against a recessive gene Genetic load = proportion of the population that does not reproduce due to presence of a gene i.e., the reduction in fitness

Selection against a recessive lethal - example What will the frequency of the recessive allele be after t generations? s = 1 to go from q=0.20  0.01 generations What could a breeder do to speed up the process?

Change in gene frequency with selection Falconer & Mackay, pg 28 no dominance partial dominance complete dominance of A 1 selection against A 2 overdominance complete dominance of A 1 selection against A 1

Change of gene frequency due to selection Complete Dominance s=0.20 q (-)=selection against A 2 (+)=selection for A 2 Falconer & Mackay, pg 30

Selection for a favorable allele Haldane’s Sieve: In nature, favorable alleles tend to be dominant For the case of s = 0.5, p 0 = 0.05

Direction of selection Other possibilities: Frequency dependent selection Multiple peak epistasis Which types of selection are common in plant breeding?