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

The effects of assortative mating and epistasis between interspecific nucle ar and cytoplasmic genomes (cytonuclear incompatibility) on levels of homoz ygote cytonuclear disequilibria are analysed in an effort to gain a better understanding of the processes affecting the genetic structure of hybrid po pulations 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 selec tion; (ii) the level of dominance of the nuclear alleles; and (iii) the lev el of disequilibria occurring at the seed stage. Assortative mating based o n nuclear genotypes conserves cytonuclear genetic associations between the adult and the subsequent seed stage. The preservation of cytonuclear associ ations between the previous adult and the current progeny stages enhances t he level of disequilibria at the, next adult stage because the initial leve l of cytonuclear disequilibria in the progeny is greater before the action of selection. Under random mating, cytonuclear genetic associations are bro ken up in each generation, so the levels of disequilibria attained by selec tion are not as high as those observed with assortative mating. The changes in disequilibria between life stages in a hybrid Iris population were exam ined to infer the strength of selection and levels of assortative mating. B ased on these estimates, it appears that the combined action of epistasis a nd assortative mating would be sufficient to maintain the high levels of cy tonuclear disequilibria observed.