Worm Breeder's Gazette 17(1): 39 (October 1, 2001)

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.

Natural variation in egg-laying behavior

Michael Ailion, James H. Thomas

Molecular and Cellular Biology Program of the University of Washington and Fred Hutchinson Cancer Research Center, and Department of Genetics, University of Washington, Seattle, Washington 98195, USA

N2 wild-type worms lay most of their eggs at the mid-to-late gastrulation stage of embryogenesis. N2 animals typically carry between 15 and 20 eggs in the first day of adulthood when grown at 20. However, this pattern of egg-laying is not conserved among all C. elegans natural isolates. We examined the egg-laying behavior of 20 other C. elegans natural isolates by assaying the stage of eggs laid and the number of eggs carried in the gonad 24 hours after the L4 stage. By these assays, the strains can be divided into two broad groups. Five other strains laid eggs at a similar stage as N2: CB3191, CB4507, CB4555, TR388 and TR389. Fifteen strains laid eggs at earlier stages than N2 and retained fewer eggs in their gonad: AB1, AB3, CB4852-CB4858, CB4932, KR314, LSJ1, PB303, PB306, and RC301. CB4856, for example, laid 100% of its eggs earlier than mid-late gastrulation, compared to 6% for N2. Furthermore, 48% of the eggs laid by CB4856 had four cells or fewer; N2 never laid eggs at such early stages. CB4856 adults typically carried four or five eggs in their gonad.

 

The fifteen strains that lay eggs at early stages all show clumping behavior, whereas the six strains that lay eggs at later stages are not clumpy. Clumping behavior in these strains is determined by a polymorphism in the npr-1 gene. We looked at the egg-laying behavior of npr-1(ad609) animals; npr-1(ad609) is a mutation in the N2 background that causes clumping by this strain. However, npr-1(ad609) animals do not lay early eggs (4% eggs laid before mid-late gastrulation). Thus, it is likely that clumping and egg-laying are partially independent, though it remains possible that the npr-1(g320) allele found in the wild strains leads to early egg-laying.

 

We began a genetic investigation of early egg-laying in strain CB4856. CB4856 F1 hybrids with N2 laid 33% of their eggs before mid-late gastrulation, and only 2% with four cells or fewer. This suggests that early egg-laying is a semi-dominant phenotype. Preliminary mapping of the early egg-laying phenotype of CB4856 suggested weak linkage to lon-2 on the X chromosome. npr-1 is tightly linked to lon-2.

 

Many questions are raised by these observations. Is early egg-laying in different natural strains determined by the same genetic polymorphism(s)? Why is there a strong correlation between early egg-laying and clumping? And perhaps the most interesting question: why is there variation in the stage of eggs laid?