Worm Breeder's Gazette 15(5): 30 (February 1, 1999)
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.
Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94143-0448
The dauer larva is a morphologically specialized and developmentally arrested third-stage larva (1). The developmental choice to enter the dauer stage is temporally and environmentally controlled (2,3). Mutations in the daf-2 gene cause animals to arrest at the dauer stage even under favorable growth conditions (4). Mosaic analysis indicates that daf-2 functions cell non-autonomously within signaling cells to control entry into and exit from the dauer stage in target tissues (5). Mutations in the heterochronic genes can affect the temporal control of dauer larva formation: lin-4 mutants reiterate L1-specific fates in extragonadal tissues and are dauer-defective, since they are never competent to enter the dauer stage (3). In addition, double mutant analysis of lin-4 and other heterochronic mutants reveals that lin-4 is required for dauer larva morphogenesis (3). We examined the phenotype of lin-4; daf-2 double mutants to explore the genetic relationship between these two genes.
daf-2(e1370) animals have dark intestines and develop as dauers at 25°C but not at 20°C or 15°C. lin-4(e912) animals are long, vulvaless and do not have gonad migration defects. We used the dark intestine, long, and vulvaless phenotypes to construct the lin-4(e912); daf-2(e1370) double mutant.
We found that lin-4(e912); daf-2(e1370) animals grown at 25°C arrested development in a dauer-like state. Like dauers, these animals had dark intestines, had gonad arms which did not migrate dorsally, and were developmentally arrested. However, unlike dauers, lin-4(e912); daf-2(e1370) animals did not exhibit dauer alae nor radial body shrinkage, and had pharynxes that pumped sporadically and were not morphologically remodeled. This dauer-like phenotype is different from the "partial-dauer" phenotype observed in other mutant backgrounds, since partial dauers have dauer alae and are transient (6). Also, this dauer-like phenotype was never observed when we analyzed the phenotype of daf-2 genetic mosaics (5).
When lin-4(e912); daf-2(e1370) animals arrested at 25°C for 3 days were shifted to 15°C, they resumed development. They reached an adult body size, their gonad arms migrated dorsally and then turned, their germline proliferated, and the animals were fertile. This shows that the lin-4(e912); daf-2(e1370) animals at 25°C were in a state of arrest that was reversible, and that the arrested animals were competent to continue development.
We conclude that lin-4 activity is required for some but not all tissues to respond to the lack of daf-2 activity. Also, we conclude that lin-4 activity is not necessary for the dauer stage arrest observed in daf-2 mutants; and that this arrest does not require that all tissues actually adopt a dauer phenotype.