During the L2, a simple pattern consisting of a single neuroblast within a row of epidermal cells is generated in the lateral ectoderm. This neuroblast, the postdeirid (Pd) neuroblast, is the V5.pa cell. The other Vn.pa homologs become seam cells. We are interested in the question of how this particular cell, V5.pa, is instructed to become a Pd neuroblast.
mu16 is a semidominant mutation found by David Waring in a Nomarski screen. In
mu16 animals, V5.pa sometimes becomes a seam cell instead of a Pd neuroblast.
mu16 is not fully penetrant: only 53% of
mu16 homozygotes completely lack a postdeirid. 11% produce an abnormal postdeirid (i.e. only 2 cells of the postdeirid cell group or a full postdeirid and an extra seam cell). The remaining 36% appear wild- type. At a low frequency
mu16 also causes 11% of
mu16 males to have a gap in their alae or an ectopic ray just anterior to the tail.
mu16/+ heterozygotes exhibit a weaker phenotype: only 14% completely lack a postdeirid cell group and only 5% of the males produce a posterior alae gap or extra ray. Based on three-factor data
mu16 maps to the cluster of LGII between
unc-4 and
bli-1. The deficiency mnDf57 covers both
unc-4 and
bli-1.
mu15/mnDf57 is indistinguishable from
mu16/+. This could mean that mnDf57 is a complex deficiency that contains the
mu16 locus. We plan to test this by putting
mu16 over other deficiencies in the region. However, if mnDf57 does cover
mu16, then:
mu16/mu16 >
mu16/+ =
mu16/Df > +/+ = +/Df This would mean that that the
mu16 locus is not haplo insufficient and that
mu16 is most likely a gain-of-function mutation. Recessive
lin-22 mutations cause additional Vn.pa cells to become Pd neuroblasts (B. Fixsen and B. Horvitz). In order to learn whether
mu16 could affect these ectopic postdeirids, we made
mu16;
lin-22 double mutants. In the
mu16;
lin-22(
mu2) double mutant, V5.pa becomes a Pd neuroblast only 50% of the time, as in
mu16 alone. In addition, there is a significant reduction in the frequency that V4.pa becomes a Pd neuroblast (87% in
lin-22 (
mu2) alone versus 65% in the double mutant). Thus
mu16 can influence cells besides V5, at least in a lin- 22 background. At this point it is unclear how
mu16 is acting or what the wildtype function of the gene might be. Because of the low penetrance of
mu16, it is not practical to look directly for intragenic suppressors that inactivate the
mu16 product. However, recently we found that when the putative
mab-5 gain-of-function mutation
e1751 is combined with
mu16, the frequency of Pd formation is only 2%. (
e1751 alone eliminates the Pd neuroblast at a low frequency of about 5%). Thus, in order to isolate loss-of-function alleles of the gene affected by
mu16 we plan to mutagenize
mu16;
mab-5(
e1751) double mutants, and screen for animals that generate postdeirids.