Genetically modified organisms (GMOs) possess novel and, often, heritable genotypes that typically would not occur without human intervention. Transgenic modifications can alter the fitness of a species by enhancing or diminishing its reproductive success and survivability. These consequences can be managed in a laboratory setting, but if GMOs are introduced into uncontrolled environments, they might pose a threat to populations and ecosystems of their unmodified conspecifics. Some GMOs have already been released into uncontrolled environments, and others are closely regulated but still at risk of escape or accidental release. It is therefore important to understand how the fitness of GMOs influences their evolutionary outcomes when introduced to a natural population.

Researchers at Purdue University (West Lafayette, IN) simulated this scenario in experimental populations using wild-type and transgenic populations of zebrafish (Evolution 69, 1143–1157; 2015). By intermating transgenic and wild-type zebrafish over 11 generations, the researchers produced a transgenic population that was similar to the wild-type population in its genetic background and fitness components, except that it fluoresced red and had lower mating success.

Mating success can be influenced by multiple factors that include intersexual mate choice and intrasexual mate competition. In this case, red fluorescence seems to appeal to female zebrafish but behavioral observations found that wild-type males usually outcompeted the transgenic males by behaving more aggressively toward both sexes.

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The researchers also carried out a long-term study of 18 populations for up to 15 generations to understand how the populations would evolve after the introduction of the red fluorescence transgene. Transgenic and wild-type zebrafish had similar mortality, but by the 15th generation all but one population had lost the transgene. “The females didn't get to choose,” said author William Muir in a press release. “The wild-type males drove away the reds and got all the mates. That's what drove the transgene to extinction.”

These findings suggest that the transgenic trait of red fluorescence would not persist in a natural population. To accompany these findings, Muir and his peers applied a demographic model that simulates evolution within a population based on estimations of fitness. This model closely predicted the decline and extinction of the transgenic trait, indicating that it might be a valuable tool for assessing the risk of genetic pollution from uncontrolled GMOs. Muir emphasized that the model does not make any specific recommendations on whether to release GMOs. “It simply says what would likely happen if we did.”