Premating barriers in young sympatric snail species

Abstract

Sympatric coexistence of recently diverged species raises the question of barriers restricting the gene flow between them. Reproductive isolation may be implemented at several levels, and the weakening of some, e.g. premating, barriers may require the strengthening of the others, e.g. postcopulatory ones. We analysed mating patterns and shell size of mates in recently diverged closely related species of the subgenus Littorina Neritrema (Littorinidae, Caenogastropoda) in order to assess the role of premating reproductive barriers between them. We compared mating frequencies observed in the wild with those expected based on relative densities using partial canonical correspondence analysis. We introduced the fidelity index (FI) to estimate the relative accuracy of mating with conspecific females and precopulatory isolation index (IPC) to characterize the strength of premating barriers. The species under study, with the exception of L. arcana, clearly demonstrated preferential mating with conspecifics. According to FI and IPC, L. fabalis and L. compressa appeared reliably isolated from their closest relatives within Neritrema. Individuals of these two species tend to be smaller than those of the others, highlighting the importance of shell size changes in gastropod species divergence. L. arcana males were often found in pairs with L. saxatilis females, and no interspecific size differences were revealed in this sibling species pair. We discuss the lack of discriminative mate choice in the sympatric populations of L. arcana and L. saxatilis, and possible additional mechanisms restricting gene flow between them.

Linking ecology, morphology, and metabolism: Niche differentiation in sympatric populations of closely related species of the genusLittorina(Neritrema)

“…This fraction was not analyzed in this study, but is expected to differ from the upper part, similarly to the upper and the lower shore L. saxatilis based on our previous results (Maltseva et al, 2016). Moreover, some laboratory and field observations suggest the possibility of a restricted gene flow between L. arcana and L. saxatilis, the existence of presumable hybrids, and interspecies copulations in wild populations (Granovitch et al, 2013;Maltseva et al, 2021;Mikhailova et al, 2009;Warwick et al, 1990). Thus, the divergence between these two species is of particular interest since the ecological specification can hardly be its driving force.…”

“…Thus, the divergence between these two species is of particular interest since the ecological specification can hardly be its driving force. Some postmating prezygotic reproductive isolation mechanisms, for example, those associated with polymorphism of gamete interaction proteins, could be expected to function in this pair of species and to contribute to their nonecological and nongeographical speciation (Lobov et al, 2018;Maltseva et al, 2021).…”

Species-Specific Proteins in the Oviducts of Snail Sibling Species: Proteotranscriptomic Study of Littorina fabalis and L. obtusata

“…Based on 2D DIGE, only differences in fructose-1,6-bisphosphate aldolase and arginine kinase between species were detected. Robust functional differences between L. obtusata and L. fabalis were inferred from proteomic and metabolomic screening [ 17 , 21 ]. Our previous proteomic studies in Littorina revealed that there are several isoforms of arginine kinase varying between specimens from different microhabitats and between species; some interspecies differences in aldolase features were detected as well [ 26 , 38 ].…”

“…This determines differential exposition to diverse stressor actions (temperature, desiccation, etc. ), as well as toxins, because L. obtusata is not only epiphytic but also phytophagous, feeding on the thalli of Fucus and Ascophyllum and displaying resistance to their toxic polyphenols (unlike L. fabalis ) [ 1 , 17 ].…”

“…L. fabalis and L. obtusata represent a pair of cryptic species, diverged very recently (~0.5–1 Mya), most probably in sympatry owing to ecological speciation [ 4 , 13 , 14 , 15 , 16 ]. These species are polyandrous dioecious internal fertilizers still living in sympatry (and accidentally commencing interspecific copulation) [ 1 , 4 , 17 ]; obviously they need additional mechanisms restricting gene flow. At the same time, there is indirect evidence of conspecific sperm precedence in these species.…”