EGG LAYING BEHAVIOR IN C. ELEGANS SOCIAL VS. SOLITARY STRAINS

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C. M. Loer^a, M. Maxwell^a, K. Kent^a & M. Ailion^b
aDept. of Biology, Univ. of San Diego; ^bInst. of Neuroscience, Univ. of Oregon

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Some wild isolates of C. elegans feed together ("social" strains) whereas "solitary" strains - including the standard wildtype N2 - do not^1,2. A single amino acid difference in the NPR-1 neuropeptide receptor mediates this profound behavioral difference; reduction or loss-of-function results in social behavior ². Social strains in the lab not only clump together, but also "border," spending most of their time in the thicker edge of a bacterial lawn. Solitary strains engage in much less bordering. Bordering is at least partly a consequence of social strains' preference for the lower oxygen concentration of the lawn edge³. We are currently investigating whether social strains display other behaviors in association with clumping and bordering. In particular, we predicted that social strains might display specific egg laying behaviors that would be advantageous in association with social behaviors. The clumping behavior of social worms results in high local density of conspecifics, which likely results in intense competition for food between individual worms. We investigated whether social adults might compensate by laying eggs away from the border of the lawn where the worms aggregate. We allowed individual young adult worms to lay eggs for 4 hours, and then examined the spatial distribution of eggs. Under 21% O₂, both wild isolate social strains (RC301, CB4856) and npr-1 mutants with an N2 background (DA650, IM222) lay >90% of eggs at the border whereas N2 lays 48% of eggs at the border. Therefore, social strains do not disperse eggs, but lay them at the border, where they spend most of their time. Low O₂ (7%) reduces the percentage of border eggs in social strains to that of N2 (53%). We have also confirmed that wild social strains lay eggs at a earlier stage of development, and retain fewer eggs in the uterus, whereas npr-1 mutants with an N2 genetic background do not⁴. For example, 92% of eggs laid by the Hawaiian social strain CB4856 are at early gastrulation or younger versus 8% for N2. As a young adult, CB4856 averages about 5 eggs in the uterus whereas N2 averages about 18 eggs. The mutant npr-1(ad609) is like N2, laying only 4% early eggs and carrying on average 13 eggs in the uterus. Preliminary genetics suggests that the early egg laying trait is semi-dominant and located on LG X. How the suite of behavioral traits associated with social behavior affect individual reproductive success and population dynamics warrants further investigation.
Refs: ¹Hodgkin & Doniach (1997) Genetics 146:149. ²de Bono & Bargmann (1998) Cell 94:679. ³Gray et al. (2004) Nature 430:317. ⁴Ailion & Thomas (2001) WBG 17(1):39.