Though skin cancer is deadly to male swordtail fish, it also has one perk: black melanoma splotches help lure females, suggesting that the melanoma gene is conserved for its beneficial role in sexual selection. The study, published inProceedings of the National Academy of Sciences, marks the first time scientists have found a cancer gene linked to a pigment pattern that functions to increase mating success in animals.
In the study, the researchers placed a female swordtail fish in the middle of a tank with two partitions. They positioned a male with the faux pattern from which melanomas form on one side, and a male without the pattern on the other. After releasing the female from an opaque tube into the tank’s center chamber, the scientists observed how much time she spent looking at each male during an eight-minute period.
This suggests that the swordtails keep the prevalence of the cancer gene in check by some of the females rejecting the melanoma males. The scientists speculate that this is because of a higher ratio of both males and females with the gene for skin cancer, which increases the likelihood of too many offspring inheriting the gene and dying off.
“Melanoma formation cuts the reproductive life cycle in half,” Fernandez said. “It has a huge cost for males.” But during the few months when the male is sexually mature and healthy, he also can produce a lot of offspring, he noted.
The swordtail melanoma has been studied since the 1920s, and scientists previously believed that fish developed the cancer only in captivity. Fernandez now hopes to conduct further studies into whether stronger exposure to the sun’s UV rays might be driving more instances of skin cancer in the wild.