I previously introduced the worm Ceanorabditis elegans (C. elegans) (“Une histoire de ver et d’ultrasons”), underlining the fact that every connexions and cells of its nervous system were known…Well, every statement in science is susceptible to change. A study published in Nature revealed the existence of two extra neurons in C. elegans. Interestingly, they are only observed in male nematode worms… Here starts the story.
C. elegans is a sexually dimorphic species with hermaphrodites and males. The first can self-fertilize whereas the second can only cross-fertilize hermaphrodites. There is a marked dimorphism of behaviours between hermaphrodites and males, both reproductive and non-reproductive. Only males perform sexual conditioning : they need to learn how to optimally locate mating partners. This is a form of associative learning whereby males use sodium chloride as a cue for hermaphrodite location. Both males and hermaphrodites normally avoid sodium chloride after associative conditioning with salt and starvation. However, males become attracted to sodium chloride after conditioning with salt and starvation if hermaphrodites are present during conditioning. This contrasting behaviour of male and hermaphrodite is linked to differences in brain development and structure in areas involved in higher order processing.
The authors of the study lead by Dr Arantza Barrios, from University College London, discovered a previously unknown pair of neurons in the head of males, which they have called MCMs (for mystery cells of the male). They proved that these two cells, which they identified as interneurons arising from glial cells, are specifically required for a male-specific switch in chemosensitory behaviour induced by sexual conditioning. “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.”
According to Arantza Barrios: “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.” It is tricky to extrapolate these results to other species. But it’s a foundation stone in building the understanding of these mechanisms.
Glia-derived neurons are required for sex-specific learning in C. elegans. Sammut M. et al, Nature 15 october 2015.