Speciation in the genus Agrodiaetus (Lepidoptera: Lycaenidae): Karyotypic diversification and reinforcement of pre-zygotic isolation

The role of chromosomal rearrangements in maintaining post-zygotic isolation between well-established species is not controversial, however little evidence supports or refutes their importance in speciation. The large genus Agrodiaetus (Lepidoptera: Lycaenidae) provides a model system to study the effect of karyotypic diversity on speciation. Agrodiaetus butterflies exhibit unusual interspecific diversity in chromosome number, from n=10 to n=134, whereas most lycaenid butterflies have n=23/24. Most aspects of their morphology, including genitalia and other characteristics are largely uniform, but different species can vary considerably in male wing color. The only other examples of diverse species karyotypes in Lycaenidae are found in Lysandra and Plebicula. These genera are closely related to Agrodiaetus. Nuclear and mitochondrial markers showed that Agrodiaetus, Lysandra and Plebicula are monophyletic lineages that acquired their karyotypic diversities independently. The genus Agrodiaetus originated approximately 3 million years ago, giving rise to 124 species recognized today. Male wing coloration is commonly used in mate recognition in butterflies, and is a labile character in Agrodiaetus. Comparative phylogenetic methods revealed that the sympatric distribution of 15 relatively young sister taxa of Agrodiaetus strongly correlates with differences in male wing color. This pattern is most likely the result of reinforcement, in which natural selection drives the evolution of enhanced species discrimination. Little evidence was found to support sympatric speciation in Agrodiaetus . Rather, karyotypic changes appear to have accumulated gradually in allopatry, prompting reinforcement when karyotypically divergent populations came into secondary contact. These data argue against differential fusion as the mechanism responsible for the differences in male wing coloration, although differential fusion contributed to augmenting the karyotypic diversity found among young sympatric Agrodiaetus species. Consequently, the more chromosomal rearrangements accumulate between diverging allopatric populations, the higher chance that these populations will persist in secondary sympatry and become true biological species. Finally, comparison of sister groups uncovered positive correlation between karyotypic diversity and net species diversification in Agrodiaetus. These results indicate that rapid karyotypic diversification in this genus has contributed to its exceptional rate of speciation. Thus chromosomal rearrangements can play an active role in the final stages of speciation.