Comparative analysis of hox gene expression in the polychaete Chaetopterus: Implications for the evolution of body plan regionalization

American Zoologist, Jun 2001 by Irvine, Steven Q, Martindale, Mark Q

Hox genes in Chaetopterus

The annelids are one of the three great 'segmented' animal phyla (along with the arthropods and chordates). Each of these groups belong to a distinct metazoan clade, the Lophotrochozoa, Ecdysozoa, and Deuterostomia, respectively. While a great deal of work has concentrated on body plan development and evolution in the arthropods and chordates, comparatively little is know about these issues in any members of the Lophotrochozoa (Shankland and Seaver, 2000). The polychaete annelid Chaetopterus is of interest because of the complex regionalization of its body plan (Fig. 1). As is typical of polychaetes, the body axis consists of a pre-segmental "head" (the prostomium and peristomium), followed by a series of many distinct "trunk" segments called setigers, each separated by a septum and having its own set of muscles, portion of the ventral nerve cord, blood vessels, nephridia, and lateral appendages called parapodia. The post-segmental posterior terminus is called the pygidium. For most annelids, polychaetes included, the morphology of segments is very similar along the body axis, a trait called homonomy. Chaetopterus is unusual for the extent of its segmental specialization, termed heteronomy, resulting in a tagmatized body plan with three functional regions, called here A, B, and C (Fig. IC) (Crossland, 1904).

The developmental mode of polychaetes is characterized by the production at hatching of an initially unsegmented trochophore larva. (In the case of Chaetopterus the trochophore is a modified form [Okada, 1957].) The segmental anlage of the larval and adult body plan is produced by a growth zone located just anterior to the pygidium (Okada, 1957; Anderson, 1973; Irvine and Martindale, 1996). In most polychaetes, the ectodermal and mesodermal derivatives of the growth zone become incorporated into segmental tissues in an anterior-posterior sequence. Chaetopterus exhibits a developmental heterochrony in the sense that cells for the entire A region (the first nine segments) and B region (five segments) are generated but the A region shows no overt signs of segmentation until after the B region (Irvine et al., 1999).

Because the Hox group of homeobox genes have been shown to be instrumental in specifying segmental identity and mediating segmental morphology in such a wide array of metazoan taxa (Ruddle et al., 1994; Slack et al., 1993), we were interested in studying Hox genes in Chaetopterus, with its tagmatized body plan and highly specialized segmental morphology. A survey of the Chaetopterus genome using the polymerase chain reaction (PCR), along with cDNA library screening, uncovered nine Hox genes (Irvine et al., 1997; Irvine and Martindale, 2000). Phylogenetic sequence analysis determined that these genes can be related to Hox genes found in another polychaete Ctenodrilus, and to Hox genes in flies and mice. From this analysis a hypothetical Chaetopterus Hox cluster was constructed which has putative orthologs of each of the Hox genes in Drosophila, except Abd-B. Subsequently we cloned cDNAs from the Hoxl-Hox5 orthologs of Chaetopterus for use in in situ hybridization experiments to determine gene expression patterns.