The heterochronic genes control the timing of many developmental events. In particular, these genes control the timing of the larval-to-adult (L/A) developmental switch by hypodermal cells (Ambros, Cell 57:49, 1989) and the timing of serotonin expression by the HSN neurons (Garriga and Horvitz, WBG 11(2):101, 1990). For example, loss-of-function alleles of
lin-14 cause precocious defects in both the L/A switch and HSN serotonin expression, whereas
lin-4 alleles cause retarded defects in both of these developmental events. We have identified three new mutations --
egl-35(
n694), lin
(n2853) and lin
(n2914) -- that control the timing of the L/A switch. Like loss-of-function alleles of
lin-14, the mutation
egl-35(
n694ts) was found to cause precocious expression of HSN serotonin (G. Garriga and B. Horvitz, unpublished results). Although
n694 alone did not cause a precocious L/A switch,
n694 could enhance the precocious L/A switch caused by
lin-14(
n179ts) at permissive (15 degrees) or semi-permissive (20 degrees) temperatures; for example, at 20 degrees the
n694;
n179 double mutant made alae precociously in the L4-stage (the presence of precocious alae is diagnostic of a precocious L/A switch), whereas the
n179 single mutant did not. Thus, in the absence of normal
lin-14 function,
egl-35 can control the timing of the L/A switch. We noticed that, unlike either single mutant, the
n694;
n179 double mutant was sterile at the non-permissive temperature of 25 degrees. We therefore could look for suppressor mutations that restored fertility to the double mutant at 25 degrees. Since both
n179 and
n694 cause precocious developmental defects, such suppressor mutations might be expected to cause retarded developmental defects. From the progeny of approximately 14,000 F1 animals, we identified 22 isolates that were fertile at 25 degrees. One isolate displayed a retarded L/A switch, as assessed by the lack of alae in adult animals. The suppressor mutation in this isolate was designated
n2853. Animals containing the
n2853 mutation alone were slightly long and uncoordinated, had a protruding vulva and burst at the vulva as adults.
n2853 animals displayed two defects that are diagnostic of retarded hypodermal cell development: young adults lacked alae and adults underwent supernumerary molts. In addition, the tails of
n2853 males had a morphology that has been termed "leptoderan" (Fitch and Emmons, Dev. Biol. 170:564, 1995), in which a tailspike protrudes from the fan. Fitch et al. (WBG 11(4):87, 1990) suggest that leptoderan-tail morphology might result from the failure of hypodermal cells to migrate late in development, a failure that can be regarded as a retarded developmental defect.
n2853 maps very close to
unc-3 on the right arm of LG X. Slack et al. (accompanying abstract) show that
n2853 is an allele of
let-7. The
n2853 mutation bears a striking resemblance to mutations in the retarded heterochronic gene
lin-29 in that both
n2853 and
lin-29 animals have protruding vulvae, burst at the vulva as adults, lack alae as adults, and undergo supernumerary molts. Moreover, neither
n2853 nor
lin-29 mutations, unlike mutations in several other heterochronic genes, cause an apparent defect in dauer development, and neither is suppressed by passage through the dauer stage. These similarities suggest that lin
(n2853) and
lin-29 may act at the same step in the heterochronic gene pathway that controls developmental timing. As noted above,
n2853 animals burst at the vulva as adults. This defect was temperature-sensitive: whereas at 15 degrees approximately 75% of animals burst, at 25 degrees all animals burst. Because animals that burst at the vulva were sterile, we could look for suppressor mutations that restored fertility to
n2853 animals at 25 degrees. Since
n2853 causes retarded developmental defects, such suppressor mutations might be expected to cause precocious developmental defects. We screened approximately 12,000 F1 animals and identified ten isolates that were fertile at 25 degrees. One isolate displayed a precocious L/A switch, as assessed by the presence of alae in L4-stage animals. The suppressor mutation in this isolate was designated
n2914.
n2914 behaved genetically as a dominant suppressor of the temperature-sensitive defect in sterility caused by
n2853 and as a recessive mutation that caused animals to be dumpy, sterile, make alae precociously in the L4-stage and have vulval abnormalities. The precocious alae made by lin
(n2914) animals were often faint and patchy.
n2914 maps between
unc-29 and
lin-11 on LG I. The possibility that
n2914 is an allele of the previously identified precocious heterochronic gene
lin-41 has not been tested.