Researchers have identified a pair of male-specific neurones in the brain of male nematodes that cause them to remember and seek sex. The findings are published in the journal Nature.
What makes the two sexes different in terms of preferences and behaviour? What are the inner workings in the brain that accommodate for the contrast? The findings of the new study might be helpful to answer these questions as they demonstrate a direct association between differing male and female behaviours and brain development and structure. The researchers also believe that brain circuits pertaining to learning in the two sexes might be revealed.
The newly-discovered male-specific neurones are allegedly needed for sex-based differences in learning. This suggests that the differences in cognition between the two sexes might have genetic implications.
“Areas of the brain involved in learning display sex differences in many animals, including humans, but how these differences directly affect behaviour is not clear,” says senior author Dr Arantza Barrios.
“We’ve shown how genetic and developmental differences between the two sexes lead to structural changes in the brain of male worms during sexual maturation. These changes make male brains work differently, allowing males to remember previous sexual encounters and prioritise sex in future situations.”
The pair of neurones have been named “mystery cells of the male” (MCMs). They change a brain circuit common to both sexes, thereby creating differences in behaviour. They come from glial cells which are companion and support cells of neurones; the MCMs are only made from those glial cells having male chromosomes. The genetic sex of these glial cells ((and not the sex of the animal, or hormones) will, therefore, determine whether the neurones are born or not.
“Our findings suggest that differences in learning and perception depend not just on the sex of the animal but also on the sex of the individual neural progenitor cells. This means that different aspects of an animal’s behavior may well develop independently of each other in some circumstances, instead of through the co-ordinated action of hormones. Of course not all behavioural differences are genetically hardwired, environment can also play an important role,” says Dr Barrios.
The sway the MCMs have on the behaviour of the worms was analysed via classic conditioning behavioural assays whereby the worms are made to learn to associate aversive or pleasant experiences (such as starvation or mates) with other stimuli (salt) so that they modify their responses. It was found that worms previously starved in the presence of salt would learn to shift from areas with high salt concentration indicating that they had learned that salt was a sign of the absence of food. On the other hand, when males were starved in environments with salt and mates, and then placed in a new environment, they would move to regions with high salt concentration. This implies that the association of salt with sex was stronger than the link between salt and absence of food. This change in behaviour was only noted in male worms, and not in hermaphrodites. It also did not occur in males who had their MCM neurones removed surgically.
As the researchers carried further experiments to reveal more features of the MCMs, they found that while the MCMs interacted with neurones found in both males and females, they would modify the circuits so that the processing of the information is different.
“Only in C. elegans, at the moment, is it possible to identify every synapse in a neural circuit in the way we have done here. Though the work is carried out in a small worm, it nevertheless gives us a perspective that helps us appreciate and possibly understand the variety of human sexuality, sexual orientation, and gender identification,” says co-author Professor Scott Emmons.
These findings might have important implications in repairing damaged brain areas as they can possibly lead scientists to find how glial cells make neurones. The scientists also hope to learn more of the specific characteristics of brain circuits monitoring the acquisition and retention of information.