Cell contact-dependent positioning of the D cleavage plane restricts eye development in the Ilyanassa embryo

In embryos of the gastropod Ilyanassa obsoleta, the first-quartet micromeres of the A, B and C lineages (1a, 1b, and 1c) are each competent to form an eye in response to signaling from the 3D cell. The first-quartet micromere of the dorsal D lineage (1d) is smaller than the others, divides at a slower rate, and lacks the ability to form an eye. These properties of 1d all depend on inheritance of vegetal polar lobe cytoplasm by its mother cell D at second cleavage. I show that they depend also on the presence of cells adjacent to D during the late four-cell stage: after ablation of the A and/or C cells before this stage, 1d inherits more cytoplasm than normal, divides more rapidly, and frequently forms an eye. In non-D lineages, cleavage plane positioning and micromere division rates are relatively insensitive to cell contacts. Compressing whole embryos during third cleavage also leads to an increase in 1d volume correlated with abnormal eye formation; this suggests that the normal effect of cell contacts is to position the D cell cleavage furrow closer to the animal pole, and the enhanced division asymmetry of the D cell contributes to the suppression of eye development.