Old lake versus young taxa: a comparative phylogeographic perspective on the evolution of Caspian Sea gastropods (Neritidae: Theodoxus )

Abstract

The Caspian Sea has been a highly dynamic environment throughout the Quaternary and witnessed major oscillations in lake level, which were associated with changes in salinity and habitat availability. Such environmental pressures are considered to drive strong phylogeographic structures in species by forcing populations into suitable refugia. However, little is actually known on the effect of lake-level fluctuations in the Caspian Sea on its aquatic biota. We compared the phylogeographic patterns of the aquatic Neritidae snail genus Theodoxus across the Pontocaspian region with refugial populations in southern Iran. Three gene fragments were used to determine relationships and divergence times between the sampled populations in both groups. A dated phylogeny and statistical haplotype networks were generated in conjunction with the analyses of molecular variance and calculations of isolation by distance using distance-based redundancy analyses. Extended Bayesian skyline plots were constructed to assess demographic history. Compared with the southern Iranian populations, we found little phylogeographic structure for the Pontocaspian Theodoxus group, with more recent diversification, homogeneity of haplotypes across the Pontocaspian region and a relatively stable demographic history since the Middle Pleistocene. Our results argue against a strong influence of Caspian Sea low stands on the population structure post the early Pleistocene, whereas high stands may have increased the dispersal possibilities and homogenization of haplotypes across the Pontocaspian region during this time. However, during the early Pleistocene, a more dramatic low stand in the Caspian Sea, around a million years ago, may have caused the reduction in Theodoxus diversity to a single lineage in the region. In addition, our results provide new insights into Theodoxus taxonomy and outlooks for regional conservation.

A revision of the extant species of Theodoxus (Gastropoda, Neritidae) in Asia, with the description of three new species

“…Previously published phylogenetic relationships among species of Theodoxus are spread between a number of different studies (e.g. Bunje and Lindberg 2007;Sands et al 2019aSands et al , 2019b. These phylogenies still lack some commonly recognised morphospecies.…”

“…To consolidate and expand upon recommendations from these studies, we constructed a new, robust and time-calibrated Bayesian phylogeny. We supplemented data from published studies available on GenBank (Bunje and Lindberg 2007;Fehér et al 2012;Sands et al 2019aSands et al , 2019b with new data produced herein to incorporate as many morphospecies and Asian localities as possible to allow for sound taxonomic inferences and distribution assessments (Suppl material 1: Table S1). Moreover, Sands et al (2019a) identified three new species, which we describe herein.…”

“…and 41 commonly recognised Theodoxus morphospecies. Of these Theodoxus morphospecies, 39 had previously been incorporated in phylogenetic assessments and 21 of them had already been earmarked as probable taxonomic synonyms (Bunje and Lindberg 2007;Sands et al 2019aSands et al , 2019b. The molecular dataset lacked sequences for T. gloeri Odabaşı & Arslan, 2015, for which fresh material could not be obtained.…”

“…When p-distances lay between these thresholds, they used the nearest supported node encompassing a monophyletic group to define the species. To delimit species in this study, we compared the phylogenetic relationship of all additional sequenced material to the sequence data (species) incorporated from Sands et al (2019aSands et al ( , 2019b.…”

“…Presently, the phylogenetic species concept through molecular approaches has proved to be a reliable tool in substantiating species diversity in Theodoxus. Three studies have presented broad-scale phylogenetic outlooks on the interspecific diversity within Theodoxus (Bunje and Lindberg 2007;Sands et al 2019aSands et al , 2019b. These phylogenetic studies have greatly improved our understanding of the relationships among species and have supported the outcomes of some more recent and detailed morphological reviews (Alba et al 2016;Anistratenko et al 2017;Glöer 2018;Wesselingh et al 2019).…”