Résumé
An important parameter in the evolution of sexual polymorphisms in hermaphroditic organisms is the way resources are allocated to male and female functions by various morphs. Detecting differences in allocation between morphs may not always be possible under optimal conditions and may require stressful conditions. Here we investigate the allocation to various functions in the hermaphroditic, highly selfing, freshwater snail Bulinus truncatus, in relation to phally polymorphism. In natural populations, two morphs, euphallic individuals (regular hermaphrodites) and aphallic individuals (missing the male copulatory organ), co-occur at various ratios. To quantify whether the loss of the male organ in aphallics may be compensated for by other functions, we compared the fecundity, size, and survival of 123 aphallic and 111 euphallic snails originating from 53 families and three natural populations under optimal (25 degrees C) and stressful (19 degrees C and 30 degrees C) temperatures. We also compared the hatching rate and survival of offspring of 10 aphallic and 10 euphallic snails per population at these three temperatures. Firstly, we confirmed a strong effect of temperature on the determination of the sexual morph. Secondly, we found that the two morphs did not differ in size, fecundity, hatching rate, or survival of offspring, suggesting no obvious cost of the male function in isolated snails, even under stressful conditions. However, a trade-off was observed: aphallic individuals laid more capsules, though with fewer eggs per capsule, than euphallic individuals. Thirdly, we found a linear variation of some parameters with the family aphally ratio, suggesting that fitness may be perceived in a frequency-dependent way. Our results are discussed with regard to the relative strength of selective and stochastic factors in the evolution of phally polymorphism.