Evidence for Lack of Inbreeding Avoidance by Selective Mating in a Simultaneous Hermaphrodite

Abstract

In the many species in which breeding between close kin yields deleterious effects (inbreeding depression), outbreeding is promoted by various mechanisms, such as kin recognition and, in hermaphrodites, barriers to self-fertilization. When the cost of inbreeding is not too high, hermaphrodites will be selected to self. The sexually reproducing, simultaneous hermaphroditic flatworm Dugesia polychroa does not self-fertilize, suggesting that selfing is too costly in this species. To ascertain whether kin recognition exists as a behavioral mechanism to avoid inbreeding in D. polychroa, experiments were designed using two genetically identical regenerates, or clonemates, from a single halved worm. The mating behavior of clonemates vs. unrelated worms was recorded, in pairs and in triads consisting of two clonemates and one unrelated worm. No clear differences in frequency or duration of copulations could be demonstrated between clonemates or unrelated worms, either in pairs or triads. Our results indicate that there is no kin recognition mechanism or selective mating for inbreeding avoidance in D. polychroa. We hypothesize that, in the field, dispersal in this species is sufficient to limit inbreeding. Additional key words: inbreeding, mate choice, clones, hermaphrodite An important factor in mate choice is relatedness. It is generally accepted that inbreeding, breeding between close kin, is unadaptive and its deleterious effects, inbreeding depression, have been observed in many species (Hepper 1986; Charlesworth & Charlesworth 1987). Inbreeding can be limited or avoided by dispersing from the natal area or by recognition of (the degree of) relatedness and controlled outcrossing (Bateson 1983). Kin recognition (Wilson 1987) is widespread in the animal kingdom, appearing in a variety of taxa, from coelenterates to arthropods to vertebrates. Recognition can take place at several levels: discrimination between members of one's own and an alien group, discrimination of classes within a group, and discrimination of individuals. The latter has been rarely reported among invertebrates, except for colonial forms such as corals and sponges (Fletcher 1987) and ascidians (Grosberg 1988). In these cases, however, recognition of self is used to make fusion or rejection decisions at the colony level. Despite these costs, inbreeding through selfing is not a Present address: Ecology and Evolution Group, Division of Botany and Zoology, Australian National University, ACT 0200 Australia. b To whom correspondence should be addressed. uncommon among hermaphrodites. In general, selfing will be selected for whenever the fitness of selfed offspring is higher than half the fitness of outcrossed offspring (Jarne & Charlesworth 1993). However, selfing even occurs in cases where outcrossing is favored and maintained by strong inbreeding depression, e.g., in freshwater gastropods (Jarne et al. 1993), ascidians (Bishop & Ryland 1993), and land snails (Chen 1993, 1994). The fact that it is still present, despite inbreeding costs, might be due to the something is better than nothing advantage when no mates are encountered due to low population density (low density model, Ghiselin 1969). We investigated recognition and discrimination against kin in the free-living turbellarian Dugesia polychroa (SCHMIDT 1861), a freshwater triclad, or planarian (Tricladida, Paludicola). These sexually reproducing worms are diploid and cross-fertilize during copulation (Peters et al. 1996). Chemoreception is well developed (Hyman 1951). Although self-fertilization is known from at least three species (Benazzi 1952, 1991; Anderson & Johann 1958), it is thought to be rare among turbellarians (Goetsch 1924; Hyman 1951) and absent in Dugesia gonocephala (Vreys et al. 1994). In D. polychroa, worms will start to produce infertile cocoons after extended isolation (>2 months; This content downloaded from 207.46.13.172 on Fri, 07 Oct 2016 06:18:33 UTC All use subject to http://about.jstor.org/terms