Relationships between hexapods and crustaceans based on 4 mitochondrial genes.

The ever-increasing use of molecular data in phylogenetic studies have revolutionized our view of the phylogenetic relationships among the major lineages of arthropods. In this context, an important contribution is offered by mitochondrial genes, and the now widely available sequences of entire mitochondrial genomes. One of the most debated issues in arthropod phylogeny is the relationship between crustaceans and hexapods, and particu- larly, whether the traditional taxa Crustacea and Hexapoda are mono- or paraphyletic. A key role is played by basal hexapodan taxa, the entognathan apterygotans (Protura, Collem- bola, Diplura), whose phylogenetic position as the sister taxa of the Insecta s. str. is not totally convincing. The phylogenetic analysis based on mitochondrial protein-coding genes suggests that there are crustacean taxa which are more closely related to the Insecta s. str. than Collembola and Diplura, therefore suggesting non-monophyly of the taxon Hexapoda as traditionally defined. Hence, Collembola and Diplura might have differentiated from different pancrustacean ancestor(s) than those from which the remaining hexapods (Insecta) arose. These results also imply a new scenario for the evolution of several morphological and physiological features of hexapods, including terrestrialization.