Amphioxus, also called lancelet or cephalochordate, is a promising model organism owning to its particularly evolu- tionary position, simple genome content and comparable body plan to that of vertebrates （Holland et aL, 2004; Bertrand and Escriva, 2011）. However, use of amphioxus as a model or- ganism has been limited for many years because of lack of an efficient genomic modification method. Recently, several revolutionary gene targeting methods that could induce directed mutations, insertions and deletions at intended target sites, have been developed （Gaj et al., 2013）.
The traditional knowledge in textbooks indicated that cephalochordates were the closest relatives to vertebrates among all extant organisms. However, this opinion was challenged by several recent phylogenetic studies using hundreds of nuclear genes. The researchers suggested that urochordates, but not cephalochordates, should be the closest living relatives to vertebrates. In the present study, by using data generated from hundreds of mtDNA sequences, we revalue the deuterostome phylogeny in terms of whole mitochondrial genomes （mitogenomes）. Our results firmly demonstrate that each of extant deuterostome phyla and chordate subphyla is monophyletic. But the results present several alternative phylogenetic trees depending on different sequence datasets used in the analysis. Although no clear phylogenetic relationships are obtained, those trees indicate that the ancient common ancestor diversified rapidly soon after their appearance in the early Cambrian and generated all major deuterostome lineages during a short historical period, which is consistent with ＂Cambrian explosion＂ revealed by paleontologists. It was the 520-million-year＇s evolution that obscured the phylogenetic relationships of extant deuterostomes. Thus, we conclude that an integrative analysis approach rather than simply using more DNA sequences should be employed to address the distant evolutionary relationship.
Epigonichtys and Asymmeron are genera of lancelet.and there is a dispute about their consolidation.Mitochondri...
LI Wei-Ye,WANG Yi-Quan Key Laboratory of the Ministry of Education for Subtropical Wetland Ecosystem Research,School of Life Sciences Xiamen University,Xiamen,China,361005
