Phylogeny of the genus Agathistoma (Mollusca: Gastropoda: Tegulidae) and phylogeography of Brazilian species: genetic and morphological diversity patterns

Abstract

The biogeography of South American marine biota is known to have been influenced by several marine transgressions that affected water currents, and modified salinity, temperature, and primary productivity. Therefore, the dispersal ability and survival of the marine organisms was affected by the new environmental conditions, with the consequent impact on their distribution and genetic structures. A result of these transgressions is known as antitropical distribution, defined by species with distributions interrupted in the equatorial region. This pattern is well represented among western Atlantic species or species pairs distributed northwest and southeast of the combined plume of the Orinoco and Amazon rivers. Currently, members of such closely related species pairs are often classified as subspecies or even placed under the same specific name, despite being separated by an intertropical latitudinal gap. Agathistoma encompass 23 extant species, occurring in tropical and subtropical waters of the eastern Pacific, on the Caribbean, on the Pacific coast of America and Atlantic coast of South America. Among Agathistoma, three occur in Brazil: A. viridulum and A. hotessierianum, which have antitropical distribution and A. patagonicum, occurring in a more restricted geographic region, but with a higher morphological variation when compared to other species of the genus. There is no knowledge of biogeographical barriers acting on the gene flow of the species, or even in their delimitations. The most recent phylogenetic study based on morphological data supported a monophyletic Agathistoma; on this analysis, the polymorphic taxon was divided into smaller subunits and tested the monophyly of the group, indicating a non-monophyletic A. hotessierianum. However, due to lack of the clear diagnostic characters, unusual distribution pattern and high intraspecific variation, there are still doubts about some species strains. Species distribution is a valuable source for understanding historical processes of diversification in the western Atlantic, since it is possible to recover, with the use of molecular data, the history of this species complex, marked by disjunctions, dispersions, and local extinctions. It is possible to study these processes in small and large areas with the biogeography of mollusks, analyzing the diversification of the Atlantic and Pacific Tropical and South America Temperate faunas in detail. This project aims to present a hypothesis of relationships among the species of Agathistoma based on morphological and molecular data, and analyse the genetic variation of A. viridulum, A. hotessierianum and A. patagonicum, to detect its evolutionary lineages and biogeographic patterns. The results will be discussed within the scope of the different hypothesis resulting diversification of marine transgressions in the Atlantic. (AU)