Asymmetric cell division is essential to generate cell diversity during development. In C. elegans, Wnt signaling regulates many asymmetric cell divisions. We have focused our analysis on DSH-2, a key Wnt signaling pathway component. Loss of both maternal and zygotic
dsh-2 function results in asymmetric neuroblast division defects and embryonic/early larval lethality, while loss of zygotic
dsh-2 function disrupts asymmetric cell division of the somatic gonadal precursor cells, Z1 and Z4. To identify genes that function with
dsh-2 in asymmetric division, we undertook a genetic screen to isolate suppressors of
dsh-2 lethality. This screen was highly successful and we isolated over 60 suppressors, all of which are dominant. These suppressor mutations could be activating mutations in downstream pathway components or in parallel signaling pathways that function in concert with DSH-2. We have focused our characterization on Sup245, Sup305 and Sup327, which suppress all
dsh-2 defects examined thus far. We have further characterized suppressor function in Z1/Z4 division. Asymmetric division of Z1/Z4 involves the reciprocal asymmetric localization of POP-1/TCF and SYS-1/b-catenin in the proximal and distal daughters resulting in a high ratio of SYS-1 to POP-1 ratio in the distal daughter and the specification of distal cell fate.
dsh-2 mutants disrupt both POP-1 and SYS-1 asymmetry (1, 2). To determine if Sup245, Sup305 and Sup327 suppress
dsh-2 by restoring asymmetric localization of POP-1 and/or SYS-1 we have analyzed the expression of two GFP reporters, Psys-1::SYS-1::GFP (qIs95) and Psys-1::POP-1::GFP (qIs74) in the
dsh-2; Sup strains. All three suppressors partially re-establish both POP-1 and SYS-1 asymmetry. Thus, a sufficient SYS-1 to POP-1 ratio is likely restored to specify distal cell fate. Genetic mapping experiments have placed both Sup305 and Sup327 on chromosome I to the left of
lin-17. Based on their genetic map position, and similar phenotypes they are likely to be allelic. Sup245 has been mapped to the left of
unc-54 on the right arm of chromosome I. To determine the molecular identity of the suppressor mutations, two strains were sent for whole genome sequencing; one containing Sup305 and a second strain containing both Sup327 and Sup245. We are currently analyzing the resulting sequence and testing potential candidates. These studies will identify novel genes that function with the Wnt signaling pathway to control asymmetric cell division. 1. Chang et al., (2005) Mech. Dev. 122:781-789 2. Phillips et al., (2007) PNAS 104:3231-3236.