Learning by interaction with the other sex
Hermaphrodites in C. elegans are attracted to salts when they are cultivated with plenty of food and salt. However, they change the behavior to avoidance of salt after experiencing starvation under the presence of salt. This behavior is thought to be beneficial for searching food by using salt as a cue. Similar food-dependent behavioral plasticity is also known for other senses such as odor and temperature. On the other hand, we found that C. elegans males change the behavior (attraction or avoidance of salt) depending on the presence or absence of the other sex, hermaphrodites. Males were attracted to salts when they were cultivated with plenty of food and salt irrespective of presence or absence of hermaphrodites. When they were starved with salt in the absence of hermaphrodites, they changed the behavior to avoidance of salt. These male behaviors are the same as those of hermaphrodites. However, after being starved in the presence of salt and hermaphrodites, the males showed attraction to salt. On the other hand, behavior of hermaphrodites did not change by the presence of males. Therefore the behavioral plasticity (sexual learning) which is affected by the presence of the other sex is specific for males. Sexual learning in C. elegans males is thought to be beneficial for transmitting the genes of males to the next generation by proritizing hermaphrodite search over food search.
By studying the mechanisms for sexual learning, the characteristics of the signals from the hermaphrodites necessary for the learning was revealed. C. elegans is known to use pheromones for inter-individual communications. It was found that sexual learning did not occur by starving males in the presence of salt and hermaphrodites that cannot procude pheromones (mutants of the daf-22 gene which is required for pheromone synthesis). Nor did sexual learning occur when males with defective copulatory organs (mutants of the mab-5 gene which is required for development of copulatory organs) were starved with hermaphrodites. These results indicated that for the sexual learning to occur, pheromone from hermaphrodites and another unknown signal from the hermaphrodites that is sensed by copulatory organ of the male are necessary.