PT - JOURNAL ARTICLE AU - Matthew L. Aardema AU - Peter Andolfatto TI - Phylogenetic incongruence and the origins of cardenolide-resistant forms of Na<sup>+</sup>, K<sup>+</sup>- ATPase in North American <em>Danaus</em> butterflies AID - 10.1101/048033 DP - 2016 Jan 01 TA - bioRxiv PG - 048033 4099 - http://biorxiv.org/content/early/2016/04/10/048033.short 4100 - http://biorxiv.org/content/early/2016/04/10/048033.full AB - Rapid species radiations can obscure phylogenetic relationships between even distantly related species and lead to incorrect evolutionary inferences. For this reason, we examined evolutionary relationships among the three North American milkweed butterflies, Danaus plexippus, D. gilippus and D. eresimus using &gt;400 orthologous gene sequences assembled from transcriptome data. Contrary to previous phylogenetic assessments, our results indicate that D. plexippus and D. eresimus are the sister taxa among these species. This result explains many previously noted phylogenetic incongruences such as an amino acid substitution in the sodium-potassium pump (Na+,K+-ATPase) of D. eresimus and D. plexippus, which increases resistance to the toxins found in these butterflies’ host plants. In accordance with a rapid radiation of Danaus butterflies, we also find evidence that both incomplete lineage sorting and post-speciation genetic exchange have contributed significantly to the evolutionary histories of these species. Furthermore, our findings suggest that D. plexippus is highly derived both morphologically and behaviorally.