In Dr. Meital Oren-Suisa's lab at the Weizmann Institute, they discovered that male worms learn less well from experience compared to females, and uncovered a neural receptor that is responsible for the difference between the sexes and is also conserved in humans.
In human society, men are often seen as risk-takers and women as more cautious. The premise of evolutionary psychology is that these differences evolved to meet the unique threats and needs of each species. While these generalizations are too binary and simplistic to accurately describe complex and multidimensional human behavior, in animals, sharp and clear differences between females and males are often found, even in simple creatures like worms. In a new study,His findings were published in the scientific journal Nature Communications., Weizmann Institute of Science scientists have shown that male worms learn less well from experience and have difficulty avoiding taking risks – even at the cost of their lives – and that this ability improves if they are given the opportunity to mate with females of the opposite sex. The scientists have also identified a protein, conserved in evolution from worms to humans, that is likely responsible for the difference in learning ability between the sexes.
Tiny worms of the type C. elegans They are a good model for studying the most basic genetic differences between the sexes, since their sex is determined by genes, without the influence of hormones and other factors. The worms are divided into two sexes, males and hermaphrodites. The latter are females that also produce male gametes and are capable of fertilizing themselves or mating with a male. The tiny worms have a simple nervous system that includes only a few hundred nerve cells, and they are the only organism so far in which all the neural connections in the brain have been mapped in both sexes. Early in life, there is no difference in the neural connections between the sexes, and these only emerge after sexual maturation. In Dr. Mittal Oren-Suisa The Department of Neuroscience at the Weizmann Institute of Science is taking advantage of the opportunity offered by the tiny worms to reveal fundamental differences between species in the brain and nervous system.
In the new study, the scientists focused on differences between species in learning processes. The tiny worms feed on bacteria, and unfortunately, they are particularly attracted to the smell of a disease-causing bacterium that is harmful to them if eaten. Will they learn to avoid it? The research team, led by doctoral student Sonu Pedikil Kurian from Dr. Oren-Suisa's group, began the experiment with a "training" phase during which worms of both sexes were grown separately on the harmful bacteria medium. After the training phase, the worms were transferred to a "test" dish where they could choose whether to eat from the toxic bacteria medium or from a less enticing medium of a nutritious, non-disease-causing bacterium. While the females quickly learned to associate the smell of the harmful bacteria with the disease it causes and chose to eat from the other medium, most of the males did not learn and continued to be attracted to the harmful bacteria, even though they became just as sick - the bacteria settled in their digestive tract, secreted toxins and triggered an immune response. When the scientists waited an unusually long time, a few of the males were finally able to learn to avoid the harmful bacteria, but this happened only after they had been infected, sickened, and many of them died.
These findings led the scientists to look for differences in nervous system activity between the sexes. In the worm, there are two types of nerve cells involved in the sensation of certain odors – one type is responsible for inducing a feeling of attraction and the other a feeling of disgust. When these nerve cells are activated, they fill with calcium ions, and because the tiny worms are transparent, these ions can be labeled and the activity of the nerve cells can be monitored. The scientists thus identified that in females, and only in them, the activity of the nerve cell responsible for the feeling of disgust increased significantly following the disease caused by the tempting bacteria; this is likely how they were conditioned to subsequently choose a different food source.
Learn from the gut
In the next stage of the study, the scientists tried to understand the source of the difference between the sexes at the genetic and molecular level. "We genetically engineered a female worm with a male nervous system and saw that its learning ability dropped," describes Pedikil Kurian. "On the other hand, for males to start associating the disease in the digestive system with the smell of the bacteria, it was not enough to switch the sex of their nervous system; we also had to switch the sex of the digestive system. This finding and others led us to hypothesize that there is communication between the digestive system and the male nervous system, for example through neuropeptides – short proteins that bind to and affect nerve cells – and that this communication leads to the suppression of their ability to learn."
With the help of the Crown Genomics Institute at the Weizmann Institute of Science, the research group examined how gene expression changed in males that survived exposure to both types of bacteria – that is, those that learned to avoid danger – and found a decrease in the expression of a receptor called npr-5 in their brains. When the scientists created males without this receptor, they learned successfully – and when they restored it in sensory neurons only, the ability to learn was lost again. From this, they concluded that the receptor was responsible for suppressing sensory learning in males.
Learning from experience and developing a sense of aversion to danger are important for survival. So why is this ability suppressed in males? "It is known that male worms will abandon food to seek a mate, and it is possible that their drive to reproduce outweighs other evolutionary pressures, such as the need to avoid danger," speculates Dr. Oren-Suisa. "An interesting point that we discovered in this context is what happened when we allowed the male worms to mate during the 'training' phase with females of the opposite sex - we saw that their ability to learn from experience improved. In fact, we could say that the receptor we identified is responsible for males prioritizing reproduction over learning from experience in their decision-making system."
The receptor that the scientists identified in worms has a counterpart in mammals and humans. The receptor in mammals is activated by a neuropeptide known as NPY, which has previously been linked to feelings of stress, control of eating, and many other processes. "It was previously shown that mice have lower levels of NPY than rats, and it was hypothesized that this is why they are more sensitive to stress in response to danger," says Dr. Oren-Suisa. "This hypothesis fits well with our findings showing that in worms, aversion to danger is accompanied by a decrease in the expression of the receptor. Human diseases such as post-traumatic stress disorder and anxiety, which involve negative feelings toward what is perceived as dangerous factors, are more common in women, and although human behavior is much more complex, the study lays the foundation for understanding differences between the sexes even in complex creatures."
Dr. Rizwanul Haque and Dr. Assaf Gat from the Institute's Department of Neuroscience also participated in the study.
More of the topic in Hayadan:
Immune response of worms
Synonyms:
Immune resistance, acquired immunity, immunological memory, response to pathogens, immune defense, differential response between species, immune system response, immune learning
