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 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 synthesis. 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 development. 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.