Parapatric genetic introgression and phenotypic assimilation: testing conditions for introgression between Hercules beetles (Dynastes, Dynastinae).

The prevalence and consequences of genetic introgression between species have been intensively debated. I used Hercules beetles as examples to test for conditions that may be associated with the occurrence of introgression. RADseq data were used to reconstruct the species tree and history of introgression between Hercules beetles. Image data from museum specimens were used to investigate the phenotypic similarity of two adaptive traits between species from two distinct climatic realms (Nearctic vs. Neotropical). Genetic introgression was identified between Hercules beetles living in geographic proximity (parapatric). Phylogenetic relatedness and phenotypic similarity did not predict nor preclude genetic introgression between species. Phenotypic assimilation in body coloration was evident between distantly related Hercules beetles codistributed in Central America, where directional introgression was also statistically supported from the putative donor to receiver lineages. The number of introgressed loci was significantly higher between species with than without phenotypic similarity. I discuss the implications of recent studies on adaptive genetic introgression by providing supporting evidence from the Hercules beetle system.