Genome-engineered C. elegans strains with sex-transformed neurons and gut

In the animal kingdom, the difference between male and female is often written into the very cells that make up the body. In many complex animals, this distinction is managed by hormones released from reproductive organs, which travel through the bloodstream to instruct distant tissues on how to behave. However, a tiny roundworm called Caenorhabditis elegans takes a different approach. In this creature, every single cell decides its own sexual identity based on the chromosomes it carries, acting independently of its neighbors. This cell-by-cell decision-making allows scientists to ask a fascinating question: if you could change the sexual identity of just one specific group of cells, like the nerves or the gut, would the rest of the animal change to match, or would that tissue simply become a male or female island within a body of the opposite sex?

For decades, researchers have tried to answer this by using genetic tools to force specific cells to act like the opposite sex. They have done this by inserting extra copies of genes that control sexual development into the worm's DNA. While this method has worked, it has been messy. The extra genetic material often sits in unstable locations, can be silenced by the cell, or accidentally turns on in the wrong places, making it difficult to know exactly what is happening. To solve this, a team of researchers at Columbia University has developed a much cleaner way to rewrite the worm's genetic code. Instead of adding unstable extra copies, they used a precise molecular editing tool to insert the instructions for changing sex directly into the worm's own natural genes. They targeted two specific locations: one that is active only in the nervous system and another that is active only in the gut. By doing this, they created worms where the brain or the gut could be switched to a male or female identity without disturbing the rest of the body.

The researchers focused on two master switches that control sexual development in the worm. One switch, called TRA-1, promotes female traits, while another, FEM-3, promotes male traits. In a normal female worm, the TRA-1 switch is on, keeping the body female. In a male, it is turned off. The team engineered four new strains of worms. In two strains, they inserted the male-promoting gene into the gut and the nervous system of female worms, effectively turning those specific tissues male. In the other two strains, they inserted the female-promoting gene into the gut and nervous system of male worms, turning those tissues female. Because these changes were made directly into the worm's own genetic blueprint, the new traits were stable and appeared exactly where the researchers intended, without the noise or errors of previous methods.

When they looked at the results, the changes were striking and precise. In the female worms where the nervous system was switched to male, the cells in the brain lost their female markers and adopted male characteristics, while the gut remained perfectly female. Similarly, when the gut of a female worm was switched to male, only the intestinal cells changed, leaving the nervous system untouched. The reverse was also true for the male worms. When their nervous system was forced to become female, the brain cells adopted female traits, and when their gut was switched, the intestine became female. The researchers confirmed these changes by watching how specific genes turned on and off. They saw that the master female switch, TRA-1, disappeared from the tissues that had been turned male and appeared in the tissues that had been turned female, exactly as the genetic instructions predicted.

Beyond just looking at genes, the team checked if these internal changes affected how the worms behaved. They knew that male worms usually move faster and explore their environment more aggressively than females. When they tested the worms with male brains but female bodies, these animals moved with the speed and boldness of males. Conversely, when they tested the worms with female brains but male bodies, the animals slowed down and became more cautious, behaving like typical females. This proved that the sexual identity of the nervous system is the primary driver of these movement differences. The gut changes also had a clear effect. In female worms, the gut produces a nutrient-rich protein needed for making eggs. When the gut was switched to male, this production stopped. When the gut of a male worm was switched to female, it began producing this protein in large quantities, sometimes so much that it leaked into other parts of the body.

The study also revealed a subtle complexity in how these tissues talk to each other. In the male worms with female brains, the researchers noticed that the female switch appeared in the brain as expected, but it also showed up in the front part of the gut, even though the gut itself had not been genetically altered. This suggests that a female brain might send a signal that tells the gut to turn on its female switches, hinting at a non-cell-autonomous influence where the nervous system can guide the identity of other tissues. However, this signal did not affect the rest of the gut, showing that the influence is limited and specific.

These new worm strains offer a powerful and reliable tool for the scientific community. By providing a way to change the sex of specific tissues without the side effects of older methods, they allow researchers to study exactly how sexual identity shapes the function of the brain and the gut. The work confirms that sexual identity is not just a property of the whole animal but a feature that can be tuned in individual organs, and that these local changes have real, measurable consequences for how the animal moves and functions. The findings suggest that the sexual identity of the nervous system is a key controller of behavior, while the identity of the gut controls reproductive physiology, and that these systems can operate independently, yet still influence one another in specific ways.