The
egl-20 gene functions in diverse aspects of anterior/posterior (A/P) patterning[1].
egl-20(+) activity is required to activate the Hox gene
mab-5 in the QL neuroblast and its descendants for proper migration. Independent of
mab-5,
egl-20 is required to position other migratory cells and to orient certain asymmetric cell divisions along the A/P axis. We mapped
egl-20 to a cosmid (W07H7) containing a Wnt gene. We sequenced this Wnt gene from a strong loss-of-function allele and found a mutation that changes a highly conserved cysteine to a serine. We are sequencing the remaining four alleles of
egl-20 and attempting to narrow down the rescuing activity to a sub-cloned region of W07H7 containing only the Wnt gene. Wnt genes define a large family of signaling proteins implicated in many developmental processes including cell proliferation, cell fate determinations, and cell polarity. In Drosophila, a signaling pathway involving the Wnt protein/ wingless, its receptor/frizzled and dishevelled (a novel protein) is required to determine correct planar polarity in adult epidermal cells. In addition to
egl-20, other homologs of wingless signaling pathway components have been isolated in C. elegans. For example,
lin-44 encodes another Wnt protein[2],
lin-17 encodes a frizzled homolog[3] and
mig-5 encodes a dishevelled homolog[4]. Like
egl-20,
lin-17 and
mig-5 have been implicated in Q cell migration[1,4]. Also,
egl-20,
lin-44, and
lin-17 are known to affect cell polarity[5,6,7].
egl-20,
lin-44 and
lin-17 function in determining the polarity of certain seam cell lineages. Mutations in the
egl-20 gene cause reversals in the polarity of the first V5 cell division [5]. Mutations in
lin-17 alone do not affect V5 polarity although they are able to suppress the reversals in
egl-20 mutants.
lin-44 does not appear to be involved in determining V5 polarity. Mutations in
lin-44 lead to polarity reversals in the T cell lineage[6 ]and mutations in
lin-17 cause a loss of polarity (symmetric divisions). Sawa and Horvitz have proposed that
lin-17/frizzled serves as a receptor for
lin-44/wingless . Since
lin-44 mutations primarily result in polarity reversals rather than symmetric divisions, they have also proposed that there exists another signal that can affect T cell polarity. We asked if
egl-20 is this signal. We found that there was a loss of T cell polarity in the
lin-44(
n1792);
egl-20(
n585) double mutant (13/13 animals), as in the
lin-17 single mutant. This result suggests that both
egl-20 and
lin-44 are signals acting through the
lin-17 receptor to determine the polarity of the T cell lineage. The roles of
egl-20 and
lin-17 in determining V5 polarity and Q cell migration remain unclear. Further molecular analysis of these genes will be informative in understanding how the wingless signaling pathway functions in such seemingly different processes. References: 1. Harris et al., Development, in press 2. Herman et al., Cell 83:101-110 3. Sawa and Horvitz, 1995 IWM abs. 4. Guo and Hedgecock, 1996 ECWM abs. 5. Whangbo et al., 1996 WCWM abs. 6. Herman and Horvitz, Development 120:1035-1047 7. Sternberg and Horvitz, Dev. Biol.130:67-73