SEAVER, E.C.*; PAULSON, D.; IRVINE, S.Q.; MARTINDALE, M. Q.: Sequence and developmental expression of the engrailed gene in the polychaete Chaetopterus

The number of times a segmented body plan has evolved in the metazoa is a currently debated issue. We are interested in understanding whether the annelids and arthropods share a common segmented ancestor and have approached this question by characterizing the expression pattern of the segment polarity gene engrailed (en) in a basal annelid, the polychaete Chaetopterus. We have isolated a member of the en class of genes from a Chaetopterus cDNA library. Not only does the predicted protein sequence have an en class homeobox, but the 1.2 kb cDNA clone also contains all domains characteristic of en proteins in other taxa. Whole mount in situ hybridization reveals that en is expressed in complex dynamic pattern during larval life, when the adult body plan is formed. The en transcript is expressed in all body regions of Chaetopterus, and is localized exclusively to mesodermal tissues and small sets of segmentally reiterated cells in the nervous system. Although expression precedes morphological segmentation, en does not exhibit a segment polarity pattern of expression in the ectoderm, which is the highly conserved pattern observed in arthropods. Our results of the expression of en in Chaetopterus do not support the homology of segmentation between annelids and arthropods.