Neural Circuitry That Mediates Behavior Governing the Tradeoffs Between Survival and Reproduction in Caenorhabditis elegans

Abstract

In all outcrossing sexual species there is a mechanism that brings two parents together. For animals, this reproductive requirement may at times conflict with other needs, such as foraging for food. This tension has been studied using the tiny (1 mm) nematode worm, Caenorhabditis elegans. In a trade off between certainty of survival and possibility of reproduction, the C. elegans male will abandon a food patch lacking mates and explore its environment to find one where mates are present. A quantitative behavioral assay has been used to study the behavioral mechanism of mate searching and nutritional, sexual, and neurohormonal pathways that influence the underlying drive state. Taking advantage of the known connectivity of the C. elegans nervous system, neural pathways have been identified that influence the male's behavior in the presence of food with and without mates.

Fig. 1

A simple experiment (unpublished) showing that C. elegans males actively find their hermaphrodite mating partners. An agar plate is set up with two dots of bacterial food and a small group of hermaphrodites are placed on one of them, where they will remain and lay eggs produced with their self-sperm. A male is placed an equal distance away from both food patches. All worms are attracted to food odors and after 5 h the males will find the food patches. But half will wind up on the one without mates. This shows there is no long-range sex pheromone gradient across the plate that attracts males. Twenty-four hours later, however, all the males are with the hermaphrodites. The males that initially found the hermaphrodites remained with them (those plates show no worm tracks on the empty food patch), while those that went to the wrong patch left it. Their tracks show they wander about the agar surface until they hit the productive food patch, where they then remain. Sex-attractant pheromones that have been discovered since this experiment was first performed act at shorter range and are not detected by this protocol (White et al. 2007; Leighton and Sternberg 2016).

Fig. 2

The quantitative assay used to study C. elegans mate-searching behavior (Lipton et al. 2004). The “leaving” assay measures the rate at which males leave a food source. In a typical assay, a number of males (usually 20) are placed individually on a dot of bacteria in the center of an agar plate and their tracks on the agar surface (slight indentations) are examined at intervals. If a male has wandered a certain distance from the food source, it is scored as a “leaver.” When the fraction remaining is plotted on a semi-log plot, it yields a straight line, a “decay curve” that shows leaving occurs stochastically with a certain probability that is the same for all males and constant during the period of the assay. There is very little lag time; males appear to know very quickly (within 20 min) that there are no hermaphrodites present. The slope of the line yields a probability of leaving per hour, P_L. If hermaphrodites are present on the food, P_L = 0. P_L is measured under different conditions, for example after starvation of the males, after removal of the gonad or particular neurons with a laser microbeam, or in mutants, to study the mechanism and regulation of the behavior.

Fig. 3

The C. elegans nervous system. Left panels: fluorescent reporter genes reveal the tracts of the entire nervous system in the hermaphrodite (upper) and a sub-set of male-specific neurons in the male (lower). Right panels: Nomarski micrographs of the sexual specialization of the adult male tail, showing structures important for copulation. In the lower panel, four sets of sensilla are indicated. Each ray sensilla contains the sensory endings of two sensory neurons. These ray sensory neurons play a role in stimulating male mate-searching.