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 ®ssipary, a combination not reported before for freshwater¯atworms. More speci®cally, 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 ®ve 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. Nonreciprocal 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.

Reproductive modes and life cycles of freshwater planarians (Platyhelminthes, Tricladida, Paludicula) from southern Brazil

“…Girardia tigrina and Girardia schubarti are planarian species that present populations with inter‐ and intraspecific variation in several respects, from genetics to reproductive behavior and regeneration capacity. G. tigrina is a widespread, native American triclad (Gee et al 1998; Tamura et al 1998) with sexual, fissiparous, and parthenogenetic populations (Vreys et al 2002) that may also differ in their ploidy (Benazzi & Benazzi‐Lentati 1976). G. schubarti is a species whose occurrence is described only in Brazil (Benazzi & Benazzi‐Lentati 1976), and its populations may also vary in ploidy and reproductive behavior (Kawakatsu et al 1984).…”

“…The southern Brazilian planarians Girardia tigrina GIRARD 1850 and Girardia schubarti MARCUS 1946 have been used in morphological, karyological, and taxonomic studies (see, e.g., Kawakatsu et al 1981Kawakatsu et al , 1982Kawakatsu et al , 1985Vreys et al 2002) and as test organisms in toxicity assays (Rivera & Perich 1994;Guecheva et al 2001Guecheva et al , 2003. However, little is known about their mating behavior, life cycles, and ecology.…”