Self-fertilization and morphological stasis in an ancient group of worldwide freshwater snails

ALDA P.; VAZQUEZ AA; DILLON, ROBERT T.; DAVID, PATRICE; JARNE, PHILIPPE; POINTIER JP; HURTREZ-BOUSSÈS, S.

Montpellier

Workshop; Simultaneous and Sequential Hermaphroditic Organisms Workshop; 2020

CEFE

Self-fertilization is generally thought to be a derived character tending to drive species towards extinction by loss of evolutionary potential over time scales on the order of 1􏰀2 Myr. In a group of freshwater pulmonate snails􏰇the lymnaeid genus Galba􏰇, however, selfing seems to have remained stable for a much longer period of time. All Galba species worldwide (except one) have also retained a cryptic phenotype, i.e. a very similar shell morphology and reproductive anatomy. Contrary to the well-documented long-term negative effects of self-fertilization, populations of Galba have not merely thrived but invaded suitable habitat on six continents. In this study, we investigate the origin of such exceptional mating system stasis, together with morphological crypsis. We develop an integrative approach combining morphological observations with molecular markers (microsatellites and DNA sequences for four genes) to analyze Galba samples using Bayesian phylogenetic reconstruction. Our analysis suggested that the most recent common ancestor of Galba already has the cryptic phenotype and evolved ca. 22 Myr ago. We conclude that crypsis in Galba may best be explained by shared morphological stasis. Galba populations live in temporary habitats which may mitigate both predation and interspecific competition. Adaptation to such habitats may impose strong stabilizing selection for a shell morphology able to resist desiccation and concomitant morphological stasis. Static reproductive anatomy may have initially reflected a shared adaptation to self-fertilization in unpredictable habitat patches. Once established, however, self- fertilization would promote rapid erosion of genetic variation to reinforce morphological stasis that may have originated for other reasons.