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.

Affiliation:
- Dept. of Genetics and Cell Biology, University of Minnesota, St. Paul, MN 55108 Dept. of Genetics and Cell Biology and Dept. of Biochemistry, University of Minnesota, St. Paul, MN 55108