Resolving the tips of the tree of life: How much mitochondrial data doe we need?

Mitochondrial (mt) DNA sequences are used extensively to reconstruct evolutionary relationships among recently diverged animals, and have constituted the most widely used markers for species- and generic-level relationships for the last decade or more. However, most studies to date have employed relatively small portions of the mt-genome. In contrast, complete mt-genomes primarily have been used to investigate deep divergences, including several studies of the amount of mt sequence necessary to recover ancient relationships. We sequenced and analyzed 24 complete mt-genomes from a group of salamander species exhibiting divergences typical of those in many species-level studies. We present the first comprehensive investigation of the amount of mt sequence data necessary to consistently recover the mt-genome tree at this level, using parsimony and Bayesian methods. Both methods of phylogenetic analysis revealed extremely similar results. A surprising number of well supported, yet conflicting, relationships were found in trees based on fragments less than ~;2000 nucleotides (nt), typical of the vast majority of the thousands of mt-based studies published to date. Large amounts of data (11,500+ nt) were necessary to consistently recover the whole mt-genome tree. Some relationships consistently were recovered with fragments of all sizes, but many nodes required the majority of the mt-genome to stabilize, particularly those associated with short internal branches. Although moderate amounts of data (2000-3000 nt) were adequate to recover mt-based relationships for which most nodes were congruent with the whole mt-genome tree, many thousands of nucleotides were necessary to resolve rapid bursts of evolution. Recent advances in genomics are making collection of large amounts of sequence data highly feasible, and our results provide the basis for comparative studies of other closely related groups to optimize mt sequence sampling and phylogenetic resolution at the "tips" of the Tree of Life