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PHYLOGENETIC RELATIONSHIPS - Spring Snail
Center Researcher:
Hsiu-Ping Liu
Collaborator:
Dr. Robert Hershler
National Museum of Natural History
Smithsonian Institution
Partners:
Smithsonian Instituion
Related Publications:
Liu, H.-P., R. Hershler, and K. Clift. 2003. Mitochondrial
DNA sequences reveal
extensive cryptic diversity within a western American springsnail. Molecular
Ecology12:2771-2782.
Hershler, R., and Liu, H.-P. Reappraisal of the North American freshwater Gastropod Subgenus Natricola Gregg & Taylor (Rissooidea: Hydrobiidae). (in press in The Veliger)
There are four projects dealing with different aspects of Spring Snail (Pyrgulopsis spp.).
1) Mitochondrial DNA sequences reveal extensive cryptic diversity within a western North American springsnail
Pyrgulopsis (>120 species), may be the most diverse element of the western aquatic fauna. Pyrgulopsis lives in springs and other permanent waters that are poorly integrated on arid western landscapes. Although the dispersal ability of Pyrgulopsis has never been studied, these gill-breathing animals have an entirely aquatic life cycle and thus are presumably unable to move among hydrographically separated habitats (fide Taylor & Bright 1987). The spread of Pyrgulopsis within aquatic systems may also be constrained by its direct mode of development (bereft of a free swimming dispersal phase) and frequent restriction to headsprings, which may reflect physiological specialization for these unique environments (e.g., O'Brien & Blinn 1999). The entirely aquatic life cycle and limited vagility of Pyrgulopsis suggest that it should be prone to differentiation on a fine geographic scale. The restriction of most species of this genus to a single spring, spring complex, or local watershed is consistent with this premise. However, more than two dozen other congeners range across one or more drainage divides. Given that contemporary gene flow among hydrographically separated populations is unlikely, these widely ranging snails may be expected to have pronounced geographic structuring and to possibly include morphologically cryptic species. The genetic structure of a widely distributed member of the genus, P. micrococcus, which ranges among five isolated drainage basins in southeastern California and southwestern Nevada (Hershler & Pratt 1990), was surveyed. The primary objectives of this study were to use mitochondrial DNA (mtDNA) markers to infer the phylogenetic relationships of populations assigned to P. micrococcus, and to test the morphology-based classification of this snail.
COI and NDI sequence variation among 29 populations of a widely ranging southwestern springsnail (Pyrgulopsis micrococcus) and 18 regional congeners was analysed. Cladistic analyses of these sequences depict P. micrococcus as a polyphyletic composite of five well supported clades. Sequence divergences among these clades and subclades implied the occurrence of as many as seven-eight cryptic species in addition to P. micrococcus.
2) A molecular phylogeny of aquatic gastropods provides a new perspective on biogeographic history of the Snake River region
Mitochondrial
DNA sequences of aquatic gastropods of the subgenus Pyrgulopsis
(Natricola) were analyzed to test a commonly accepted hypothesis
concerning the early history of the Snake River in the northwestern United
States. Distributions of Natricola and other regional biota were
previously used to infer that the Snake River flowed to the Pacific through
southeastern Oregon and northern California during the Neogene prior to
its capture by the Columbia River in the late Pliocene (2 Ma). A molecular
phylogeny based on partial sequences of COI and NDI (1149 bp) indicates
that the Natricola clade is restricted to the modern Snake-Columbia
River basin and the Great Basin of Oregon whereas northern California
populations previously assigned to this subgenus belong to other lineages.
The Natricola clade is not deeply subdivided into Oregon Great
Basin and Snake River basin units consistent with late Pliocene fragmentation
of the hypothesized paleodrainage, but instead is shallowly structured
and contains multiple transitions among these two geographic areas. The
strongly supported sister relationship between Natricola and a
species from northern Nevada (P. imperialis) is consistent with
a recent proposal that the ancestral Snake River did not flow through
southeast Oregon but instead flowed south to the Humboldt River (northern
Nevada). Within the context of this hypothesis, the multiple transitions
between the Snake River basin and Oregon Great Basin that occurred within
Natricola may be attributed to a late Pleistocene connection between
these drainages that was unrelated to the early course of the Snake River.
3) Taxonomic Reappraisal of Species Assigned to the North American Freshwater
Gastropod Pyrgulopsis (Natricola) (Rissooidea: Hydrobiidae)
The hydrobiid gastropod subgenus Natricola (currently placed in synonymy with Pyrgulopsis) was erected for three species from the northwestern United States which have a large shell and long accessory process on the penis. Two of these snails are restricted to Snake River drainage (P. idahoensis, P. robusta) while the third ranges among Snake River and Great Basin drainage in southeast Oregon (P. hendersoni). A possible fourth species of this subgenus was recently discovered in the Columbia River. In this project the taxonomic status of these closely similar and little studied snails is re-evaluated using mitochondrial and nuclear DNA markers and morphological characters.
4) Springsnails (Pyrgulopsis), a Model Group for Testing Hypotheses of Historical Biogeography of Western North America
Traditionally, biogeography of western North American fishes has been attributed to dispersal facilitated by a highly integrated late Pleistocene ("pluvial") drainage. Recently, Minckley et al. (1986) accepted greater antiquity (Oligocene-Miocene) of this fauna and consequently emphasized the complex role that tectonic events had in molding distributions, both by creating barriers that split ancestral biota (vicariance), and by rafting of fishes on displaced crustal fragments (also see Hendrickson 1986). They provided a geological cladogram of western areas (Minckley et al. 1986, figs. 15.4, 15.5) which, although preliminary, was partly congruent with the limited phylogenetic data then available for regional fishes. While this work set the stage for a new era in the study of regional biogeography, recognition of general patterns of historical biogeography has not followed because few pertinent phylogenetic hypotheses have since been generated for groups other than fishes and salamanders.
Hydrobiid snails share with fishes and salamanders many attractive features for evolutionary and biogeographic studies, and also provide distinct advantages over them. The regional age of hydrobiids is minimally Paleogene, and thus these organisms are suitable for evaluation of old (pre-Pleistocene) biogeographic relationships. Hydrobiids are obligately aquatic, have limited vagility, and presumably are incapable of dispersing across terrestrial barriers. Hydrobiids exhibit a much greater degree of local endemism in the West than fishes and salamanders, and they often form "species flocks" in spring systems. These tiny snails often persist in poorly watered western basins devoid of fishes and (aquatic) salamanders and therefore they can provide a more comprehensive biogeographic signal than these vertebrate counterparts. While it is unrealistic to expect extant species distributions of any group to perfectly preserve ancient geographic separations, the above features suggest that hydrobiids are a near ideal tool for studying biotic response to potential vicariance events in the West.
Despite the unique and compelling features of western hydrobiids, an absence of robust phylogenetic hypotheses has limited their use in evolutionary and biogeographic studies, although they have figured prominently in the development of provocative, albeit non-phylogenetic scenarios of regional drainage history. This study has two main goals. First is to evaluate monophyly and phylogenetic structure of this group using mtDNA sequences and morphological characters. Second is to use the resulting phylogenetic hypothesis for an analysis of historical biogeography of western North America.
