Origin and diversification of the soldier meadow springsnails (Hydrobiidae: Pyrgulopsis), a species flock in the northwestern Great Basin, United States

The large, western North American hydrobiid gastropod genus Pyrgulopsis (commonly known as springsnails) includes a series of locally endemic faunas that are thought to be species flocks. Although these assemblages are of interest from the evolutionary, biogeographic and conservation perspectives, their monophyly and phylogenetic relationships have yet to be rigorously evaluated. Here we present a molecular phylogeny (based on mitochondrial sequence data) of a putative flock of four thermal spring-dwelling springsnails that is distributed in many sites in Soldier Meadow and a single locality in Bog Hot Valley (northwestern Nevada). Our analyses support monophyly of this assemblage ('Soldier Meadow clade') and a close relationship with other regional species and suggest that the invasion of thermal habitats by these springsnails occurred independently of other such radiations within the genus. The divergence of the Soldier Meadow clade relative to its sister group is substantial (6.7910.36% for COI, 10.35-15.88% for NDI), suggesting a split in the early Pliocene, based on the application of a COI clock for Pyrgulopsis. The splits within the Soldier Meadow clade into three main subunits also appear to be old events, based on their 5.78-8.54% COI divergence relative to each other. These findings are consistent with a long history of springsnail evolution in Soldier Meadow, which is intriguing given that this basin was flooded by Lake Lahontan during periods of the early and middle Pleistocene. We suggest that progenitors of the contemporary fauna survived in high elevation springs that may have been present in the basin during these pluvial periods and subsequently colonized contemporary habitats following the termination of the extreme Lake Lahontan highstands. We speculate that the broadly disjunct population (of P. militaris) in Bog Hot Valley, which is consistently nested within the Soldier Meadow clade in our phylogenetic analyses, is either a vicariant relict of a spring zone that may have once extended between these two areas; or was founded by a past `jump' dispersal event from Soldier Meadow. Phylogeographic structure of springsnail populations in Soldier Meadow bears the strong stamp of geologically recent, allopatric diversification, perhaps reflecting the short time that basin floor habitats have been occupied. We describe a new species (P. varneri) for a series of recently discovered populations that are monophyletic, substantially divergent and morphologically distinctive. Additional studies will be necessary to confidently assess the taxonomic status of morphologically distinctive P. limaria and P. umbilicata, which are shown herein to be little divergent genetically; and a recently discovered minute springsnail that is morphologically divergent yet closely similar genetically to P. notidicola.