Abstract
Polar lobes, anucleate vegetal pole protrusions formed by Ilyanassa obsoleta embryos, serve as a mechanism for shunting morphogenetic determinants to one cell during the first two cleavages. Polar lobe material becomes segregated in the CD cell during first cleavage and in the D cell during second cleavage, resulting in a very unequal fourโcell stage. Larval structures including external shell, foot, operculum, statocysts, and eyes develop only when polar lobe material is present. Treatment with the anionic detergent sodium dodecyl sulfate (SDS) before and during the first cleavage inhibited polar lobe formation and equalized cleavage, as the lobe material was distributed to two cells. No polar lobes formed during second cleavage in SDSโequalized embryos, and the fourโcell stage consisted of four equal cells with reduced cell contacts. SDS irreversibly inhibited polar lobe formation without affecting cytokinesis. Although 27% of the larvae from SDSโequalized embryos had one or more lobeโdependent structures duplicated, morphogenesis was impaired: more than 40% of such larvae failed to form shell and/or statocysts. When cells were separated after equalized first cleavage and raised as pairs, the pairs of resulting larvae duplicated lobeโdependent structures with the same frequency as whole equalized embryos. Possible explanations for impaired morphogenesis in SDSโtreated embryos are discussed.