Mate searching in Caenorhabditis elegans: a genetic model for sex drive in a simple invertebrate

Abstract

Much of animal behavior is regulated to accomplish goals necessary for survival and reproduction. Little is known about the underlying motivational or drive states that are postulated to mediate such goal-directed behaviors. Here, we describe a mate-searching behavior of the Caenorhabditis elegans male that resembles the motivated behaviors of vertebrates. Adult C. elegans males, if isolated from mating partners, will leave the area of a food source and wander about their environment in an apparent search for a mate. When mating partners are present on the food source, males do not wander but remain with them. This behavior is sexually dimorphic for C. elegans and two additional male/hermaphrodite species studied; for these species, hermaphrodites leave food significantly slower than males. In contrast, for three male-female species examined, both males and females left food, in two cases with similar frequency, suggesting coordinate evolution of behavioral dimorphism with hermaphroditism. We use a quantitative behavioral assay to show that C. elegans male mate searching is regulated by signals from hermaphrodites and by physiological signals indicating nutritional and reproductive status. We identify genes in the serotonin, insulin, and sex determination pathways that affect the rate of mate searching. These genes may contribute to physiological and reproductive regulatory mechanisms. Our results establish C. elegans as a model genetic animal with a simple nervous system in which neural pathways leading to a motivated behavior may be genetically dissected.

Figure 1.

Wild-type males leave food. When C. elegans males are isolated on a food source away from mating partners, they eventually leave the food source and wander about their environment. These photographs show the tracks a male left on the agar surface of a leaving assay plate after being placed at time 0 in the center of a 0.9-cm-diameter bacterial lawn in the middle of a 9-cm-diameter agar plate. The position of the male on the 4 hr plate is indicated by the white arrow. When the male's track reaches a distance 3 cm from the edge of the food source (dashed white circle), the male is scored as a leaver (5 hr plate). The white rectangle is a raised area of plastic on the bottom of the Petri plate.

Figure 2.

Quantitative measurement of leaving behavior. To determine the probability of leaving food, a number of animals (typically 20) are scored for their time of leaving as shown in Figure 1. A plot is shown of representative data for two assays, one with wild-type C. elegans males and one with wild-type hermaphrodites. The log fraction of nonleavers versus time is plotted, and the data are fitted to the first-orderequation N(t)/N₀ = exp(-P_Lt), where N(t)/N₀ is the fraction of total worms that remain nonleavers at time t and P_L, the rate constant, represents the probability of leaving per unit time. A reasonable fit to a straight line indicates that to a first approximation, the probability of leaving per unit time is the same for each individual and is constant over the period of the assay.

Figure 3.

Representative leaving assays under various conditions. A, Male leaving is blocked by hermaphrodites and slowed by other males. Single wild-type males were tested in the presence of five wild-type hermaphrodites or paralyzed [unc-51(e364)] males. In the male plus hermaphrodite experiment, at the time indicated by the arrow, the hermaphrodites were removed from the plates. Leaving probability returned to that of males alone without measurable delay, indicating the hermaphrodite signal is on the hermaphrodite body or is labile in the environment. B, Leaving behavior in a male-female species, C. remanei. Virgin females leave food, but mated females do not. C, Food deprivation decreases leaving. Wild-type C. elegans males were kept on plates without food for the times shown, then tested for leaving behavior. D, Effect of the reproductive system on leaving behavior. Either the entire gonad was ablated by laser microsurgery (GA) or the germ line only was ablated (GL). E, Effect of signaling through the insulin pathway on male leaving behavior. A strong loss-of-function mutation in the insulin receptor daf-2(e1370ts) inhibits leaving. A null allele in the downstream transcription factor daf-16(mgDf50) increases leaving and is epistatic to daf-2(e1370ts). Animals were raised to the L4 larval stage at 20°, shifted to 25° to mature, and tested at 25°. F, fog-2(q71) hermaphrodites (functional females) leave food. Leaving is decreased by mating. Data are shown for three independent tests. Alone among all genotypes and conditions we tested, leaving data for fog-2 animals did not yield a straight line for unknown reasons. The q71 allele gave the strongest effects, but we also tested q70, q123, oz40, and oz170 with similar results (data not shown).