Theoretical studies of the influence of apomixis and polyploidy on genetic diversity and cytonuclear disequilibria in plant populations.

摘要

The research contained within consists of three theoretical investigations of the nuclear and cytonuclear effects of facultative apomixis and polyploidy. First, the maintenance of genetic variation and deviations from Hardy-Weinberg frequencies at a diallelic locus under mixed mating with apomixis and constant viability selection was analyzed. Analytical proofs show that: (1) at most one polymorphic equilibrium exists, (2) polymorphism requires overdominant or underdominant selection, and (3) a simple, modified overdominance condition is sufficient to maintain genetic variation. In numerical analyses, the potential for maintaining both alleles increases with increasing apomixis or outcrossing and decreasing selfing. Simulations also indicate that equilibrium levels of heterozygosity will often be statistically indistinguishable from Hardy-Weinberg frequencies and that adults, not seeds, should usually be censused to maximize detecting deviations.; Next, the cytonuclear effects of generalized mixed mating, including all combinations of selfing, outcrossing, and apomixis, the asexual production of seeds, for diploids were analyzed. Then, a cytonuclear framework for tetraploid populations in which a diallelic nuclear marker exhibits tetrasomic inheritance was developed. This second system requires two separate parameterizations. In both systems, dynamical solutions for the disequilibria under generalized mixed mating, with any combination of apomixis, selfing and outcrossing, were derived and analyzed. Double reduction was also taken into account in the tetrasomic system. For both diploids and tetraploids, all disequilibria ultimately decay to zero, unless nuclear and cytoplasmic alleles are nonrandomly associated and outcrossing is absent, in which case, permanent associations result. Selfing and apomixis retard the decay of disequilibria (or approach to equilibrium), and often to the same extent. In contrast, double reduction can accelerate the loss of tetraploid cytonuclear associations, but only negligibly in hybrid zones; nevertheless, this loss is never faster than in diploids. Practical guidelines for experimental design and data analysis are given. Cytonuclear disequilibrium dynamics under mating system alone furnish a quantitative baseline for null hypotheses against which to test for the presence of other evolutionary forces.