Worm Breeder's Gazette 7(1): 45

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More on Heterochronic Mutants

V. Ambros, B. Horvitz

lin-14.  In the last newsletter (Vol.  6, No.  1), we described 
heterochronic mutants that define the gene lin-14.  Gene dosage 
experiments have indicated that the dominant lin-14 alleles, which 
cause reiteration of early larval characteristics, are hypermorphic (
cause elevated levels of wild-type lin-14 function); the recessive 
alleles, which cause the deletion of early larval developmental events 
and precocious expression of later events, are hypomorphic or amorphic 
(cause reduced or eliminated lin-14 function).  These observations 
support the hypothesis that the level of lin-14 function determines 
the cell fates of certain cells: high lin-14 levels define events that 
occur early during wild-type postembryonic development, and low levels 
define later events.  Although most recessive lin-14 alleles are 
pleiotropic and fail to complement each other for all defects, several 
of these defects are independently mutable: 16 alleles delete L1-
specific cell divisions of the lateral hypodermis, with precocious 
expression of subsequent stages; three alleles have normal L1 
development, but L2-specific divisions are deleted and L3 and L4 
divisions are precocious; one mutant, isolated by Meredith Kusch, has 
normal L1, L2 and L3 development, but the lateral hypodermal seam 
cells form adult alae one stage earlier than in wild type.  This 
independent mutability of the timing of adult alae formation and of 
the lateral hypodermal cell lineage pattern is further exemplified by 
another lin-14 recessive mutant in which the cell lineages are 
neotenic, reiterating L2 divisions, but the seam cells cease division 
and form adult alae precociously (during the late third larval stage). 
Thus, it appears that lin-14 may have at least three activities that 
independently control specific cell fates within given lineages: 
whether L1 or L2 division patterns occur in the first larval stage; 
whether L2 or L3 division patterns occur during the second stage; and 
whether adult alae are formed late during the third or fourth stage.  
In temperature-sensitive recessive mutants, the ts periods for 
determining which division pattern is generated by each of the blast 
cells in the first and second stage are short intervals just prior to 
the division of each cell.  However, the ts period for the timing of 
adult alae formation is during the first larval stage, at least two 
cell divisions before that characteristic is expressed.
New Heterochronic Genes.  Based upon the phenotypic characteristics 
of lin-14 alleles (e.g.  abnormal vulvae, lack of lateral alae), new 
heterochronic mutants were sought by examining egg-laying defective 
strains isolated in our laboratory.  Four new mutants have been 
identified.  There are three alleles of lin(n333) II.  These neotenic 
mutants undergo supernumerary molts and seem to reiterate cell 
division patterns normally specific for the L3.  Vulval morphogenesis 
is abnormal, and seam cells never form adult alae.  The one allele of 
lin(n719) I was isolated by Chip Ferguson.  This recessive mutant has 
the same precocious phenotype as the lin-14 mutants that delete L2-
specific cell divisions.  The n719 mutation suppresses lin-14 and lin-
4 neotenic mutants and can enhance lin-14 precocious mutants.  This 
mutation may define a gene that interacts with lin-4 and lin-14 in 
controlling the temporal pattern of expression of specific cell fates.
Dauer Recovery Suppresses Some Heterochronic Mutations.  During 
routine handling of heterochronic mutant strains, we have noticed that 
dauer larvae from genotypically mutant strains often recover to become 
phenotypically normal, i.e.  normal in the timing of vulval 
morphogenesis and of adult alae formation, as well as, in the case of 
certain neotenic strains, in the number of larval molts.  Such 
phenotypic rescue occurs for both neotenic and precocious mutants of 
lin-14 and for lin(n719) I and has allowed us to mate strains 
otherwise too severely abnormal in vulval development.  This 
phenomenon suggests that there exists a system that controls the 
timing of developmental events during dauer recovery and that this 
system can bypass the heterochrony induced by the mutations we have 
defined so far.