Biology Reference
In-Depth Information
“corrected” development. Mutants that alter
lin-14
expression can form
dauer larvae either early, at the L1 stage, or late, at the L3 stage (
Liu &
Ambros, 1989
). When these animals emerge from the dauer, they still
undergo two additional larval stages; however, despite having undergone
a total of three or five larval stages, they can be fully functional, egg-laying
competent animals. This is something they would not have achieved if they
had developed without arrest. A curiosity is that the postdauer animal seems
to synchronize its somatic development, which has undergone one too few
or one too many larval stages already, with its gonad development, whose
progress is unaffected by the heterochronic genes. Thus, the first postdauer
larval stage is sped-up or slowed-down, presumably to get in sync with the
gonad. The mechanism of this resynchronization is entirely unknown.
11. LAUNCHING LARVAL DEVELOPMENT
By definition, the postembryonic phase of the worm life cycle begins
when the worm hatches from the egg. With this embryo to larva transition,
life for the worm changes dramatically; direct exposure to the environment
brings with it the ability to forage, feed, and grow. But what launches the
larval developmental programs? A growing body of work indicates that post-
embryonic developmental programs are genetically repressed until appropri-
ate signals are received from the local milieu.
The key trigger is nutritional. Larval progression initiates only in the pres-
ence of a food source, typically provided by a lawn of
E. coli
in the lab. When
E. coli
is present, development begins and proceeds apace, in a stereotypic
fashion with high temporal synchrony between individuals. Without food,
the L1 larva arrests both somatic and germline development, entering a state
known as L1 arrest or L1 diapause, and it can survive more than 2 weeks. If a
food source becomes available, development resumes synchronously
throughout the animal with stage-specific programs appropriately scheduled
relative to each other and to the molts, suggesting endocrine involvement. As
for postdauer development, what is particularly impressive here is that the
same cell division pattern and sequence occurs regardless of the interruption.
Diverse developmental programs are suspended during L1 arrest, and the
heterochronic gene pathway is not activated. The
lin-4
miRNA fails to
accumulate, and consequently, LIN-14 levels remain high (
Arasu et al.,
1991; Feinbaum & Ambros, 1999
). The
lin-4
response appears transcrip-
tional because this expression pattern is recapitulated by assays of promoter
