Meeting Abstract
S9.8 Wednesday, Jan. 6 D quadrant specification in a leech (Helobdella; sp.): comparison with other spiralians WEISBLAT, D.A.*; CHO, S.J.; LYONS, D.C.; VALLéS, Y.; WANG, J.K.; Univ. of California, Berkeley; Univ. of California, Berkeley; Duke University; Univ. of California, Berkeley; Univ. of California, Berkeley weisblat@berkeley.edu
In spiralian development, D quadrant specification is key to forming the second embryonic axis. In unequal cleavers, e.g. clitellate annelids (oligochaetes and leeches), the D quadrant is established by the segregation of determinants to one cell during the first two mitoses. In equal cleavers, by contrast, the cells of the 4-cell stage are equipotent; the D quadrant is specified later, by a stochastic process involving cell-cell interactions. One possible mechanism of D quadrant specification entails a “master regulator” which, when segregated to the prospective D quadrant, elicits the complex developmental program associated with the D quadrant fates. Alternatively, one might postulate an “emergent process” in which the features of the D quadrant fates arise as outputs of a network of cellular and molecular interactions, and that quantitative differences between cells are more important than the qualitative differences implicit in the master regulator model. I will summarize our current understanding of D quadrant specification in Helobdella, which suggests a hybrid of these alternatives. While unequal cleavage is homologous among clitellates, the cell biological mechanisms of the unequal first and second cleavages differ between Helobdella and Tubifex (an oligochaete), as is the process by which determinants segregate within the zygote. Curiously, the Notch, Wnt and MAPK signaling pathways are deployed in the 2-cell embryo of Helobdella; are they required for D quadrant specification? Could this reflect an evolutionary acceleration of the stochastic D quadrant specification process in an equally cleaving spiralian ancestor? Insights gained from studying Helobdella should aid in understanding D quadrant specification in other spiralians.
