Data from: Hybridization patterns between two marine snails, Littorina fabalis and L. obtusata

Characterizing the patterns of hybridization between closely related
species is crucial to understand the role of gene flow in speciation. In
particular, systems comprising multiple contacts between sister species
offer an outstanding opportunity to investigate how reproductive isolation
varies with environmental conditions, demography and geographic contexts
of divergence. The flat periwinkles, Littorina obtusata and L. fabalis
(Gastropoda), are two intertidal sister species with marked ecological
differences compatible with late stages of speciation. Although
hybridization between the two was previously suggested, its extent across
the Atlantic shores of Europe remained largely unknown. Here, we combined
genetic (microsatellites and mtDNA) and morphological data (shell and male
genital morphology) from multiple populations of flat periwinkles in
north-western Iberia to assess the extent of current and past
hybridization between L. obtusata and L. fabalis under two contrasting
geographic settings of divergence (sympatry and allopatry). Hybridization
signatures based on both mtDNA and microsatellites were stronger in
sympatric sites, although evidence for recent extensive admixture was
found in a single location. Misidentification of individuals into species
based on shell morphology was higher in sympatric than in allopatric
sites. However, despite hybridization, species distinctiveness based on
this phenotypic trait together with male genital morphology remained
relatively high. The observed variation in the extent of hybridization
among locations provides a rare opportunity for future studies on the
consequences of different levels of gene flow for reinforcement, thus
informing about the mechanisms underlying the completion of speciation.