Before you fob off nematodes as boring, the humble-looking roundworm has given us plenty of fascinating science to pore over. Just the other day, for example, IFLScience reported the discovery of variations in just one gene that makes males more attractive to others, prompting homosexual behavior. Now, scientists have found two neurons, only present in the brains of males, which not only change male brain circuitry, but also help them remember and prioritize sex (how could they possibly forget that?).
This find is interesting, because it offers us an insight into the neural basis of sex-based differences in behavior. We know that males and females behave differently, but this is not restricted to just courtship and mating: the sexes also display dimorphism in processes such as learning, which is reflected in observable differences in the brain regions responsible. How such sex-specific differences in the nervous system influence behavior, though, was unclear.
To find out more, researchers from University College London designed some neat experiments involving the model organism Caenorhabditis elegans, or the nematode. These worms have a pretty fascinating sex life as it stands. Rather than having male and female, members of the species are either hermaphrodites, basically modified females that can self-fertilize with their own sperm, or males that can only reproduce if they cop off with a hermaphrodite. Yep, the males are kinda useless here.
Consequently, the males exhibit an abundance of reproductive behaviors that aren’t seen in the hermaphrodites, and those are down to nervous system differences that develop as the organism matures sexually. The researchers were able to pinpoint some of these variations by fluorescently tagging and monitoring cells of the nervous system in the males.
This led them to a pair of previously unidentified brain cells, which the team has unimaginatively coined “mystery cells of the male,” or MCMs. These neurons, it turns out, have their origins in a specialized type of cell called glia, also found in humans, which serve an array of active and support roles in the brain.
To find out how these may impart behavioral differences, the team subjected nematodes to classic conditioning experiments, in which an animal learns to associate a positive or negative experience with another stimulus, thus leading to behavioral modification. This involved starving the poor worms in the presence of salt, so that eventually they learnt to associate a lack of food with salt and thus moved away from it when placed in a novel environment containing varying salt levels. Both of the sexes were found to be able to learn this way.
But what they did next highlighted some differences in learning between the sexes. They repeated the experiment, but this time they kept a mate around during the conditioning. And when the males were subsequently put in a novel environment with varying amounts of salt, unexpectedly they didn’t run away screaming from the salty areas; in fact, they actively moved towards them, seeking out sex. This indicates that the association of salt with sex was stronger than that of salt and starvation. When it came to the hermaphrodites, however, the same patterns of behavior were not observed, nor in males who had their MCMs experimentally removed.
Diving a little deeper, with the assistance of electron microscopy, scientists at Albert Einstein College of Medicine found that MCMs formed connections with neurons found in both males and hermaphrodites, but their presence in the former leads to an alteration in brain circuitry that changes the way data is processed. These findings have been published in Nature.
Obviously, we can’t generalize these findings to humans, but it’s an interesting to start to better understanding the neural basis that underpins some sex-specific behavioral differences.