Biology Reference
In-Depth Information
lacks at the beginning of postembryonic development is the ability to repro-
duce. Over four larval stages, 53 cells divide and differentiate to ultimately
form the gonad and reproductive system, as well as additions and rearrange-
ments to the epidermis, muscles, and nervous system. Each larval stage ends
with a molt when the epidermis secretes a collagenous cuticle, and the old
cuticle is shed. At the last transition, somatic differentiation is complete and
the animal becomes sexually mature: either a self-fertilizing hermaphrodite
or a male.
The stage-to-stage transitions during larval development appear rela-
tively subtle. The larva grows roughly continuously in size with little change
in overall morphology. Each molt is preceded by a period of lethargus, when
the worm seems to sleep while its cuticle changeover takes place. Emerging
from the old cuticle, the worm resumes foraging and pumping food into its
pharynx, and growth continues. By the fourth stage, about 400 cells have
been added to the epidermis, nervous system, gonad, and muscle. The adult
hermaphrodite that emerges from the last molt consists of 959 somatic cells
and scores of germ cells, and is about 1 mm long. The adult will increase in
size, but will only add germ cells, not somatic cells, throughout the rest of its
roughly 2-3-week life.
If environmental conditions deteriorate, larval development can undergo
a controlled and reversible suspension that is coordinated throughout the
animal. The trigger is an increased population density, exacerbated by
diminishing food supply and slightly warmer temperatures. Under such
conditions, early stage larvae have the opportunity to transition into a mor-
phologically distinct form, called the dauer larva, which is specialized for
long-term survival under less than optimal growth conditions. The decision
to make this choice occurs during the first larval stage (L1) but the morpho-
logical shift is not complete until midway through larval development, at the
end of the second stage (L2). Survival as a dauer extends far beyond the
normal lifespan of the worm due to numerous physiological adjustments.
As the dauer disperses, the crowding dissipates (sensed by the drop in a
worm-produced pheromone), and if nutritional conditions improve, the
larva morphs back into the feeding/foraging condition and picks up devel-
opment where it left off. In this instance, the transition into and out of dauer
must be coordinated with the transition that normally occurs in continuous
development from the L2 to the third stage (L3).
Two features of
C. elegans
make it a particularly powerful experimental
system for investigating developmental transitions. First, the position and
division of every cell is observable in living specimens due to the worm's
