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transparency. Because cell behavior is remarkably the same from animal to
animal, the lineage of every cell in the animal is traceable through larval
development. But what makes
C. elegans
exceptionally informative is the
genetics: forward genetic screens have identified a diversity of mutants
and these mutants can be combined to tell the tale of how the genes function
together.
3. HETEROCHRONIC MUTANTS
Among the many
C. elegans
mutants isolated in genetic screens were a
few that eventually were grouped together because they altered the succes-
sion of larval developmental events. The gross phenotypes of these mutants
are not very different from those of many developmental mutants in the lin
or “cell lineage abnormal” class: They are unable to lay eggs, having either a
missing or defective egg-laying system (
Ferguson & Horvitz, 1985
). But cell
lineage analysis revealed that this subset displayed features not found in other
mutants: the normal order of stage-specific lineage patterns is altered
throughout the animal (
Ambros & Horvitz, 1984
). In one class of mutants,
development was precocious, where stage-specific events were skipped, and
in another development was retarded, where events were reiterated
(
Fig. 6.1
). The whole group was termed “heterochronic mutants,” for their
abnormal developmental timing. Initially, the mutations in this collection
defined just four genes:
lin-4
,
lin-14
,
lin-28
, and
lin-29
. Many others were
to follow.
The mere existence of these mutants allowed two insights: First, the tem-
poral progress of many cell lineages is under direct genetic control, rather
than being an emergent property of spatial patterning and tissue-specific reg-
ulation. Secondly, the mutants revealed that the stage-specific sublineages
were modular, that is, what happens at one stage does not necessarily depend
on what happened at a previous stage. One commentator compared the
modularity of the
C. elegans
larval stage patterns to parasegments in
Drosoph-
ila
embryonic development, with the parasegment boundaries being repre-
sented by the molts (
Thummel, 2001
). Indeed, there is likely a mechanistic
connection between temporal patterning and the molting cycle.
The epidermis replaces its cuticle four times, once per larval stage. Dur-
ing the intermolt period, certain epidermal blast cells divide in patterns that
are characteristic of the lineage at each stage. For instance, lateral epidermal
blast cells V1 to V4 each divide once in the first larval stage producing a
daughter that joins the epidermal syncytium and one that remains a blast cell
