Sex can be fatal for male swordfish

Ohio University scientists have found that the skin cancer gene in swordfish plays a beneficial role in sexual selection.

They found that the splotches of black melanoma gene arise from attractive natural markings that lure female mates.

Andre Fernandez and Molly Morris from Ohio University studied three populations of female swordtails, tiny freshwater fish native to North and Central America.

They found that two of them preferred males whose tails were painted to resemble the skin cancer spots.

The researchers also studied specimens of swordtail fish with real melanomas, which confirmed that the cancer gene is switched on only in the tissue with the dark pigment.

This is the first time that 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 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.

The project is based on previous research in the Morris lab, which used the same tests to show that female swordtails are strongly attracted to males with dark vertical bars.

To avoid any bias the female might have for a particular side of the tank, Fernandez then switched the males.

Two days later, he conducted the trials again, this time changing which male received the painted skin cancer spot.

The researchers found that the female consistently chose the male with the dark pigmented marking in two of the three populations.

The study also suggested that the swordtail fish population also keeps the prevalence of the cancer gene in check. A third population of females in the study rejected the males painted with the pattern that can form melanomas.

The researchers believe that’s because the third group had 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.

Usually, swordtail fish live for 1.5 to 2 years in the wild and sexually mature at 4.5 months. The ones with the skin cancer gene can develop melanomas at about 7 months and die a few months later.

“Melanoma formation cuts the reproductive life cycle in half. It has a huge cost for males,” Fernandez said.

However, he noted that during the few months when the male is sexually mature and healthy, he also could produce a lot of offspring.

The study is published in the Proceedings of the National Academy of Sciences. (ANI)

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