RESOURCE-ALLOCATION AND STRESSFUL CONDITIONS IN THE APHALLIC SNAIL BULINUS-TRUNCATUS

An important parameter in the evolution of sexual polymorphisms in her maphroditic organisms is the way resources are allocated to male and f emale functions by various morphs. Detecting differences in allocation between morphs may not always be possible under optimal conditions an d may require stressful conditions. Here we investigate the allocation to various functions in the hermaphroditic, highly selfing, freshwate r snail Bulinus truncatus, in relation to phally polymorphism. In natu ral populations, two morphs, euphallic individuals (regular hermaphrod ites) and aphallic individuals (missing the male copulatory organ), co -occur at various ratios. To quantify whether the loss of the male org an in aphallics may be compensated for by other functions, we compared the fecundity, size, and survival of 123 aphallic and 111 euphallic s nails 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 offsp ring of 10 aphallic and 10 euphallic snails per population at these th ree 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 survi val of offspring suggesting no obvious cost of the male function in is olated snails, even under stressful conditions. However, a trade-off w as observed: aphallic individuals laid more capsules, though with fewe r eggs per capsule, than euphallic individuals. Thirdly, we found a li near variation of some parameters with the family aphally ratio, sugge sting that fitness may be perceived in a frequency-dependent way. Our results are discussed with regard to the relative strength of selectiv e and stochastic factors in the evolution of phally polymorphism.