Worm Breeder's Gazette 10(1): 109

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

lin-29 Activity: A Tight Threshold May Control the Switch from Larval to Adult Seam-Cell Fate

A. Papp and V. Ambros

lin-29 is considered a retarded heterochronic mutant because its L4 (
and later) seam cells appear to assume an L3 fate.  In lin-29 mutants, 
development occurs normally until just prior to the L4 molt, at which 
point the seam cells, which normally differentiate, continue to divide.
Concomitantly, supernumerary molts occur, and larval cuticles are 
synthesized rather than an adult cuticle.  Thus, animals go through an 
'L5' stage, and often an 'L6.'  The stage- and tissue-specificity of 
the defects caused by apparent lin-29 null alleles suggests that lin-
29 is specifically required for cessation of molting, cessation of 
seam cell division and the switch from larval to adult cuticle 
In addition to this role in wild-type development, epistasis studies 
between mutations of lin-29, and the heterochronic genes lin-14 and 
lin-28, show that lin-29 function is required for the expression of 
adult seam cell fates in precocious and retarded development, and 
suggest that these genes act via lin-29 to specify the normal timing 
of the switch from larval to adult hypodermal cell fate.  Unlike the 
phenotype of other heterochronic mutations, lin-29 defects are not 
suppressed by development from dauer larvae.  Thus, the larval-adult 
switch appears to require lin-29 during both non-dauer and post-dauer 
To further elucidate the nature of lin-29's action in the larval-
adult hypodermal switch, we have performed a screen for mutations that 
either revert the lin-29 phenotype to wild-type, or 'reverse' it, 
causing an epistatic precocious adult cuticle formation.  Since we 
were interested in isolating both revertants and 'reversants,' the 
screen was designed to detect the formation of adult cuticle at any 
stage.  The phenotypic expression of rol-1(e91), an adult-specific 
roller, can be regulated by heterochronic genes.  Precocious mutants 
begin to roll at the first stage in which they produce adult cuticle, 
while the lin-29 utant never rolls because 
lin-29 mutants fail to make adult cuticle.
Among approximately 25,000 progeny of EMS mutagenized lin-29(n546)
rol-1(e91) hermaphrodites, nine independent revertants were isolated 
containing suppressor mutations.  They are all recessive, autosomal, 
unlinked to lin-29, and together define four complementation groups: 
mab-1, mab-13, and mab-14 (see Hodgkin, Papp, and 
Ambros, this issue).  They are allele specific to lin-29(n546) in that 
they do not suppress the lin-29 phenotype of the 4 other existing lin-
29 alleles, even when these alleles are in trans to n546.
Since lin-29 is a recessive mutation, one might expect that a single 
copy of lin-29(n546) in a homozygous suppressor background would be 
sufficient to alleviate or reduce lin-29 defects, even if the other 
allele was lin-29 null.  Thus, the fact that animals homozygous for 
suppressor, and heterozygous for n546 and other lin-29 alleles, are 
not suppressed presents a curious situation.  Either all of the other 
alleles produce some neomorphic product which poisons n546 suppression,
or some critical level of lin-29 activity is required for the wild-
type phenotype.  Clearly a single wild-type lin-29 allele is adequate, 
but perhaps a single suppressed n546 is not.  To test this possibility,
we put n546 over mnDF87, a lethal deficiency which covers lin-29, to 
see if this combination is suppressible.  These heterozygotes have 
much lower viability than n546 homozygotes, but we can say that no 
suppressed heterozygotes have been seen so far.  This implies that one 
suppressed n546 allele produces some low level of lin-29 activity 
insufficient for the larval-adult switch, but that two suppressed 
copies are sufficient for complete switching.  In each genetic 
background described above, animals have either the full lin-29 
phenotype, or are fully wild-type for the larval-adult switch.  This 
is in contrast to certain lin-14 mutations which produce animals with 
patches of both larval and adult cuticle (showing that seam cells have 
the ability to differentiate independently), and to the incomplete and 
variable suppression of tra-2 by members of the same set of mab loci (
J.  Hodgkin, personal communication).  It is consistent with a model 
in which a very quantized lin-29 activity is required for the switch 
in cell fate.