Population structure and genetic diversity of the only extant Baroninae swallowtail butterfly,Baronia brevicornis,revealed by ISSR markers

Due to its relict nature, the unique Baroninae swallowtail, Baronia brevicornis, is considered a “living fossil”. It is also one of the most enigmatic butterfly species with contentious origins and peculiar ecological characteristics. The aim of this study is to evaluate the genetic diversity and population structure of this endemic species of butterfly in Mexico. We sampled populations in two areas within its restricted geographical range in central Mexico and the isolated subspecies population in the state of Chiapas. Three ISSR primers produced 66 loci, indicating a high genetic diversity (P = 100 %, H _e  = 0.22) and variation range in these populations (62 % < P < 85 %, 0.18 < H _e  < 0.25). The Chiapas population presented the lowest values. The observed high values can be explained by the population dynamic of this species characterized by a very high density of individuals over very limited areas. Variation between populations appears to reflect both the age of colonization and locality perturbation level. Two methods of genetic structure analysis (Self-Organizing Map and Structure analysis) match to define three clusters. Natural and anthropogenic barriers may explain the separation between two clusters (cluster 1 and 2) of central Mexico but an unexpected result revealed that the Chiapas population is not genetically distinguishable from the central Mexico populations (cluster 3) leading us to hypothesize a possible “recent” separation or anthropogenic introduction. Habitat and host plant specificity probably limits the exchange of individuals between populations thus increasing fragmentation and leading to a complex genetic structure. We should put in place population monitoring schemes at different spatial scales, combining field occurrences and genetic tools, in order to reduce extinction susceptibility and keep track of recolonization events for this enigmatic species.