Meeting Abstract

37.1  Sunday, Jan. 5 08:00  Cell differentiation in cleavage arrested embryos of the comb jelly Mnemiopsis – Do they have a cleavage clock? FISCHER, AHL*; PANG, K; HENRY, JQ; MARTINDALE, MQ; MBL, Woods Hole; SARS Bergen, Norway; University of Illinois, Urbana; Whitney Lab for Marine Bioscience, Univ. Florida afischer@mbl.edu

Ctenophores, an early branching group of gelatinous marine invertebrates, have a highly stereotyped cleavage pattern and a precocious specification of blastomere fates. We investigated the role of cell autonomous specification and developmental timing of two distinct cell types in the lobate ctenophore Mnemiopsis leidyi. Light emitting photocytes and comb cells with motile compound cilia form around 9 hours after spawning. Most comb plate cilia normally appear in derivates of the E lineage, while photoprotein is formed in derivatives of the M lineage. Using Cytochalasin B, we found that even if cytokinesis is blocked at the 8-cell, 16-cell -and later stages comb plate cilia and photoprotein still form. Our work supports and extends previous studies demonstrating that the cleavage program is causally involved in the spatial segregation and/or activation of factors giving rise to photocytes and comb cells. These factors are segregated independently to the appropriate lineage at the 8- and the 16-cell stage and have features of a ‘clock’ since the comb plate-like cilia appear at roughly the same developmental time in cleavage arrested embryos as they do in controls. Timed treatment with the DNA-synthesis inhibitor Aphidicolin suggests that nuclear division, possibly affecting DNA-cytoplasmic ratios, appears to be important in the timing of differentiation markers. Evidence suggests that the 60-cell stage, just prior to gastrulation, represents the time of zygotic gene activation. Such cleavage clock regulated phenomena appear to be widespread amongst the Metazoa and these cellular and molecular developmental mechanisms probably evolved early in metazoan evolution.