Male Worm Brains are Wired to Choose Sex Over Food
When faced with a choice between food and sex, male nematodes suppress their ability to locate food in order to focus on finding mates. The findings, published in Current Biology last week, suggests that subtle changes in the brain’s circuitry could dictate differences between male and female behavior.
The microscopic, soil-dwelling roundworm called Caenorhabditis elegans is a model organism for understanding fundamental biological mechanisms, especially those involving the nervous system. These nematodes come in two sexes: males and hermaphrodites. While the latter, so-called “modified females” are able to self-fertilize, they’re also mating partners for males. According to previous observations, males and hermaphrodites act differently when exposed to food. Hermaphrodites tend to stay by the food source, while males will wander around, presumably in search of a mate.
A team led by Douglas Portman from the University of Rochester conducted a series of experiments with C. elegans, focusing on the activity of a single pair of neurons called AWA. These neurons control smell, one of the critical senses that determine how they understand and navigate the environment.
The researchers discovered that sexual identity regulates the expression of a receptor called ODR-10, which binds to the chemical scent given off by food. Hermaphrodites produce more ODR-10 receptors, making them especially sensitive (and attracted) to the presence of food. Fewer of these receptors are active in males, so their ability (and likely their desire) to find food is suppressed. When starved, however, males produce much higher receptor levels, allowing them to focus on finding food, at least for the moment. Pictured here: a male on top, a hermaphrodite on the bottom.
To confirm the role of genetic differences between these two sexes on behavior, the team observed activity of genetically engineered C. elegans in a petri-dish. First, they modified some males to overexpress the ODR-10 receptor, making them more sensitive to food smells. They were placed at their own individual food sources near the periphery of the dish, while the hermaphrodites stayed with the food source at the center. As an additional obstacle, a ring of food separated the males from the hermaphrodites.
Normal males left their own food source and made their way to the center of the dish, where they mated with hermaphrodites. The engineered males were less successful at finding a mate — likely because they were more interested in eating. The genetic profile of the resulting offspring revealed that normal males out-produced genetically engineered males, 10 to one. In a second experiment, hermaphrodites with suppressed ODR-10 receptors acted like normal males and abandoned their food source.
“While we know that human behavior is influenced by numerous factors, including cultural and social norms, these findings point to basic biological mechanisms that may not only help explain some differences in behavior between males and females,” Portman says in a news release, “but why different sexes may be more susceptible to certain neurological disorders.”