Mitochondrial Dna Clocks and the Phylogeny of Danaus Butterflies

Molecular clocks based on sequence change in mitochondrial (mt) DNA have been useful for placing molecular phylogenies in their historical context, thereby enhancing evolutionary insight. Nonetheless, despite their importance to phylogeographers, the methodology is controversial. Here we report on two mitochondrial clocks for the butterfly genus Danaus based on sequences from the cytochrome c oxidase subunit I (COI) and small subunit 12S rRNA (12S) genes. Both clocks are, within the context of Danaus, reliable time-keepers, mutually consistent and, respectively, in agreement with a crustacean COI clock and a molluscan 12S clock. Though we have no fossils with which directly to calibrate sequence divergence rates for Danaus, the 12S molluscan and COI crustacean clocks chosen for comparison were calibrated to radiometrically dated geomorphological events. Our results indicate that the Danaus COI clock evolves approximately four times faster than the 12S clock. Differences between rates of sequence change in terminal sister-taxa are small and likelihood ratio tests do not reject a hypothesis that evolution has been clock-like. The species Danaus chrysippus is paraphyletic and, therefore, invalid. Danaus probably split from its sister-genus Tirumala around 4.9 ± 0.3 million years ago in the early Pliocene.