FINITE ISLAND MODEL FOR ORGANELLE AND NUCLEAR GENES IN PLANTS

Recurrence equations for genetic diversities and differentiation were developed for hermaphrodite plant species in an island model of popula tion structure. This was made possible by the definitions of diversiti es at all hierarchical levels from gamete to total population and by t he definition of migration rates specific to plants for both nuclear a nd cytoplasmic genomes. Mating system was also incorporated. Numerical computations were used to compare equilibrium values of differentiati on obtained with our equations with those predicted by classical formu las. We show that the differences (sometimes high) result from the int erpretations of the definition of gene diversity in a population of fi nite size. We interpret it as the probability that two genes sampled w ith replacement are different alleles (instead of without replacement) . The effects of several parameters (ploidy level, mode of inheritance , outcrossing rate, population size) on genetic subdivision were evalu ated. Contrary to the situation in animals, plant migration is intrins ically asymmetrical because a gene transmitted to the next generation through the male gamete may migrate in the pollen grain and in the see d, whereas a gene transmitted through the female gamete can migrate on ly in the seed. As a consequence, mode of inheritance (in the case of cytoplasmic genes) and outcrossing rate have strong impacts on subdivi sion, especially when pollen migration is larger than seed migration ( a likely situation in many plant species). Parameters estimated in a s urvey of oak populations (Quercus robur L.) were used to examine wheth er our understanding of a real situation could be improved by the mode l. In particular, the rate of return to equilibrium was studied after a perturbation, i.e. a temporary decrease of population sizes (a bottl e-neck).