Genetic characterization of flat periwinkles (Littorinidae) from the Iberian Peninsula reveals interspecific hybridization and different degrees of differentiation

Abstract

The flat periwinkles, Littorina fabalis and L. obtusata, are two intertidal gastropods that offer a promising system for local adaptation and ecological speciation studies. Taking advantage of 17 recently developed specific microsatellite loci, we performed the first comprehensive genetic characterization of these sister species in the Iberian Peninsula. Special attention was given to L. fabalis by sampling populations representing the three morphotypes/ecotypes described from the region, which mainly differ in the habitat they are confined to (i.e. algae/seagrass 'host' species and level of wave exposure). Our analysis revealed, for the first time, the existence of hybrids between L. fabalis and L. obtusata in a natural population, which suggests that despite their strong genetic differentiation, reproductive isolation is not complete. Furthermore, the differentiation between L. fabalis ecotypes is lower than between these and L. obtusata; and their population genetic structure appears to be mainly governed by geography. However, ecological and demographic factors seem to have also played an important role, as suggested by the higher population differentiation in one of the ecotypes. Altogether, this work provides an important baseline for further studies on flat periwinkles as a model to explore how reproductive barriers evolve and interact with each other across a speciation continuum.

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

“…Contact between L. obtusata and all different L. fabalis ecotypes has been observed during fieldwork for this and previous studies (Carvalho et al, ) but, given the distribution of both species, those involving the FI ecotype are the most common. How this diversity in terms of geographic context of divergence and local environmental conditions influences the prevalence of hybridization between L. obtusata and L. fabalis remains unclear.…”

“…Environmental change, in particular, has been suggested as the cause of temporal changes in hybridization rates between two trout species, with a recent increase related with changes in precipitation and water temperature (Muhlfeld et al, ). At least some of the populations analyzed in this study face marked environmental fluctuations, where exposure to strong wave action may lead to substantial demographic oscillations (Carvalho et al, ; Reid, ), contributing to temporal instability in potential for hybridization. Furthermore, the recent decline of fucoid macroalgae inhabited by flat periwinkles at the southern limit of their distribution (Nicastro et al, ), including the Iberian Peninsula, could contribute to the increase of hybridization in this region.…”

“…Characterizing the patterns of hybridization between closely related species pairs is an important step toward understanding the role of gene flow in speciation (Abbott et al, 2013). Introgressive hybridization between flat periwinkles was previously suggested based on mtDNA analyses (Kemppainen et al, 2009), together with the detection of hybrids between the two species in Cabo do Mundo using microsatellites (Carvalho et al, 2016). However, no systematic characterization of hybridization across multiple populations in different geographic contexts has been performed.…”

“…From the battery of 17 microsatellite loci developed for L. fabalis (Carvalho et al, ), 12 markers (four of each di‐, tri‐, and tetra‐nucleotides repeat motifs) were selected based on their informativeness for species discrimination, as well as on their genotyping reliability after combining them into two multiplexes (Appendix ). Amplification reactions were performed as in Carvalho et al () and run on a ABI 3730 sequencer (Applied Biosystems) at STABVIDA. Genotyping was carried out using P eak S canner v1.0 (Applied Biosystems) but later manually inspected.…”

“…The percentages were calculated in respect to the sample size (N Analyzed) for each location. Carvalho et al, 2016). Alternatively, the geographic variation in hybridization could reflect a snapshot of temporal variation within sites.…”