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B. Horvitz, unpublished results).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.