Fate maps of the first quartet micromeres in the gastropod Ilyanassa obsoleta.

Cell fate specification in the gastropod mollusc Ilyanassa obsoleta involves both cell autonomous and inductive mechanisms, which depend on determinants localized first in the polar lobe and then in the D quadrant of the embryo. A complete cell lineage is lacking for this embryo and is essential for a critical interpretation of previous experimental results and an analysis of the mechanisms at the molecular level. Lineages of the first quartet micromeres were followed using Lucifer Yellow dextran as a tracer. The tracer was injected into individual first quartet micromeres using iontophoresis and patterns of fluorescence were analyzed in the larva after 8 days of development. Fluorescence was limited to head structures, including eyes, tentacles and velum. Structures on the left side were derived from 1a and 1d micromeres; 1a gave rise to the left eye, including the lens. Right side structures were derived from the 1c micromere and 1b contributed to the apical plate between the eyes and symmetrically to both sides of the velum. First quartet lineage data are compared with results from previous cell ablation experiments and with lineage data from other species.