Evolutionary mitogenomics of Chordata: The strange case of ascidians and vertebrates

The availability of almost one thousand complete mitochondrial genome (mtDNA) sequences of chordates provides an almost unique opportunity to analyse the evolution of this genome in the phylum Chordata, and to identify possible divergent evolutionary trends followed by the three chordate subphyla: Vertebrata, Cephalochordata and Tunicata. Here, we review some genome-level features of mtDNA, such as genetic code, gene content, genome architecture and gene strand asymmetry, mostly focusing on differences existing between tunicates and remaining chordates. Indeed, tunicate mtDNAs show a surprisingly high variability in several genome-level features, even though the current tunicate taxon sampling is absolutely insufficient and is focused mainly on the class Ascidiacea. On the contrary, a stabilization of the mtDNA structural and evolutionary features is observed in both cephalochordates and vertebrates, where genome-level features are almost invariant. Thus, different evolutionary dynamics, probably related to divergent functional constraints, have modelled the overall mtDNA structure and organization of the three chordate subphyla.