Organization of Hox genes in ascidians: present, past, and future.

Hox genes have been regarded to play a central role in anterior-posterior patterning of the animal body. Variations of Hox genes among animal species in the number, order on a chromosome, and the developmental expression pattern may reflect an evolutionary history. Therefore, it is definitely necessary to characterize Hox genes of wide variety of animal species, especially the species occupying key positions in the animal phylogeny. Ascidians, belonging to the subphylum Urochordata, are one of the sister groups of vertebrates in the phylum Chordata. Recent studies have shown that nine Hox genes of Ciona intestinalis, an ascidian species, are present on two chromosomes in the genome. In this review, we discuss the present state of Hox genes in ascidians, focusing on their novel chromosomal organization and expression pattern with unique features and how the novel organization has evolved in relation to the unique body plan of ascidians.