MAPK in Ilyanassa

Molluscan embryogenesis is a classic example of mosaic development – most cell deletions result in partial larvae with predictable defects. This is because the cells of the embryo are specified at an early stage and rarely regulate to restore the missing structures. Mosaic development is often equated with cell-autonomous modes of cell fate specification such as cytoplasmic localization. However, in the embryo of the gastropod Ilyanassa obsoleta, the cells that give rise to most ectodermal structures are specified conditionally, by cell-cell interactions. Embryological manipulations have demonstrated that a particular cell in the D quadrant lineage of the embryo functions as an organizer of axial patterning by inducing various other cells to assume their proper spatial pattern of cell fates (Cather, 1971; Clement, 1976). Despite the importance of this induction in the axial patterning of the embryo, interest in the evolution of axial patterning in animal embryos and a long history of experimentation aimed at uncovering patterning molecules in molluscan embryos, nothing is known about the molecular mechanisms that underlie this event. Nonetheless, a century of experiments on molluscan embryos make specific predictions about the expected behavior of the molecules used by the molluscan organizer. The D quadrant organizer functions at a early stage, when there are relatively few cells in the embryo, and the regularity of the preceding cell divisions allows unambiguous identification of all of these blastomeres. The first cell division is accompanied by the formation of a polar lobe – an anucleate extrusion of cytoplasm from the vegetal pole (see Fig. 1 for diagram of early Ilyanassa development). The contents of the polar lobe are shunted into one of the blastomeres at the fourcell stage. This produces one cell that is larger than the other three. The larger blastomere that inherits the polar lobe becomes the founder cell of the dorsal lineage known as the D quadrant. This event specifies the dorsal-ventral axis of the embryo. The three smaller blastomeres present at the four cell stage become the founder cells of different lineages, the A, B and C quadrants, respectively. During the next three cleavage cycles, each macromere divides towards the animal pole to produce a micromere and the four micromeres produced during a given cleavage cycle are referred to as a quartet. After the fifth cleavage cycle, there are three quartets of micromeres and their derivatives in the animal hemisphere and four vegetal macromeres. These micromeres give rise to all of the ctodermal structures of the larva, including velum, eyes, statocysts, shell, foot and operculum (Render, 1997). While the micromeres have predictable fates, they are not specified until later in development, when they are induced by the D quadrant macromere. Ablation experiments have shown that the D quadrant macromere is required to establish the proper cell fates among the micromeres. Removal of the polar lobe prevents the specification of D quadrant macromere. The larvae that result from polar lobe ablation lack a dorsal-ventral axis (van den Biggelaar and Guerrier, 1983). They have disorganized ciliated velar bands, everted stomodea and esophageal tissue. They lack eyes, shell, heart, intestines, statocysts, opercula, larval kidneys and certain muscles (Atkinson, 1971; Clement, 1952). Killing he D macromere at the four, eight, or 16-cell stages (e.g. ablation of D, 1D or 2D) has the same effect as removal of the polar lobe, demonstrating that the requirement for the D 45 Development 128, 45-56 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV5424