Biology Reference
In-Depth Information
lin-28
mutants, like
lin-14
mutants, are precocious, but instead of skip-
ping first-stage events, they skip second-stage patterns (some cells also skip
third-stage events).
lin-29
mutants are retarded like
lin-4
animals, but their
problems do not begin until the L4 stage. The animal becomes adult, but its
epidermis remains larval. Eventually, other heterochronic mutants were
identified that showed precocious or retarded development starting at one
stage or another. Some heterochronic mutants have stronger null pheno-
types than others, and a surprising number affect the L2, and most affect
the transition to adulthood, directly or indirectly. The overall theme, how-
ever, is that their loss-of-function phenotypes are either precocious or
retarded, and the defect begins at a particular stage.
The organization of the heterochronic pathway emerges when individ-
ual mutants are combined into multiply-mutant strains (
Ambros, 1989
). For
example, when a null allele of
lin-4
, which reiterates the L1 fates, is put
together with a null allele of
lin-14
, which skips the L1 fates, the resulting
double-mutant animal resembles the
lin-14
mutant in all respects. By the
logic of epistasis analysis, this result suggests that
lin-4
normally negatively
regulates
lin-14
in some way. When a
lin-29
mutant is combined with
any known precocious mutant, the resulting animal is always retarded for
late-stage developmental events, indicating that
lin-29
is the most down-
stream gene in the pathway.
In this way, the heterochronic pathway has been built up as new genes
are added. In most cases, the mutations are strong and the double phenotypes
clear. These results give us the core pathway (
Fig. 6.2
, top). In other cases,
however, the gene mutations are weaker (either incompletely penetrant,
incompletely expressive, or both), or, the mutations affect a subset of cells
or tissues affected by the stronger genes. In these cases, how the genes fit into
or onto the core pathway is much less clear. Nevertheless, certain themes of
organization have emerged from the genetics. Combined with molecular
knowledge gained from cloning and expressing the genes, certain mecha-
nisms that seem particular to developmental timing have emerged.
Among the first mutant animals with heterochronic phenotypes were
some that resembled the
lin-4
loss-of-function mutants, but were dominant
in their genetic behavior (
Ambros & Horvitz, 1987
). These are gain-of-
function alleles of
lin-14
in which portions of the 3
0
untranslated region
(UTR) of the gene's mRNA are deleted, leaving the open reading frame
intact (
Wightman, Burglin, Gatto, Arasu, & Ruvkun, 1991
). These alleles,
together with analysis of
lin-4
, were key to making the most significant dis-
covery in the pathway: microRNAs.
