Biology Reference
In-Depth Information
Figure 6.1 Cell lineages of precocious and retarded heterochronic mutants. Post-
embryonic development is indicated on the left from hatching to adulthood,
encompassing four larval stages. Thick lines represent the molting cycle with gray boxes
indicating the lethargus period and the short horizontal lines indicating the molts. Thin
lines are lineage diagrams of representative lateral and ventral epidermal cells. Lateral
epidermal blast cells terminally differentiate into seam cells of the adult, indicated by
three horizontal lines. Ventral cells give rise to the vulva. Dashed lines indicate addi-
tional cells not shown. Lineages of representative precocious and retarded mutants
are shown. Precocious mutants skip stage-specific lineage events (in this case, those
of the L2) and may cease the molting cycle prematurely. Retarded mutants reiterate
stage-specific events (e.g., L2), and, as in the example shown, cease molting inde-
terminantly after five or more cycles.
(
Fig. 6.1
). This blast cell divides twice in the second larval stage, and once
again in the third and fourth stages. At adulthood, these cells fuse to form
another syncytium, the lateral seam, which secretes a cuticle structure called
adult lateral alae. Tracking of these and other lineages in heterochronic
mutants reveals their temporal patterning defects in detail (
Ambros &
Horvitz, 1984
).
lin-4
null mutants reiterate the L1 pattern in subsequent stages and ter-
minal differentiation of the lateral epidermal blast cells never occurs. In the
ventral epidermis, the three “vulva precursor cells” (VPCs) normally divide
in the L3 to produce the 22 cells of the vulva, but do not do so in
lin-4
mutant animals. A recessive mutation in
lin-14
, on the other hand, causes
L1 patterns to be skipped, and all subsequent nongonadal lineages, including
the VPCs, advance precociously. In these animals, the lateral epidermal blasts
divide twice in the first stage, which they would normally do in the second
stage, and differentiation occurs at the end of the L3, one stage early. Thus,
lin-4
and
lin-14
mutants show opposite effects on the L1, resulting in
retarded and precocious development, respectively.
