A PROBLEM WITH THE ESTIMATE OF SELF-FERTILIZATION DEPRESSION IN THE HERMAPHRODITE FRESHWATER SNAIL BULINUS TRUNCATUS: THE EFFECT OF GROUPING.

Abstract

The evolution of self-fertilization has been the subject of both numerous theoretical and empirical studies (reviewed by Jain 1976; Charlesworth and Charlesworth 1987; Uyenoyama et al. 1993; Jarne and Charlesworth 1993). These studies have shown that inbreeding depression, that is, the lowered fitness of inbred as compared with outbred individuals, is an important factor determining this evolution. Although most research has focused on plants, some studies have been performed on hermaphrodite animals (see Jarne and Charlesworth 1993) and particularly freshwater snails (see Jarne et al. 1993). Differences in survival rates between progeny of selfing and outcrossing individuals have been shown and interpreted as resulting from inbreeding depression. Selfing snails also have a lower fecundity than outcrossing snails. The whole effect (fecundity of parents and inbreeding depression of their progeny) has been referred to as self-fertilization depression (Jarne et al. 1991). However, specific problems may arise when estimating self-fertilization depression in animals. In experiments on hermaphroditic freshwater snails, selfed progenies have been produced usually by isolated virgin adults, and outcrossed progenies have been produced by grouped adults (review in Jarne et al. 1993). The effect of the mating system (outcrossing) and the effect of grouping are therefore associated when analyzing the results of outcrossing. It has been shown for various species of freshwater snails that both population density and size influence growth, survival, and fecundity (Chernin and Michelson 1956a,b; Thomas et al. 1974; Thomas and Benjamin 1974; Ahmed et al. 1986; Chaudhry and Morgan 1987). Increased density and population size is generally associated with a reduced fecundity and could lead to an underestimate of self-fertilization depression. In this paper, we investigate whether previous estimates of self-fertilization depression may have been biased by a grouping effect. By using aphallic individuals of the hermaphrodite freshwater snail Bulinus truncatus (see below), we can estimate the effect of grouping selffertilized for isolated and grouped snails. This is done by comparing the fitness of isolated snails with that of individuals at various group size and densities. Bulinus truncatus is a self-fertile hermaphrodite with two sexual morphs, euphallic and aphallic individuals, occurring in natural populations. Euphallic snails can self-fertilize as well as outcross as males or as females. Aphallic snails are characterized by the absence of the male copulatory organ (Larambergue 1939). They can selffertilize and receive sperm from an euphallic partner. However, they cannot transfer their own sperm to a partner (Larambergue 1939). No mutual exchange of sperm occurs during copulation in hermaphrodite freshwater snails: one individual acts as the female when its partner acts as the male. The determinism of aphally is still unclear; some results indicate a genetic determinism (Larambergue 1939), whereas others point strongly to the role of temperature (Schrag and Read 1992). A consequence of aphally is that purely aphallic populations can only self-fertilize. By using aphallic snails, the consequences of grouping can therefore be estimated without interference of the mating system. The aim of the present work is to test (1) whether a grouping effect occurs in the conditions previously used to analyze the self-fertilization depression (about 150 mL of water per snail with group size of 5 to 10 snails) (Jarne and Delay 1990; Jarne et al.