THE EVOLUTION OF DIMORPHISM AND SEPARATE SEXES IN SCHISTOSOMES

The fixation of gonochorism (separate sexes) from an hermaphrodite anc estor in the Schistosomatidae may have been the result of selection fo r dimorphism, under the particular constraint of having to expel eggs from the vertebrate circulatory system. The hermaphroditic system, whi ch exists in blood parasites closely related to Schistosomatidae, e.g. Spirorchidae and Sanguinicolidae, may result from a weaker constraint caused by the different pathways of egg output, and a weaker aggregat ion of parasites in the host population that will act against the fixa tion of gonochorism. The importance of these two factors relative to t wo others (inbreeding depression and allocation to female function in hermaphrodites) in the evolution of gonochorism in blood parasites is evaluated using an analytical model.