Self-fertilization in the white-lipped land snail Triodopsis Albolabris

A reconstruction of sexual modes throughout animal evolution

BACKGROUND

Although most extant animals have separate sexes, simultaneous hermaphrodites can be found in lineages throughout the animal kingdom. However, the sexual modes of key ancestral nodes including the last common ancestor (LCA) of all animals remain unclear. Without these data, it is difficult to infer the reproductive-state transitions that occurred early in animal evolution, and thus a broad understanding of the evolution of animal reproduction remains elusive. In this study, we use a composite phylogeny from four previously published studies, two alternative topologies (ctenophores or sponges as sister to the rest of animals), and multiple phylogenetic approaches to conduct the most extensive analysis to date of the evolution of animal sexual modes.

RESULTS

Our analyses clarify the sexual mode of many ancestral animal nodes and allow for sound inferences of modal transitions that have occurred in animal history. Our results also indicate that the transition from separate sexes to hermaphroditism has been more common in animal history than the reverse.

CONCLUSIONS

These results provide the most complete view of the evolution of animal sexual modes to date and provide a framework for future inquiries into the correlation of these transitions with genes, behaviors, and physiology. These results also suggest that mutations promoting hermaphroditism have historically been more likely to invade gonochoristic populations than vice versa.

SELF-FERTILIZATION VERSUS CROSS-FERTILIZATION IN THE HERMAPHRODITIC FRESHWATER SNAIL BULINUS GLOBOSUS

Self-fertilization depression of fitness in the freshwater hermaphroditic snail Bulinus globosus, an intermediate host of the parasitic trematode Schistosoma, has been studied in a strain originating from Niger. B. globosus is an outcrosser that can self-fertilize when isolated before any copulation has occurred. The self-fertilization depression has been estimated during two successive generations. In the first generation, selfing was compared to outcrossing. Within each mating system group, selfing and outcrossing were compared again in the second generation. A striking difference was shown in favor of cross-fertilization for the number of eggs laid, the survival at birth of young snails and the number of snails reaching sexual maturity. The overall self-fertilization depression is 0.920 after two generations of selfing. We discuss the relative role of selfing and outcrossing in the evolution of freshwater snail populations.

INBREEDING DEPRESSION, NEUTRAL POLYMORPHISM, AND COPULATORY BEHAVIOR IN FRESHWATER SNAILS: A SELF-FERTILIZATION SYNDROME

This paper examines the consequences of self-fertilization on life-history traits and neutral genetic polymorphism in natural populations of three species of hermaphrodite freshwater snails: Biomphalaria straminea, Bulinus globosus, and the aphallic species Bulinus truncatus. Life-history traits (fecundity, growth, hatching rate, and survival of offspring) are compared under laboratory conditions between isolated (obligatory selfing) and paired (outcrossing possible) snails in one population of B. straminea and B. globosus and two populations of B. truncatus. The genetic polymorphism of the same four populations is analyzed using electrophoretic markers in B. straminea and B. globosus and microsatellite markers in B. truncatus. In B. truncatus and B. straminea, isolated snails have a higher fecundity than paired snails, whereas the contrary is observed in B. globosus. For all populations, no difference in hatching rate and offspring survival is detected between the two treatments. Genetic analyses using microsatellite markers conducted in B. truncatus on progeny of paired snails reveal a high selfing rate in spite of high copulation rates, highlighting the difficulties of obtaining outcrossing in highly selfing snails. The high survival of selfed offspring in B. truncatus and B. straminea indicates that inbreeding depression is limited. The extent of inbreeding depression in B. globosus is less clear. Overall, fitness decrease in this species is limited to fecundity. The extent of allozyme polymorphism is very limited whereas a much higher variability is observed with microsatellites. Biomphalaria straminea and B. truncatus populations are also characterized by very low observed heterozygosities and large heterozygote deficiencies, whereas the B. globosus population does not exhibit such a deficiency. Overall these results allow the definition of a self-fertilization syndrome in hermaphrodite freshwater snails: selfing populations (such as those of B. straminea and B. truncatus studied here) are characterized by high selfing rates in spite of copulations, limited deleterious effects of selfing, limited neutral genetic polymorphism, and large heterozygote deficiencies.

Breeding system, shell size and age at sexual maturity affect sperm length in stylommatophoran gastropods

BACKGROUND

Sperm size and quality are key factors for fertilization success. There is increasing empirical evidence demonstrating that sperm form and function are influenced by selective pressures. Theoretical models predict that sperm competition could favour the evolution of longer sperm. In hermaphrodites, self-fertilizing species are expected to have shorter sperm than cross-fertilizing species, which use sperm stored from several mating partners for the fertilization of their eggs and thus are exposed to intense sperm competition. We tested this hypothesis by comparing original data on sperm length in 57 species of simultaneously hermaphroditic stylommatophoran gastropods from Europe and South America with respect to the species' breeding system. We used 28S rRNA nuclear and COI mitochondrial sequence data to construct a molecular phylogeny. Phylogenetic generalized linear models were applied to examine the potential influence of morphological and life-history characters.

RESULTS

The best-fit model revealed that the breeding system and age at sexual maturity influence sperm length in gastropods. In general, species with predominant cross-fertilization had longer sperm than species with predominant self-fertilization or a mixed breeding system. Across species with shells (snails), sperm length also increased with shell size.

CONCLUSIONS

Our study provides evidence that sperm length in stylommatophoran gastropods is influenced by the risk of sperm competition, as well as by age at sexual maturity and shell size. This finding extends present knowledge of sperm evolution to a group of so far poorly studied simultaneous hermaphrodites.