Conservation Genetics of the Comal Springs Riffle Beetle (Heterelmis comalensis) Populations in Central Texas, with Examination of Molecular and Morphological Variation in Heterelmis Sp. Throughout Texas
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The Comal Springs Riffle Beetle, Heterelmis comalensis, is an endangered endemic species, known to occur in only two spring complexes in the Texas Hill Country, Comal Springs and San Marcos Springs. We surveyed molecular genetic variation in H. comalensis and three congeners from Texas using mitochondrial and nuclear sequence markers and amplified fragment length polymorphisms (AFLPs), to determine whether H. comalensis is experiencing reduced genetic variability within and among populations in its limited geographic range, and to delineate evolutionarily significant units (ESU)s for the species. A morphological analysis of a critical character used to distinguish Heterelmis species in the current taxonomic key was then conducted. The concordance between the taxonomic characters used to delineate Heterelmis species and the patterns of molecular genetic variation found within and among them was then assessed. Mitochondrial DNA (mtDNA) analyses revealed high levels of genetic variation within and differentiaion among three of the seven H. comalensis localities sampled, particularly when compared to the common flight- and drift-dispersed species of Heterelmis, suggesting isolation among these populations, with little to no current gene flow. However, the other four H. comalensis localities sampled were genetically invariant according to mtDNA data. Partially supporting mtDNA results, AFLP analyses clustered the seven H. comalensis populations into two groups, those with high and those with low genetic diversity. Sequencing of the single-copy nuclear gene ITS revealed a single haplotype within H. comalensis and its most closely related congener, suggesting recent divergence of the two species. Finally, this study found that nominal species designations using the current taxonomic key are incongruent with morphological and molecular data, necessitating amendment to the key. Although genetic variation is exceptionally high in three of the H. comalensis populations, common summer droughts in Texas in combination with human withdrawal of water threaten the springs that H. comalensis inhabit, and may have caused severe population bottlenecks in the four invariant localities sampled.