Homeobox genes in the ribbonwormLineus sanguineus: Evolutionary implications

Abstract

From our current understanding of the genetic basis of development and pattern formation in Drosophila and vertebrates it is commonly thought that clusters of Hox genes sculpt the morphology of animals in specific body regions. Based on Hox gene conservation throughout the animal kingdom it is proposed that these genes and their role in pattern formation evolved early during the evolution of metazoans. Knowledge of the history of Hox genes will lead to a better understanding of the role of Hox genes in the evolution of animal body plans. To infer Hox gene evolution, reliable data on lower chordates and invertebrates are crucial. Among the lower triploblasts, the body plan of the ribbonworm Lineus (nemertini) appears to be close to the common ancestral condition of protostomes and deuterostomes. In this paper we present the isolation and identification of Hox genes in Lineus sanguineus. We find that the Lineus genome contains a single cluster of at least six Hox genes: two anterior-class genes, three middle-class genes, and one posterior-class gene. Each of the genes can be definitely assigned to an ortholog group on the basis of its homeobox and its f lanking sequences. The most closely related homeodomain sequences are invariably found among the mouse or Amphioxus orthologs, rather than Drosophila and other invertebrates. This suggests that the ribbonworms have diverged relatively little from the last common ancestors of protostomes and deuterostomes, the urbilateria.

HOX genes in the sepiolid squid Euprymna scolopes : Implications for the evolution of complex body plans

“…Hox2͞proboscipedia orthologs have also been identified in two polychaetes (14,44). One explanation for these differing results using PCR methods could be the presence of an intron (19). In addition to Hox2͞proboscipedia orthologs, earlier studies yielded different results with respect to posterior group genes.…”

“…A posterior class gene most similar to AbdB was found in oligochaeta (44), and two posterior Hox genes were found in the polychaete Nereis (14). A member of the posterior group of Hox genes in the ribbonworm could not be identified with PCR methods, but eventually was isolated by using low-stringency hybridization methods (19). The identification of posterior group genes in Lophotrochozoan and Ecdysozoan protostomes and in deuterostomes supports the existence of the posterior group of Hox genes in the last common ancestor of deuterostomes and protostomes.…”

“…2). Similarly, the Esc-Post-2 homeodomain displays very high sequence similarity to the previously identified Post-2 sequences, in particular to the nemertean Lineus sanguineus Hox9 gene (14,19,48). Both neighbor-joining analysis (Fig.…”

Organization of Hox genes in ascidians: Present, past, and future

“…Nonchordate Hox clusters have been reported with Drosophila (Lewis, 1978;Von Allmen et al, 1996), mosquito (Devenport et al, 2000;Powers et al, 2000), red flour beetle (Brown et al, 2002), silk moth (Ueno et al, 1992), a grasshopper (Ferrier and Akam, 1996), nematode, Caenorhabditis elegans (Wang et al, 1993;Van Auken et al, 2000), ribbon worm (Kmita-Cunisse et al, 1998), and sea urchin (Popodi et al, 1996;Martinez et al, 1999). Of these bilaterians, vertebrates have multiple Hox clusters, whereas all invertebrate species that have been examined contain a single Hox gene cluster.…”

Function and specificity of synthetic Hox transcription factors in vivo

“…Hox genes have persisted in the animal kingdom; they are found in animals as diverse as worms and humans (5,6) and the Homeodomain (HD), a helix-turn-helix DNA-binding domain, has been strikingly conserved in animals since before the bilaterian split (1,7,8). Sequence-specific binding of Hox proteins has been studied for the Drosophila Sex combs reduced (Scr) (9,10), Antennapedia (Antp) (11,12) and Ultrabithorax (Ubx) (11,12) HDs.…”