THE EVOLUTION OF PHALLY POLYMORPHISM IN BULINUS-TRUNCATUS (GASTROPODA, PLANORBIDAE) - THE COST OF MALE FUNCTION ANALYZED THROUGH LIFE-HISTORY TRAITS AND SEX ALLOCATION

In the hermaphrodite freshwater snail Bulinus truncatus, two sexual mo rphs, euphallic (regular hermaphrodites) and aphallic individuals with out a male copulatory organ, co-occur at various ratios in natural pop ulations. Both aphallic and euphallic individuals can reproduce by sel fing, but when outcrossing aphallic individuals can only play the fema le role. A comparison of life-history traits and sex allocation in the se two forms provides the opportunity to investigate the evolution and maintenance of sexual polymorphisms. This study was performed to test whether a reallocation of resources from the lost male function to th e female function occurs in aphallic snails at the level of both sex o rgans (sex allocation) and life-history traits. In a first experiment we compared life-history traits over a whole life-cycle under selfing between the two sexual morphs. In a second experiment, the sex organs were weighed to test for a difference in sex allocation between the tw o morphs. No difference in resource allocation to female function betw een the two morphs was observed in either experiment. This is in contr ast to patterns frequently observed in sexually polymorphic plants, an d in a previous study performed on aphally in the same snail species. We discuss the genetic and physiological hypotheses that could explain these results, and their consequences for the evolution and maintenan ce of phally polymorphism in B. truncatus.