Evidence for unconditional sperm transfer and sperm-dependent parthenogenesis in a hermaphroditic flatworm (Girardia tigrina) with fissipary

Abstract

Sperm-dependent parthenogens are expected to promote higher female fecundity at the expense of the production of excessive numbers of sperm. They are also expected to trade sperm strictly, in order to compensate for the loss of nutrients invested in sperm, that otherwise could be allocated to the female function. In this paper, we present evidence that in a natural population of the freshwater planarian Girardia (= Dugesia) tigrina, sperm-dependent parthenogenesis is alternated with fissipary, a combination not reported before for freshwater flatworms. More specifically, we found that most individuals that engage in a copulation have no or only few testes and sperm available, despite the presence of a fully developed penis, indicating that male allocation is fairly low in this (dense) population. Female fecundity on the contrary is rather high, with isolated individuals producing up to six fertilized cocoons over a 3-week period, releasing five young per cocoon on average. An analysis of the mating behaviour further shows that matings in this species are simple and direct with no distinct courtship behaviour. Copulations last for about 36 min and occur at a rate of at least one copulation a day. Bilateral sperm transfer is the rule, but a considerable number of copulations are unidirectional or without sperm transfer. Non-reciprocal inseminations mainly result from the fact that one or both partners has no sperm stored prior to copulation, indicating that strict sperm trading plays no role in this population of G. tigrina.