The canonical
unc-20 allele,
e112, was described almost 25 years ago by Brenner.
unc-20(
e112) mutants display a temperature-sensitive coiling phenotype. More recently Phil Anderson's laboratory has identified six new alleles of
unc-20 that cause a phenotype similar to
unc-20(
e112) when heterozygous in a
smg-1 mutant background. The six newer alleles are homozygous L1 lethal, suggesting that
unc-20 provides an essential function. Mutations in
unc-20 cause defects in HSN axon pathfinding and extension, AVKR axon pathfinding, VD/DD axon pathfinding (Yishi Jin, personal communication), and VC axon anatomy (T. Rakow and B. Schafer, personal communication). Therefore, we propose that
unc-20 functions in growth cone migration and/or guidance. In order to understand the function of
unc-20 we are cloning the gene.
unc-20 maps to the left arm of the X chromosome between
unc-2 and
unc-78. Genetic mapping of
unc-20 demonstrated that it lies to the left of
fax-1. This provides a righthand breakpoint (cosmid F56E3) for the probable location of
unc-20. Furthermore, the
unc-20(
e112) mutation is fully complemented by the
gm27 deletion that removes the entire region flanking
fax-1, indicating that
unc-20 lies on cosmid C15C7 or to its left. Mapping of
unc-20 relative to the bwP1 polymorphism located to the left of
fax-1 in cosmid K08C6 indicates that
unc-20 is to the right of, or very tightly linked to, bwP1.
unc-20 also maps to the right of the spectrin gene located on nearby cosmid K10B3 (K. Norman and D. Moerman, personal communication). Taken together, these data map
unc-20 to a relatively small region covered by approximately two cosmid clones. Transformation rescue experiments to locate the
unc-20 gene are ongoing. To learn more about the role of
unc-20 in HSN axon guidance and extension, we are performing temperature-shift experiments to determine if
unc-20 is required for HSN pathfinding and extension during early larval development (the time period during which HSN axon outgrowth occurs).