Development of Iiyanassa obsoleta embryos after equal distribution of polar lobe material at first cleavage.

Ilyanassa obsoleta embryos cleave unequally via the formation of polar lobes, which contain materials essential for the development of larval shell, foot, operculum, statocysts, and eyes. Polar lobe material is shunted to the CD cell during first cleavage and to the D cell during second cleavage. Treatment with cytochalasin B (CB) before first cleavage prevents the formation of the polar lobe and leads to equal cleavage and the equal distribution of lobe material. At second cleavage each cell forms a polar lobe, resulting in a four-cell stage with two large (D) cells and two smaller (C) cells. Embryos equalized with CB frequently display duplications, 68% duplicating two or more larval structures. Embryos with adjacent D cells (CCDD) duplicate statocysts more frequently than embryos with opposite D cells (CDCD), perhaps due to enhanced inductive interactions. When equal cells are separated after first cleavage, resulting larvae develop like CD halves from control embryos. When equal halves are analyzed as pairs and compared with whole, equalized embryos, they duplicate shell, foot, and operculum more frequently. This difference is probably due to masking of duplications of these structures in whole, equalized embryos rather than to general inhibitory interactions between the two D quadrants. These results are discussed with respect to proposals that interactions between D quadrants in equalized embryos may alter developmental capabilities.