Abstract
Abstract Phally polymorphism in snails offers an opportunity, hitherto neglected, to study the evolution of sexual polymorphism in animals. It is characterized by the co-occurrence in natural populations of regular (euphallic) individuals and aphallic individuals with no male copulatory organ. Both sexual morphs can self-fertilize but, when outcrossing, aphallic individuals can only play the female role. Using simple models, we investigated the influence of selfing rate, inbreeding depression and allocation to female function on the evolution of aphally, assuming different kinds of inheritance of aphally. These models show that high selfing rates favour the maintenance of aphally and permit polymorphic situations at equilibrium for some sets of these parameters, except with cytoplasmic inheritance. We then review the empirical data sets on aphally, mainly available in the freshwater snailBulinus truncatus, to evaluate the determination of aphally and the parameters of the models as well as the potential role of other factors, including polyploidy, parasitism, population structure and dynamics. In the light of both theoretical and empirical results, we propose likely scenarios for the evolution of aphally.
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