The spatial and temporal expression of Ch-en, the engrailed gene in the polychaete Chaetopterus, does not support a role in body axis segmentation

We are interested in understanding whether the annelids and arthropods shar ed a common segmented ancestor and have approached this question by charact erizing the expression pattern of the segment polarity gene engrailed (en) in a basal annelid, the polychaete Chaetopterus. We have isolated an en gen e, Ch-en, from a Chaetopterus cDNA library. Genomic Southern blotting sugge sts that this is the only en class gene in this animal. The predicted prote in sequence of the 1.2-kb cDNA clone contains all five domains characterist ic of en proteins in other taxa, including the en class homeobox. Whole-mou nt in situ hybridization reveals that Ch-en is expressed throughout larval life in a complex spatial and temporal pattern. The Ch-en transcript is ini tially detected in a small number of neurons associated with the apical org an and in the posterior portion of the prototrochophore. At later stages, C h-en is expressed in distinct patterns in the three segmented body regions (A, B, and C) of Chaetopterus. In all segments, Ch-en is expressed in a sma ll set of segmentally iterated cells in the CNS. In the A region, Ch-en is also expressed in a small group of mesodermal cells at the base of the chae tal sacs. in the B region, Ch-en is initially expressed broadly in the meso derm that then resolves into one band/segment coincident with morphological segmentation. The mesodermal expression in the B region is located in the anterior region of each segment, as defined by the position of ganglia in t he ventral nerve cord, and is involved in the morphogenesis of segment-spec ific feeding structures late in larval life. We observe banded mesodermal a nd ectodermal staining in an anterior-posterior sequence in the C region. W e do not observe a segment polarity pattern of expression of Ch-en in the e ctoderm, as is observed in arthropods. (C) 2001 Academic Press.