The role of unequal cleavage and the polar lobe in the segregation of developmental potential during first cleavage in the embryo of Chætopterus variopedatus.

The inequality of the first cleavage division of the Chætopterus embryo is caused by the production of a small polar lobe and the internal shifting of the first cleavage spindle. This division produces a two-celled embryo containing a small AB and a large CD blastomere. These blastomeres have different morphogenetic potentials. Only the larvae resulting from isolated CD blastomeres are able to form bioluminescent photocytes, eyes and lateral hooked bristles. The removal of the polar lobe during first cleavage does not have a great effect on development. These lobeless embryos display a normal pattern of cleavages through the time of mesentoblast formation. The resulting larvae are essentially normal, however they do not form functional photocytes. If the CD cell is isolated after the removal of the first polar lobe, the resulting larva is virtually identical to those formed by the intact CD cell except it lacks the photocyte cells. These results indicate that two separate pathways are involved in the segregation of developmental or morphogenetic potential which takes place during first cleavage. One set of factors, which are necessary for photocyte formation, are associated with the first polar lobe. Other factors that are necessary for the formation of the eyes and lateral hooked bristles are segregated by the unequal cleavage which results from an internal shifting of the cleavage spindle. The removal of a large portion of the vegetal region of the embryo during first cleavage leads to the production of larvae which display a decreased ability to form eyes and lateral hooked bristles. These embryos frequently display an abnormal pattern of cleavages. They do not form the primary somatoblast or the mesentoblast. These results indicate that the vegetal region of the CD cell of Chætopterus is analogous to polar lobes which have been studied in other species, and is therefore important in the specification of the D quadrant. These features of the first cleavage of Chætopterus are a combination of those displayed by forms with direct unequal cleavage and other forms which cleave unequally through the production of large polar lobes. The significance of these findings is discussed relative to the origins of these different types of unequal cleavage.