脊索動物か,脊索を持った動物たちか : 古生物学と分子生物学の統合

抄録

Vertebrates, urochordates, and cephalochordates have been classified into the phylum Chordata because of possessing a notochord and a dorsal hollow nerve cord in common. However, their phylogenetic relationships within deuterostomes or even within chordates are still controversial. Nucleotide sequences of ribosomal RNAs or deduced amino acid sequences of mitochondrial genes have provided phylogenetic relationships that support monophyly of deuterostomes, but failed to do so for chordates . Molecular analyses of the earliest development of notochord and nerve cord in vertebrates, ascidians, and lancelets (amphioxus) have clarified that although genes involved in this event resemble each other between the groups, processes underlying the establishment of body plan are rather diversified. Despite that advances and accumulation of data in molecular studies on chordate phylogeny have amplified our knowledge, they seem not to converge on support to homology of the notochord and nerve cord among chordates or to monophyly of this group. Early Cambrian vertebrates and urochordates from China, mainly from the Chengjiang fossil beds, suggest that by that time all of the three extant chordate groups had appeared with the body plan comparable to that of modern forms. Since there is no record of chordates from strata lower than the Yu'anshan member that contains Chengjiang beds, early chordates might have diverged quickly. Divergent molecular mechanisms underlying early development of modern chordates and the seemingly short-spanned radiation of early chordates implicate alternative scenario on chordate phylogeny. To step forward, combination of palaeobiology, earth sciences and molecular biology is essential.