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

Porcelio, while Dfd is expressed in the two homologous gnathal segments of both fly and crustacean. On the other hand the pb and Scr homologs have somewhat shifted expression domains between the two groups (Fig. 3A, B) (Abzhanov and Kaufman, 1999a, b).

In the trunk of flies, the Antp, Ubx, abdA, and Abd-B genes mediate the morphology of segments along the body axis and the morphological boundary between thorax and abdomen. It has been shown that fine temporal and spatial regulation of these genes is responsible for much of the variation in segmental morphology within and between segments (e.g., Castelli-Gair and Akam, [1995]; and other examples reviewed in Akam [1998a]). However, looking between arthropod groups for more gross comparisons of expression patterns can be informative as to the correlation of Hox expression domains with major differences in body plan. For example, in the branchiopod crustacean Anemia, expression domains of the homologs of Antp, Ubx, and abd-A overlap in the entire homonomous thorax of the animal (Averof and Akam, 1995) (Fig. 3C). The anterior expression borders are staggered colinearly and can be correlated with the differing morphology of the gnathal and first thoracic segments. Each of these genes has a posterior boundary at the caudal limit of the thorax. Artemia Abd-B is expressed only in the two genital segments directly posterior to the thorax. However, no expression of these genes was found in the Artemia abdomen, leading to the hypothesis that the entire Drosophila thorax and abdomen is homologous to the Anemia thorax and genital segments alone (Averof and Akam, 1995).

In the other arthropod subphylum, the Chelicerata, Hox gene expression has been sampled in three species of spiders and a mite (Abzhanov et al., 1999; Damen et al., 1998; Damen and Tautz, 1998, 1999; Telford and Thomas, 1998a,b). Taken together, the spider data (Fig. 3D) gives a fairly complete picture of Hox gene expression along the body axis (ignoring possible interspecific differences in gene expression that might confound this simplification). In this group the major sites of expression are initially in the ventral ectoderm and later in the appendages. The chelicerate body plan is characterized by two tagmata, the prosoma, including head structures, and the opisthosoma. The pattern of expression in the opisthosoma for the Hox6 through Hox8 genes is reminiscent of the insect and crustacean pattern of the trunk. While there is evidence of independent gene duplication in chelicerates, which have been found in PCR surveys (Abzhanov et al., 1999; Cartwright et al., 1993), the general pattern remains one of colinear staggered anterior expression borders with expression extending along the whole body axis caudally. A striking characteristic of the spider data is the posterior termination of expression for Hoxl through Hox5 genes at the prosoma/ opisthosoma boundary (Damen and Tautz, 1999; Abzhanov et al., 1999). These coincident posterior boundaries are not seen in another chelicerate, the mite Archegozetes, where the expression domains of the Hox3, Hox4, and Hox6 orthologs overlap the prosoma/opisthosoma boundary (Telford and Thomas, 1998a, b). This difference may relate to differences in regionalization of the body axis between spiders and mites, including the less pronounced morphological transition between the two tagma in the oribatid mite examined.