HOMEOBOX GENES IN THE RIBBONWORM LINEUS-SANGUINEUS - EVOLUTIONARY IMPLICATIONS

From our current understanding of the genetic basis of development and pattern formation in Drosophila and vertebrates it is commonly though t that clusters of Hox genes sculpt the morphology of animals in speci fic body regions, Based on Hox gene conservation throughout the animal kingdom it is proposed that these genes and their role in pattern for mation evolved early during the evolution of metazoans. Knowledge of t he history of Hox genes will lead to a better understanding of the rol e of Hox genes in the evolution of animal body plans. To infer Hox gen e evolution, reliable data on lower chordates and invertebrates are cr ucial. Among the lower triploblasts, the body plan of the ribbonworm L ineus (nemertini) appears to be close to the common ancestral conditio n of protostomes and deuterostomes. In this paper we present the isola tion and identification of Hox genes in Lineus sanguineus, We find tha t the Lineus genome contains a single cluster of at least six Hox gene s: two anterior-class genes, three middle-class genes, and one posteri or-class gene, Each of the genes can be definitely assigned to an orth olog group on the basis of its homeobox and its flanking sequences. Th e most closely related homeodomain sequences are invariably found amon g the mouse or Amphioxus orthologs, rather than Drosophila and other i nvertebrates, This suggests that the ribbonworms have diverged relativ ely little from the last common ancestors of protostomes and deuterost omes, the urbilateria.