title: The Effects of Habitat Isolation on Fine-Scale Genetic and Geographic Structure of Populations of Two Threatened Endemic Insects in Southern Nevada, Pseudocotalpa giulianii and Icaricia shasta charlestonensis
Creators: name: Tovar, Kristen
Publisher: name: University of Nevada, Las Vegas
publisherIdentifier: https://ror.org/0406gha72
publisherIdentifierScheme: ROR
Year: 2023

description: Assessing the role geographic isolation and ecological specialization have on phylogeographic patterns contributes to our understanding of the evolutionary history of a species and the processes that erode genetic diversity. I used mitochondrial and nuclear genomic sequences to assess whether habitat isolation has shaped the fine-scale patterns of present-day genetic structure and diversity in two threatened insect species endemic to southern Nevada. Pseudocotalpa giulianii (Coleoptera; Scarabaeidae) is a dune obligate scarab beetle endemic to only two small, isolated sand dunes in Nye County, Nevada, with a usable habitat of less than 4.2 km sq that is impacted by on-going degradation. Analysis of the pattern of divergence of five mitochondrial DNA haplotypes revealed a high level of divergence between the two sand dunes, separated by only 7 km of inhabitable desert (Fst: 0.077, Nm: 0.08). A phylogenetic tree constructed from a dataset of DNA sequence SNPs (single nucleotide polymorphisms) from 146 beetles representing three species within the genus Pseudocotalpa showed genetic divergence within a species is 0.24644 for P. giulianii, 0.18148 for P. andrewsi, and 0.17900 for P. sonorica. Within P. giulianii the genetic divergence in the Lava Dune (LD) population was 0.003967. The tree also showed Lava Dune individuals grouping together monophyletically on their own branch that is nested within the Big Dune beetles. The Mount Charleston blue butterfly (Lepidoptera; Lycaenidae, Icaricia shasta charlestonensis) is a subspecies endemic to the Spring Mountains (Clark Co., Nevada), found primarily along alpine ridges above Lee Canyon and Kyle Canyon at elevations between 2500 and 3500 meters. I. s. charlestonensis is listed as endangered due to threats to its habitat, its small population size, and extremely limited larval host plant range. Sequencing of mitochondrial DNA (mtDNA haplotypes) and nuclear DNA (SNPs) from butterflies sampled at four locations provided two genetic data sets that were used to elucidate fine-scale genetic divergence among butterfly subpopulations in upper Kyle Canyon and upper Lee Canyon. Analysis of molecular variance using eight mtDNA haplotypes did not identify any genetic structure (ΦST 0.0101) between populations. In contrast, the DNA dataset containing more than 101,000 SNP genetic markers provided evidence of substantial genetic structuring present among I. s. charlestonensis subpopulations within the Spring Mountains (Fst: 0.094, Nm: 6.326). A phylogenetic tree created using the SNP dataset suggests there is one monophyletic grouping of butterflies within the South Loop subpopulation, distinct from the other three locations in Lee Canyon Phylogenetic analysis, Principal Coordinate Analysis, identification of 3 or 4 distinct genetic units using the software STRUCTURE and estimates of genetic parameters reveal the underlying patterns of genetic structure. The Kyle Canyon subpopulation of butterflies, geographically isolated with no intervening habitat is genetically distinct with little evidence of mixing with the other three subpopulations in Lee Canyon. Genetic exchange among subpopulations (gene flow) appears to be greatest among the sites that are geographically proximate in Lee Canyon. Overall, this endangered subspecies exhibits fine-scale genetics structure within the Spring Mountains, a finding with important implications for future management of butterfly habitat and genetic diversity. 
In this study I demonstrate how the evolutionary history of each species has been shaped by fine-scale patterns relating to ecological specialization. Resources that these specialized species have evolved to exploit include the host plants of the butterfly Icaricia shasta charlestonensis and limited sand accumulation utilized by Pseudocotalpa giulianii. The geographic structure of these resources shape the phylogeographic structure and genetic diversity of these endemics with limited distributions in southern Nevada. Population and phylogenetic analysis of P. giulianii and I.s. charlestonensis revealed relatively low connectivity among some subpopulations over relatively short geographic distances and evidence that connectivity and genetic exchange is closely tied to the ecological distribution of resources within their respective ranges.