Untangling the contribution of genetic and environmental effects to shell differentiation across an environmental cline in a marine snail

Abstract

The rough periwinkle Littorina saxatilis is characterised for showing shell phenotypic variation associated to environmental clines and this is found in parallel across shores. In order to disentangle the additive genetic effects and the environmental factors involved in this variation, we analysed natural populations subjected to a gradient in wave exposure and crab predation in 3 different Rias (primary estuaries) from Galicia (NW Spain). In addition to previously described “Crab” and “Wave” ecotypes, we studied a new phenotype (“Crab+”) associated to the most sheltered habitats of the gradient. We obtained estimates of heritability and Qst (between population genetic differentiation) in embryos and Pst (between population phenotypic differentiation) in adults for shell morphology traits. Heritability estimates were close to one for shell size and relative shell aperture, suggesting a low impact of environmental factors in embryos. Estimates of Qst between ecotypes were significant and typically much higher than published Fst estimates from neutral markers, indicating a role for selection on shaping additive genetic variation. Estimates of Pst between ecotypes for adults were substantially larger than embryos´ Qst, suggesting also a role of phenotypic plasticity in ecotype differentiation. Our results suggest that Pstvs Fst comparisons should be taken with caution because they might not reflect real additive genetic effects, but Pstvs Qst comparisons could shed light into the role of phenotypic plasticity.