Riffle beetles (Elmidae) are ecologically diverse insects with aquatic larvae and plastron-respiring adults. Riffle beetles of the genus Heterelmis are found in rivers and karst-springs throughout the southwestern USA and Mexico. In riverine habitats, lineages have functional flight wings aiding dispersal, while spring-associated lineages have vestigial wings incapable of flight. Additionally, while riverine lineages can thrive in habitats over a range of fluctuating temperatures and dissolved oxygen levels, spring endemics, including the endangered H. comalensis, are stenothermal with extremely limited tolerance to environmental change. While genetic diversity has been investigated within and among the two spring complexes where H. comalensis occurs, patterns of genetic diversity and population structure has not previously been studied at the genus level or over a larger geographic range. The objective of this research is to use a genotyping-by-sequencing approach to characterize patterns of population structure to gain a better understanding of genetic diversity in an ecologically diverse genus of riffle beetles, as well as to inform effective conservation management strategies for H. comalensis. We collected and extracted DNA from 381 individuals representing spring-associated and riverine Heterelmis populations across the southwestern USA and Mexico and generated genotyping-by-sequencing data to calculate genetic differentiation using hierarchal Bayesian modeling.
Results/Conclusions
Using ENTROPY, I genotyped 381 individuals at 15,678 single nucleotide polymorphism (SNP) loci. ENTROPY bar plots of individual admixture proportion were analyzed for k=2-10. These clustering analyses have revealed useful insights into Heterelmis population structure. Most notably, the Fern Bank Springs population consistently clusters with the endangered H. comalensis rather than H. glabra. In the k=8 analysis, the San Marcos Springs population clusters independently, indicating that there is greater differentiation between the San Marcos Springs H. comalensis population and the Comal Springs H. comalensis population than there is between the Comal Springs H. comalensis population and the Fern Bank Springs population. Additionally, the Indian Springs population clusters independently across all k values rather than clustering with H. glabra as predicted. This study has generated a higher resolution dataset of Heterelmis genetic diversity than was previously available and has uncovered previously undetected population structure. For genera such as Heterelmis with endangered taxa narrowly endemic to few sites, it is imperative to understand patterns of genetic diversity and gene flow to inform agencies to implement effective conservation management strategies. Future studies will aim to understand ecological and environmental drivers of this observed diversity.