Genetic variation in the Desert Springsnail (Tryonia porrecta): implications for reproductive mode and dispersal

Population genetics and conservation of recently discovered springsnails in Arizona

Establishing baseline geographical distributions of extant genetic diversity is increasingly important for future conservation efforts of freshwater species. We analyse the mitochondrial cytochrome c oxidase subunit I (COI) gene to taxonomically characterize 233 samples from recently discovered springsnail populations throughout 17 sites in Arizona, USA. A total of 28 unique COI haplotypes were recovered, with the number of haplotypes ranging from 1 to 4 by population in Arizona. Phylogenetic analyses resulted in haplotypes from 13 of 17 locations in Arizona being successfully identified to species, with five described and three undescribed species in the genus Pyrgulopsis (Hydrobiidae). Future work will require in-depth morphological work to clarify the taxonomic status of these putatively novel species. Importantly, among recovered species, we identified haplotype diversity of the critically endangered Three Forks springsnail, Pyrgulopsis trivialis, in the eastern Gila watershed, which will inform wildlife managers in deciding which source populations to use in reintroduction efforts. We discuss possible causes for observed population structure of Arizona's springsnail populations, with suggestions for the future sampling schemes necessary for the conservation of this uniquely important freshwater mollusc.

Patterns of Rotifer Diversity in the Chihuahuan Desert

Desert aquatic systems are widely separated, lack hydrologic connections, and are subject to drought. However, they provide unique settings to investigate distributional patterns of micrometazoans, including rotifers. Thus, to understand rotifer biodiversity we sampled 236 sites across an array of habitats including rock pools, springs, tanks, flowing waters, playas, lakes, and reservoirs in the Chihuahuan Desert of the USA (n = 202) and Mexico (n = 34) over a period of >20 years. This allowed us to calculate diversity indices and examine geographic patterns in rotifer community composition. Of ~1850 recognized rotifer species, we recorded 246 taxa (~13%), with greatest diversity in springs (n = 175), lakes (n = 112), and rock pools (n = 72). Sampling effort was positively related to observed richness in springs, lakes, rivers, and tanks. Nestedness analyses indicated that rotifers in these sites, and most subsets thereof, were highly nested (support from 4 null models). Distance was positively correlated with species composition dissimilarity on small spatial scales. We predicted species richness for unsampled locations using empirical Bayesian kriging. These findings provide a better understanding of regional rotifer diversity in aridlands and provide information on potential biodiversity hotspots for aquatic scientists and resource managers.

Assembly of a micro-hotspot of caenogastropod endemism in the southern Nevada desert, with a description of a new species of Tryonia (Truncatelloidea, Cochliopidae)

Newly obtained and previously published sequences of the cytochrome c oxidase subunit I (COI) gene were analyzed to examine the biogeographic assembly of the caenogastropod fauna (belonging to the families Assimineidae, Cochliopidae, and Hydrobiidae) of an isolated spring along the lower Colorado River in southern Nevada (Blue Point Spring). Based on available COI clock calibrations, the three lineages that comprise this fauna are 2.78–1.42 million years old, which is roughly coeval or slightly younger than the age of Blue Point Spring (inferred from local fossil spring deposits). Two of the lineages—endemic Pyrgulopsiscoloradensis and Assimineaaff.infima—are most closely related to snails in the Death Valley area (well to the west) and likely colonized Blue Point Spring by transport on birds. A single haplotype was detected in both of these snails, suggesting that they may have only recently colonized Blue Point Spring. The third lineage—endemic Tryoniainfernalis, newly described herein based on morphological and molecular evidence—is most closely related to a geographically proximal species in a lower Colorado River tributary (Tryoniaclathrata); the split between these taxa may be the product of vicariance (severance of a prior drainage connection) or a separate jump dispersal event. The considerable genetic diversity in Tryoniainfernalis (three haplotypes differing by 0.6% mean sequence divergence) suggests a possibly lengthy history of local differentiation. Our findings also identify Blue Point Spring as a new micro-hotspot of groundwater-dependent biodiversity in Nevada and will assist ongoing efforts to protect and conserve these imperiled ecosystems.

Phylogeography and species biogeography of montane Great Basin stoneflies

Sky islands are ideal systems for determining the effects of climatic oscillations on species distributions and genetic structure. Our study focused on montane stonefly populations in the Great Basin of western North America. We used niche-based distribution modelling, phylogeography and traditional species-based biogeography to test several hypotheses as follows: (i) genetic differentiation among Doroneuria baumanni populations will be independent of hydrologic connectivity (headwater model); (ii) Sky islands were colonized when habitat was more continuous and populations likely experienced multiple expansions and contractions; (iii) Colonization events were coincident with the late Pleistocene and Holocene; and (iv) Shared topography and climate history will result in concordant patterns of genetic differentiation in D. baumanni and occurrences of 32 stonefly species across the region. Overall, Φ(ST) 's and coalescent-based estimates of migration were consistent with the headwater model. Maximum likelihood and Bayesian gene trees identified three major nonoverlapping east-west clades. Distribution modelling indicated more suitable habitat in the Great Basin during the Last Glacial Maximum than at present, but none during the last interglacial period. Demographic analyses showed evidence of population expansion in one of the three major east-west clades. Intra-clade divergence times (60,000-183,000ybp) were well within the late Pleistocene while among-clade divergence times (499.000-719,000ybp) were deeper. Genetic differentiation in D. baumanni and distributions of stonefly species were significantly concordant. These results imply that climatic oscillations have played major roles in shaping the genetic structure and distributions of Great Basin stoneflies, but that divergence among clades occurred much earlier than our late Pleistocence/early Holocene predictions.

Demographic stability metrics for conservation prioritization of isolated populations

Systems of geographically isolated habitat patches house species that occur naturally as small, disjunct populations. Many of these species are of conservation concern, particularly under the interacting influences of isolation and rapid global change. One potential conservation strategy is to prioritize the populations most likely to persist through change and act as sources for future recolonization of less stable localities. We propose an approach to classify long-term population stability (and, presumably, future persistence potential) with composite demographic metrics derived from standard population-genetic data. Stability metrics can be related to simple habitat measures for a straightforward method of classifying localities to inform conservation management. We tested these ideas in a system of isolated desert headwater streams with mitochondrial sequence data from 16 populations of a flightless aquatic insect. Populations exhibited a wide range of stability scores, which were significantly predicted by dry-season aquatic habitat size. This preliminary test suggests strong potential for our proposed method of classifying isolated populations according to persistence potential. The approach is complementary to existing methods for prioritizing local habitats according to diversity patterns and should be tested further in other systems and with additional loci to inform composite demographic stability scores.