Population structure and genetic diversity of one of the smallest-ranged grass skipper butterflies in North America, the Mardon skipper (Polites mardon)

Insect conservation faces several unique challenges, including the determination of dispersal behavior, population status and even geographic range for species that are cryptic or difficult to identify. For many insects, relationships among nominal taxa are also poorly resolved or have not been investigated with modern tools. Here, we investigate population genetic structure for the Mardon skipper butterfly (Polites mardon), with a geographic range that includes only a small number of locations in Oregon, Washington, and Northern California. Grass skippers are relatively understudied in comparison to other butterfly lineages, and there are gaps in our understanding of basic life history and population biology. Individuals (n = 98) were non-lethally sampled by removing two legs at seven sites across the range of the species and sequenced with a reduced complexity (GBS or RADseq) approach. Despite low extraction yields (average concentration: 1.50 ng/ul), we obtained genotypes at 33,801 loci that were used in analyses that detected a high degree of population structure. Relationships among populations differed from subspecific designations, with the California populations appearing either distinct (k=3) or more closely related to the Oregon populations (k=2). Our data also support that the Puget Sound population is distinct from the other Washington populations sampled (k=4). These findings suggest that dispersal between populations is low, likely because of long term isolation combined with more recent fragmentation and short dispersal distances, which informs our understanding of skipper butterfly movement ecology. Ultimately, these findings will support management and help inform future protection efforts.