Evolutionary implications of a high selfing rate in the freshwater snail Lymnaea truncatula

Self-compatible hermaphroditic organisms that mix self-fertilization and outcrossing are of great interest for investigating the evolution of mating systems. We investigate the evolution of selfing in Lymnaea truncatula, a self-compatible hermaphroditic freshwater snail. We first analyze the consequences of selfing in terms of genetic variability within and among populations and then investigate how these consequences along with the species ecology (harshness of the habitat and parasitism) might govern the evolution of selfing. Snails from 13 localities (classified as temporary or permanent depending on their water availability) were sampled in western Switzerland and genotyped for seven microsatellite loci. F(IS) (estimated on adults) and progeny array analyses (on hatchlings) provided similar selfing rate estimates of 80%. Populations presented a low polymorphism and were highly differentiated (F(ST) = 0.58). Although the reproductive assurance hypothesis would predict higher selfing rate in temporary populations, no difference in selfing level was observed between temporary and permanent populations. However, allelic richness and gene diversity declined in temporary habitats, presumably reflecting drift. Infection levels varied but were not simply related to either estimated population selfing rate or to differences in heterozygosity. These findings and the similar selfing rates estimated for hatchlings and adults suggest that within-population inbreeding depression is low in L. truncatula.

Asexual multiplication of larval parasitic worms: a predictor of adult life-history traits in Taeniidae?

The hypothesis that asexual multiplication inside the intermediate host and adult life-history traits within the final host are independent is tested among Cestoda. Using phylogenetic relationships among the Cestoda species, we can show that asexual multiplication appears to have been lost and recovered several times throughout Taeniidae evolution; this allows a comparison of the adult life-history traits of species with and without asexual multiplication at the larval stage. The adult trait considered is the size of the parasite, since numerous life-history traits, such as fecundity and longevity, are correlated with size. If adult traits are independent of whether the larval stage reproduced asexually or not, we expect no difference in the adult size of the proliferative (i.e. with asexual multiplication) and non-proliferative species. The results are inconsistent with this hypothesis. In contrast, species with asexual multiplication in the intermediate host seem to have smaller adult size, reflecting a trade-off. We propose that ecological factors involving intraspecific and interspecific competition in the final host might be responsible for this trade-off. The role of these parameters in the evolution of life-history traits and more precisely in the acquisition of asexual multiplication is investigated with comparative analysis and discussed in the context of host-parasite interactions.

Differential influence of Pomphorhynchus laevis (Acanthocephala) on the behaviour of native and invader gammarid species

Although various species of acanthocephalan parasites can increase the vulnerability of their amphipod intermediate hosts to predation, particularly by altering their photophobic behaviour, their influence on the structure of amphipod communities and the success of invader species has so far received little attention. We compared the prevalence and behavioural influence of a fish acanthocephalan parasite, Pomphorhynchus laevis, in two species of amphipods, Gammarus pulex and Gammarus roeseli in sympatry in the river Ouche (Burgundy, eastern France). There, G. pulex is a resident species, whereas G. roeseli is a recent coloniser. Both uninfected G. pulex and G. roeseli were strongly photophobic. although less so in the invading species. However, there was no significant difference in reaction to light between infected and uninfected G. roeseli, whereas infected G. pulex were strongly photophilic. We discuss our results in relation to the parasite's ability to manipulate invading host species, the possibility that resistant individuals have been selected during the invasion process, and the role that acanthocephalan parasites can play in shaping the structure of amphipod communities.

Reproductive and mate choice strategies in the hermaphroditic flatworm Echinostoma caproni

Due to the important role that mating systems play in the evolution of species, we investigate the selfing rate and mate choice in the simultaneous hermaphroditic parasite Echinostoma caproni (Trematoda). The echinostomes were maintained in two situations in mice: (1) double infections where the two individuals do or do not belong to the same geographic area isolate, and (2) triple infections where two of the three individuals originate from the same isolate and the third one originates from a different isolate. This experimental design permits analysis of intra- and interisolate selfing rates and of mate preference. We predict, in the first experiment, no difference between intra- and interisolate selfing rates. In the second experiment we expect a preferential outcrossing between individuals originating from the same isolate in order to avoid hybrid breakdown. The results obtained corroborate our predictions and emphasize the important and synergistic roles of selfing, inbreeding depression, and hybrid breakdown in the evolution of echinostome reproductive strategies.

Chemical communication and mate attraction in echinostomes

Mate attraction is widespread among animals and appears to facilitate mating and to prevent hybridisation between closely related species. In this study we investigated mate preference between two geographical isolates of Echinostoma caproni (Trematoda, Platyhelminth) and another species of the genus Echinostoma E. sp. Because previous experiments showed a partial reproductive isolation between echinostome isolates, we examined the possibility that such isolation resulted from differential mate attraction. We compared intra-isolate, inter-isolate and interspecific pairings using two in vitro experimental designs. In the first experiment we compared mate attraction of two individuals belonging to or not belonging to the same isolate, while in the second experiment we examined mate choice when individuals were in the presence of individuals from both the same isolate and from a different isolate or a different species. Distances between worms were measured over a period of 90 min. Results from both experiments suggested that mate attraction was similar for intra-isolate, inter-isolate or interspecific combinations. This lack of mate preference in vitro would therefore support an alternative hypothesis of a reproductive isolation through sperm selection.

Differences in adult excretory-secretory products between geographical isolates of Echinostoma caproni

Because of the potentially important role of parasite excretory-secretory (ES) products in regulating intermate relationships and host-parasite interactions, we investigated intraspecific variability of adult ES products from a trematode species. Adults from 3 geographical isolates of Echinostoma caproni were collected and maintained under in vitro conditions. ES products were collected at 4, 8, and 22 hr of in vitro maintenance. In order to test for interspecific variability, ES products from a different echinostome species (Echinostoma sp.) were collected in a similar way. Major ES polypeptides were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation and silver staining. The polypeptide patterns of total ES products appeared very stable in time for each isolate. ES products from Echinostoma sp. showed a distinct polypeptide profile with none of the major bands being shared with the E. caproni isolates. Although polypeptide patterns from the 3 E. caproni isolates shared most major bands, isolate-specific bands could be observed. Two isolates exhibited a band at 85 and 119 kDa, respectively, whereas the third isolate was characterized by the absence of both bands. These results, together with the previously reported role of ES products in molecular signaling raise the question of the importance of intraspecific ES product differences in evolutionary processes such as assortative mating or local host adaptation.

Experimental evidence of hybrid breakdown between genetically distinct populations of Echinostoma caproni

In this paper we investigate whether the assortative mating between individuals of the same genetic entity (i.e. coming from the same geographical area) of Echinostoma caproni, reported in a previous paper, may be explained by a post-zygotic isolating mechanism. The fecundity of the adults of 2 parental genetic entities and of their hybrids (i.e. F1, F2, F3) was quantified through 3 successive generations. Whereas the number of eggs released by F1 hybrids is similar to that of the mid-parent, that of recombinant hybrids (F2-3 hybrids) is significantly lower than that of F1 and that of the mid-parent. Since these results seem to demonstrate hybrid breakdown, 2 important factors maintaining reproductive isolation, i.e. pre-mating reproductive isolation and low hybrid fecundity, may influence the evolution of E. caproni.