At the sea shores of the European part of Northern Atlantic, periwinkles of the genus Littorina (subgenus Neritrema) are represented by two sister groups of closely related species: “obtusata” (L. obtusata, L. fabalis) and “saxatilis” group (L. saxatilis, L. arcana, L. compressa). Moreover, most of the species, especially, L. saxatilis, tend to form ecotypes along shore’s ecological gradient. All these species live in sympatry and incomplete restriction of gene flow was shown within “obtusata” and “saxatilis” although, cryptic groups are fully isolated from each other. Based on both partial reproductive isolation and tendency of species to form ecotypes, species within cryptic complexes could be accepted as results of ecological speciation processes (possibly not fully finished yet). Sibling species of the subgenus Neritrema are well studied as a model system for ecological speciation and adaptation. Analysis of reproductive isolation mechanisms in this model system promises to turn up very fruitful. It could elucidate when does reproductive isolation form during ecological speciation and is it a key event for speciation. Traditionally the appearance of a eproductive barrier is attributed to reinforcement, natural selection against hybrid forms (this may be regarded as a kind of postzygotic barrier). Nevertheless, there are just a few direct studies on mechanisms of reproductive barriers in the Littorina model
system. Our studies of mating behavior in Neritrema species showed that there is no reproductive isolation before insemination in any level: all mating variant can be found in natural populations. Thus, if reproductive isolation is implemented in prezygotic level, there should be gamete incompatibility (according to the wide Mayr’s definition). Such mechanisms are well studied in insects but not in internally fertilized molluscs. We identified the LOSP protein potentially involved in gamete interaction and related to formation of reproductive barriers. LOSP initially detected in L. obtusata sperm is a paraspermal protein, present in all Neritrema species. Most probably it is released after insemination and may be involved in reproductive isolation between Neritrema species via sperm competition mechanisms. Thus, comparison of LOSP diversity, on the one hand, and physiological similarity of initially specified subpopulations, on the other hand, can reveal: do ecological
adaptation and genetic flow restriction form simultaneously?