SELFING AND OUTCROSSING IN A PARASITIC HERMAPHRODITE HELMINTH (TREMATODA, ECHINOSTOMATIDAE)

Echinostomes are simultaneous hermaphrodite trematodes, parasitizing t he intestine of vertebrates. They are able to self- and cross-insemina te. Using electrophoretic markers specific for three geographical isol ates (strains) of Echinostoma caproni, we studied the outcrossing rate from a 'progeny-array analysis' by comparing the mother genotype with those of its progeny. In a simultaneous infection of a single mouse w ith two individuals of two different strains, each individual exhibits an unrestricted mating pattern involving both self- and cross-fertili zation. The association in mice of two adults of the same strain and o ne adult of another strain shows a marked mate preference between indi viduals of the same isolate. From mice coinfected with one parent of t he three isolates, each parent was shown to be capable of giving and r eceiving sperm to and from at least two different partners. Mating sys tem polymorphism in our parasitic model is thus discussed in the conte xt of the theories usually advanced.