Trechus (Coleoptera: Carabidae) of Appalachia: A phylogenetic insight into the history of high elevation leaf litter communities

Elevation gradients provide a wealth of habitats for a wide variety of organisms. The southern Appalachian Mountains in eastern United States are known for their high biodiversity and rates of endemism in arthropods, including in high-elevation leaf-litter taxa that are often found nowhere else on earth. Trechus Clairville (Coleoptera: Carabidae) is a genus of litter inhabitants with a near-global distribution and over 50 Appalachian species. These span two subgenera, Trechus s. str. and Microtrechus Jeannel, largely restricted to north and south of the Asheville basin, respectively. Understanding the diversification of these 3–5 mm flightless beetles through geological time can provide insights into how the litter-arthropod community has responded to historical environments, and how they may react to current and future climate change. We identified beetles morphologically and sequenced six genes to reconstruct a phylogeny of the Appalachian Trechus. We confirmed the Asheville Basin as a biogeographical barrier with a split between the north and south occurring towards the end of the Pliocene. Finer scale biogeography, including mountain-range occupancy, was not a reliable indication of relatedness, with group ranges overlapping and many instances of species-, species group-, and subgeneric sympatry. This may be because of the recent divergence between modern species and species groups. Extensive taxonomic revision of the group is required for Trechus to be useful as a bioindicator, but their high population density and speciose nature make them worth additional time and resources.

Methods
The data set is a nexus formatted file of aligned molecular sequence data from 6 regions of five genes: 28S, Wingless, CAD, Topoisomerase, 18S, and COI (2 fragments). The barcoding region of COI was sequenced using Illumina and Nanopore multiplexed next-generation sequencing techniques. The other markers were sequenced through standard Sanger sequencing. Sequences were quality-filtered using BBTools and ONTBarcoder, base-checked in Geneious, and aligned using MAFFT.