Worm Breeder's Gazette 14(4): 62 (October 1, 1996)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
|1||HHMI, Dept. Biology, MIT, Cambridge, MA 02139, USA. Current address: NemaPharm, Inc., 470 Landsdowne St., Cambridge, MA 02139|
|2||HHMI, Dept. Biology, MIT, Cambridge, MA 02139, USA|
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the mutation egl-35(n694ts) was found to cause precocious expression of HSN serotonin (G. Garriga and B. Horvitz, unpublished results).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.