Worm Breeder's Gazette 11(3): 35
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.
The second-stage larva can enter the developmentally arrested dauer stage in response to starvation, higher growth temperatures, and dauer- inducing pheromone. To understand the genetic basis for this developmental switch, two kinds of mutant have been studied: (1) dauer- constitutive mutants that form dauer larvae even in abundant food, and (2) dauer-defective mutants that are unable to form dauer larvae when starved. A genetic pathway for dauer larva formation has been defined by mutations in more than 25 genes, and this pathway is thought to represent sensory reception of the environmental cues and transduction of that signal to target tissues to initiate morphogenesis of the dauer larva. Some of the genes positioned early in the pathway are involved in the development or function of sensory neurons. Dauer- defective mutations in the late steps of the pathway, like daf-12, may represent defects in the reception of the neural signal by target tissues, or the response of target cells to the signal. Cloning and sequencing genes in the genetic pathway is aimed at understanding the gene functions. Three Tc1 transposon-insertion mutants of daf-12, and three revertants representing transposon excisions, were isolated. When BglII digested genomic DNA was probed with Tc1, a 3.1 kb fragment was detected in the mutants but not in the revertants. DNA containing this fragment was eluted from the gel and cloned into a bluescript vector. DNA flanking the Tc1 insertion was then used to probe a Southern blot, which showed a 1.6 kb (Tc1) increase in fragment size in the mutants, when compared with parental or revertant DNA. This flanking DNA was sequenced, and two zinc finger domains were found in two exons. The amino acid sequence in these domains, rich in Cys, Lys, Phe, and Arg residues, is 43% to 52% identical to the DNA binding domains of human steroid hormone receptors. The overall encoded 148 amino acids in the sequenced region has 29% to 39% identity to members of this gene family. Hence, daf-12 may be a dauer-inducing hormone receptor and transcriptional activator that binds to control regions of downstream genes in response to a dauer-inducing steroid hormone, resulting in dauer larva morphogenesis. Northern analysis and cDNA library screening are in progress to determine the size and abundance of daf- 12 mRNA, and to obtain a complete DNA sequence.