The Spatial and Temporal Expression of Ch-en, the engrailed Gene in the Polychaete Chaetopterus, Does Not Support a Role in Body Axis Segmentation

Abstract

We are interested in understanding whether the annelids and arthropods shared 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 an en gene, Ch-en, from a Chaetopterus cDNA library. Genomic Southern blotting suggests that this is the only en class gene in this animal. The predicted protein sequence of the 1.2-kb cDNA clone contains all five domains characteristic of en proteins in other taxa, including the en class homeobox. Whole-mount in situ hybridization reveals that Ch-en is expressed throughout larval life in a complex spatial and temporal pattern. The Ch-en transcript is initially detected in a small number of neurons associated with the apical organ and in the posterior portion of the prototrochophore. At later stages, Ch-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 small 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 chaetal sacs. In the B region, Ch-en is initially expressed broadly in the mesoderm 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 the ventral nerve cord, and is involved in the morphogenesis of segment-specific feeding structures late in larval life. We observe banded mesodermal and ectodermal staining in an anterior-posterior sequence in the C region. We do not observe a segment polarity pattern of expression of Ch-en in the ectoderm, as is observed in arthropods.

Cephalochordata

“…However, not all polychaetes are the same. engrailed is not similarly expressed in the polychaete Chaetopterus (Seaver et al 2001 ). Taken together, these results have in the main supported the idea of a segmented bilaterian ancestor, but have far from settled the question (Minelli and Fusco 2004 ).…”

Evolutionary Dynamics of the wnt Gene Family: A Lophotrochozoan Perspective

“…Whole-mount ISH was performed according to previously published protocols for Capitella (Seaver et al 2001;Seaver and Kaneshige 2006). Larvae were hybridized for 72 h at 65°C witha probe concentration of 1 ng/ll for all three genes.…”

Vasa and nanos are coexpressed in somatic and germ line tissue from early embryonic cleavage stages through adulthood in the polychaete Capitella sp. I

“…I colony was maintained in the laboratory using published culture methods (Grassle and Grassle 1976) and broods were recovered as described previously (Seaver et al 2005). Embryos and larvae were dissected from brood tubes, fixed in 3.7% formaldehyde in filtered sea water at 4°C for 16-24 h and then processed for whole-mount in situ hybridization according to published protocols (Seaver and Kaneshige 2006;Seaver et al 2001). Juveniles and adults were treated with the same conditions as embryos and larvae with the exception that proteinase K treatment was increased from 3 to 10 (juveniles) or 20 min (adults), and for adult stage in situ experiments, the volume of all washes and hybridizations was increased from 0.5 to 1 ml.…”