Meeting Abstract

P1.24  Monday, Jan. 4  Coordinating cell cleavage pattern and fate determination in the leech Helobdella LYONS, Deirdre C*; WEISBLAT, David A; University of California, Berkeley dclyons1@gmail.com

Spiral cleavage is a unique cell division program seen in the embryos of animals as diverse as annelids, molluscs and polyclad flatworms. Comparing spiral cleavage in extant species provides insight into how changes in the program have influenced body plan evolution. An experimentally accessible example of evolutionary flexibility within the spiral cleavage program is D quadrant specification. In spiral cleavage, the first two cell divisions are roughly meridional with respect to the animal-vegetal axis, establishing four embryonic quadrants (A-D). The D quadrant is of particular interest because, by convention, D is defined as the dorsal quadrant, which produces bilaterally symmetric trunk mesoderm and ectoderm. Helobdella embryos undergo a modified version of spiral cleavage in which unequal first and second cleavages are critical for setting up the axes of the adult. Prior to first cleavage, cytoplasmic rearrangements form domains of yolk-free cytoplasm (teloplasm) which is segregated to the D macromere at the 4-cell stage. Teloplasm has been shown to contain the determinants that render the D macromere capable of generating the precursors of segmental mesoderm (DM) and ectoderm (DNOPQ). The extent to which teloplasm plays a passive role in development (merely being sequestered into the largest cell) versus an instructive role (e.g. influencing the cleavage pattern itself) is an outstanding question. Here we report a systematic analysis of teloplasm movements and interaction with the mitotic apparatus up to the 7-cell stage, when DM and DNOPQ are born. These data reveal that teloplasm movements during the early cell cycles are highly dynamic and very stereotyped and support the hypothesis that teloplasm is indeed influencing the spiral cleavage program in this species.