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

These examples lend evidence to a view that both anterior and posterior expression boundaries can change in small steps within lineages and correlate with observed differences in morphology. If these small changes in Hox gene regulation continue in the evolution of various groups they could reflect a gradual mechanism for generating the pattern observed in more broad phylogenetic comparisons (Akam, 1998a).

Future directions

Further exploration of the involvement of regulatory changes in Hox gene expression will require sampling patterns from more diverse animal groups with differing body plan organization. The polychaete annelids are an ideal group for these types of studies because of the radically different body plans derived within various polychaete families. In particular, a comparison could be made between the Hox expression patterns seen in Chaetopterus with those of a more homonomous species from the same order, such as Polydora cornuta. This comparison would provide evidence to test the hypothesis, for example, that posterior boundaries of Hox expression are involved in changes in body plan regionalization. If the posterior boundaries of expression for respective orthologs differ in segmental register, and especially if they also correlate with the more subtle morphological transitions in Polydora, it would lend support to the hypothesis. If, on the other hand, the segmental register of posterior boundaries is the same in both species, the changes in body plan regionalization seen in Chaetopterus must be ascribed to downstream events, or different molecular genetic pathways altogether. This is not to say, however, that if the hypothesis is supported that changes in Hox gene regulation are the direct cause of the morphological differences. Rather, the Hox gene expression pattern differences are a read-out of changes in a developmental pathway likely to be involved in the evolutionary events leading to changes in body plan regionalization. As described in part above, these types of comparisons are underway in a subphylum of arthropods with much morphological variation between families, the crustaceans, owing to the comparative studies undertaken most extensively by the Kaufman, Akam, and Patel laboratories.

Moving beyond Hox genes as markers for the developmental pathway changes resulting in morphological evolution will then require looking to the actual regulatory mechanisms behind the Hox gene expression patterns currently being sampled. One approach to this work is the dissection of cis-regulation using cross-species transgenic analysis (e.g., Carr et al., 1998). A recent study by (Locascio et al., 1999) examined the cis regulation of the ascidian CiHox3 gene in reciprocal transgenic reporter experiments between Ciona and mouse. Regulatory elements identified in Ciona were tested in transgenic mouse embryos, and known regulatory sequences from mouse were inserted in the ascidian genome. The authors were able to conclude that though mouse Hox3 regulatory elements failed to reproduce any neural expression in Ciona, an ascidian enhancer fragment could produce a segmental expression pattern in the mouse hindbrain. Further studies of this sort could help determine the specific regulatory changes responsible in part for divergent morphology between animal groups. Another ongoing approach is the comparative analysis of genomic sequences to identify differences in enhancer elements followed by the site-directed knock-in of the enhancer from one species into another to study its functional significance. An example of this approach is an analysis of sequences of the Hoxc-8 early enhancer in mammals, which revealed a specific cis-element deletion in the baleen whales (Shashikant et al., 1998). A study is now in progress to insert this whale enhancer into the homologous genomic location in the mouse to assay its functional consequence (F H. Ruddle and H. Juan, personal communication). While these types of studies have only just begun, they promise to help reveal the actual evolutionary events behind regulatory changes in Hox gene regulation and their involvement in the evolution of regionalized body plans.