Removal of the polar lobe leads to the formation of functionally deficient photocytes in the annelid Chaetopterus variopedatus

The light emitting photocytes of Chaetopterus variopedatus larvae are bilaterally situated within the ectoderm of the post-trochal region. Their histological appearance is similar to that of the adult photocytes. The larval photocytes contain a large quantity of membranous secretory vesicles (photosomes), which probably contain the photoluminescent protein. The two-cell Chaetopterus embryo contains a small AB and a large CD blastomere. Previous studies have shown that only the "larvae" resulting from isolated CD blastomeres are able to luminesce. Consistent with these findings, morphologically distinct photocytes are only found in the CD larvae. The removal of the small polar lobe that forms during first cleavage leads to the production of a "larva" that is unable to produce light. All delobed larvae contain morphologically distinct photocytes, which are identical to those in normal larvae except they appear to contain only a small quantity of photosomes. Experimental equalization of first cleavage leads to the production of a double embryo. While photocytes are found in both of the duplicated post-trochal regions, usually only one of these is capable of emitting luminescence. Apparently, the highly localized vagetal material (determinants) responsible for functional light emission is distributed to both halves in only a few cases when first cleavage is experimentally "equalized". These results indicate that the determinative action of the polar lobe is not required for the formation of the photocytes themselves, but rather for their ability to function as emitters of light. The determinants in the polar lobe of Chaetopterus may control some aspect of the photoluminescence reaction itself, such as the production of the photoprotein.