Consequences of cytonuclear epistasis and assortative mating for the genetic structure of hybrid populations

Abstract

The effects of assortative mating and epistasis between interspecific nuclear and cytoplasmic genomes (cytonuclear incompatibility) on levels of homozygote cytonuclear disequilibria are analysed in an effort to gain a better understanding of the processes affecting the genetic structure of hybrid populations and the nature of isolating barriers between species. Cytonuclear incompatibility has a direct effect on the level of disequilibria between maternally inherited cytoplasmic and biparentally inherited nuclear markers at the adult stage that is largely a function of (i) the strength of selection; (ii) the level of dominance of the nuclear alleles; and (iii) the level of disequilibria occurring at the seed stage. Assortative mating based on nuclear genotypes conserves cytonuclear genetic associations between the adult and the subsequent seed stage. The preservation of cytonuclear associations between the previous adult and the current progeny stages enhances the level of disequilibria at the next adult stage because the initial level of cytonuclear disequilibria in the progeny is greater before the action of selection. Under random mating, cytonuclear genetic associations are broken up in each generation, so the levels of disequilibria attained by selection are not as high as those observed with assortative mating. The changes in disequilibria between life stages in a hybrid Iris population were examined to infer the strength of selection and levels of assortative mating. Based on these estimates, it appears that the combined action of epistasis and assortative mating would be sufficient to maintain the high levels of cytonuclear disequilibria observed.