Elucidating the invasion history of introduced bullfrogs in New Mexico using population genetic approaches

The American Bullfrog, Rana (Aquarana) catesbeiana, is an extremely successful invader that has spread globally in the last century, impacting vulnerable ecosystems. In the western U.S., bullfrogs were intentionally introduced in the early 1900s, but little is known about their subsequent colonization into the state of New Mexico. We evaluated a single mitochondrial gene region, cytochrome b, using population genetic approaches to investigate the invasion history of bullfrogs across their introduced range. Specifically, our objectives were to (1) assess the level of genetic diversity and identify haplotypes within bullfrog populations in New Mexico, (2) compare the genetic diversity of native and invasive bullfrog populations, (3) infer the number of introductions into New Mexico, and (4) identify potential native source populations. Using haplotype and nucleotide diversity estimates, we found moderate genetic variation within New Mexico (H _d = 0.648, π = 0.0036) with higher diversity at sites with increased human activity. However, there was significantly lower genetic diversity in introduced populations compared to native populations, consistent with expectations of recent colonization. Based on haplotype diversity estimates and BLAST results, we found a total of eight haplotypes across New Mexico, of which six haplotypes were found across the native and other introduced ranges. Pairwise ΦST revealed minimal differentiation between New Mexico sites, consistent with introduction from a single source population. Lastly, the analysis of molecular variance (AMOVA) conveyed that bullfrogs from the Northwest (Northwest: 0%, P = 0.6411) and Southwest (Southwest: 0%, P = 0.5124) invasive regions showed no significant differences compared to New Mexico populations, suggesting either recent connectivity or similar origins. This study reinforces the importance of managing the movement of invasive species and demonstrates how evaluating the genetic composition of an invasive species can reveal key points of its invasion history.