In C. elegans, the heterochronic genes
lin-4,
lin-14,
lin-28,
lin-29 and
lin-42 control the timing of lateral hypodermal seam cell terminal differentiation (the larval-to-adult (L/A) switch). Among these five genes,
lin-29 is the most direct regulator of the L/A switch;
lin-4,
lin-14,
lin-28 and
lin-42 are required to restrict LIN-29 accumulation in the hypodermis to the L4 stage. We have performed genetic screens (see Abrahante et al., WBG 14(1): 91) to ask if there are additional temporal regulators of
lin-29. We identified two new members of the heterochronic pathway that we have designated
lin-57 and
lin-58. Similar to
lin-14,
lin-28 and
lin-42,
lin-57 mutants execute the L/A switch precociously. However,
lin-57 is unique among these genes in that it is the only one to which
lin-4 is epistatic:
lin-4;
lin-57 double mutants fail to execute the L/A switch. In order to better understand the role of
lin-57 in the heterochronic gene pathway, we have initiated its molecular cloning.
lin-57 resides on the X chromosome, in a 1.5 mu interval between
mec-2 and
stP33. A YAC clone, Y23B4, rescues the
lin-57 mutant phenotype. We are currently testing fosmid and cosmid clones that contain portions of Y23B4 for rescuing activity, and we are constructing a lambda sub-library of Y23B4 DNA to obtain additional clones from this region.
lin-58 mutants exhibit a precocious hypodermal phenotype; 57% of the seam cells terminally differentiate during the L3 molt as judge by cell fusion.
lin-58 maps to an approximately 2.5 mu interval between
lon-3 and
unc-76, a region fully covered by YACs. To understand better the position of
lin-58 in the heterochronic pathway, we have performed epistasis analysis with the known heterochronic genes. In
lin-29;
lin-58 mutants the seam cells do not execute the L/A switch, and thus the double mutants resemble
lin-29 mutants. This result indicates that in wild-type animals
lin-58 acts through
lin-29 to prevent precocious seam cell terminal differentiation during the L3 molt.
lin-14;
lin-58 and
lin-28;
lin-58 double mutants exhibit an enhanced precocious phenotype. In these double mutants a higher percentage of seam cells terminally differentiate precociously than in either single mutant.
lin-42;
lin-58 double mutants show a mild enhancement of the precocious hypodermal phenotype but also exhibit a striking and highly penetrant synthetic vulval eversion defect, not seen in either single mutant. This phenotype suggests that the wild-type products of these two genes function redundantly with respect to vulva development.