The role of founder effects on the genetic structure of the invasive bullfrog (Lithobates catesbeianaus) in China

Founder effects and genetic drift can reduce the genetic diversity and alter the genetic composition of introduced species during the processes of population establishment and spread. Thus, founder effects are of particular concern for introduced commercial populations (usually founded from few individuals) and for the natural populations they interact with. Bullfrogs were initially introduced in China for aquaculture purposes and escapes from farms have established many feral populations. Most of the bullfrog farms currently operative have been founded from a limited number of descendents from the original introductions, providing an excellent framework to elucidate the importance of founder effects and genetic diversity in the establishment and persistence of invasive species introduced for commercial purposes. We sequenced a region of the mitochondrial cytochrome b gene in 510 samples collected from feral and farm individuals across China and compared them to populations in their native range. Only two haplotypes (H43 and H7) were identified, and H43 identified in this study for the first time was present at high frequency in both feral and commercial populations. We show a significant difference in the relative frequency of the two identified haplotypes in commercial and feral populations, and suggest that sequential founding events are responsible for the emerging widely distributed new haplotype and the observed differences in genetic structure between bullfrog populations. Our findings indicate that lack of genetic diversity does not necessarily impair the colonizing ability of invasive species and highlight the potential threat posed by introduced commercial populations, given their unique genetic makeup.