Guys who think high visibility is the path to romantic success might want to reconsider, at least if they take the doubtful path of getting dating advice from fish. Male zebrafish with a gene that makes them appealing to females, but attracts unwelcome attention from rivals, come off second best.
Glofish, available in six colors, are the only transgenic animals legally for sale to the public in the United States. Purdue University's Professor William Muir thought these modified versions of the model species Danio rerio would make a good test of the relative importance of mate choice and competition with other males.
Together, these make up sexual selection, one of the drivers of evolution. "Mating success is actually a stronger force of evolution than survival of the fittest," says Muir. "If an organism can't get a mate, it can't pass its genes on. In terms of evolution, whether it survives or not doesn't matter." Yet, as he notes in Evolution, "For many species, the choosy sex (most often females) prefers mates that are most successful in mate competition; thus, the two components of sexual selection reinforce each other." However, in some species, the characteristics females prefer are “unrelated to, or even in opposition to mate competition.”
Muir sought to tease apart the importance of factors within sexual selection using Starfire Red males, whose color comes from a fluorescence gene from sea anemones. Other studies have established that female zebrafish, transgenic or not, like gingers, choosing red Glofish males over boring normal mates when given the chance, although Muir says the reasons are unknown. Nevertheless, the red fish seem to father fewer offspring.
Red glofish and their wild type equivalents. Karol Głąb via Wikimedia commons.
Unfortunately, being lit up like a Christmas tree also made the transgenic zebrafish targets for other males, who aggressively chased them away.
Muir tested 18 populations of fish, each starting with a mix of the transgenic and wild-type males and females, and observed them over 15 generations—which included more than 18,500 individuals. Muir notes that genetic modification often decreased an organism's survival, saying, "Natural selection has had billions of years to maximize an organism's fitness for its environment. Changing its genetics in any way almost always makes an organism less fit for the wild. You've 'detuned' it."
At least one predator avoids the transgenic fish, but in the wild many more forces may come into play. However, in their tanks the Glofish survived as well as the wild-type zebrafish—the males just couldn't pass on their genes. "The females didn't get to choose," Muir said. "The wild-type males drove away the reds and got all the mates. That's what drove the transgene to extinction."
The wild-type males were three times as likely to chase a red male away from a potential mate than the other way around, although the authors admit they are uncertain why. Since the males were much less choosy, the red females bred as much as the wild-type, but this was not enough to save the gene. By the end of the experiment, only one of the 18 tanks had any red fish.
The rule that male-male competition outweighs mate choice may not be universal across species. However, Muir says the work is important for modeling the effects of what could happen if transgenic species escape into the wild. Specifically, the fluorescent fish are unlikely to prove invasive, even if any reach the tropical waters in which zebrafish thrive.
Perhaps more significantly, the study marks a first in testing the contribution of different evolutionary processes. “I've lectured on evolution for 25 years and never found a study that linked the mechanisms of evolution with the pattern of evolutionary outcomes," said co-author Professor emeritus Richard Howard. "This study puts the whole story together."