When a species lives in two different types of habitats, individuals with traits best suited to each habitat will thrive and reproduce, naturally selecting offspring with those traits. But what about mobile aquatic species that live in a wide range of temperatures and latitudes? How do they maintain their genetic differences if individuals are free to mix and interbreed?

New research published in Science by Cornell and the University of Connecticut finds that chromosomal inversions, which occur when a piece of chromosome containing tens of thousands of genes breaks off, rotates 180 degrees, and reattaches to the same chromosome, play a central role in shaping these advantageous adaptations.

“Each chromosomal inversion links together a large set of genes, effectively forming a genetic switch with two states (inverted or non-inverted). The surprising thing here is that multiple ‘switches’ can combine to generate smooth, continuous variation, not just on-off differences,” says the Cornell associate professor. Nina Overgaard Therkildsen.

The study focused on Atlantic silversides, a small species of fish that lives along the Atlantic coast of the United States. The fish has long been a model for scientists seeking to understand how natural selection and adaptation work in the ocean. Therkildsen lab collaborated with Hannes Baumannassociate professor at UConn, and David Conoverprofessor emeritus at the University of Oregon.

The researchers came up with a plan to create conditions that would not occur in nature, Baumann says.

“In this case, fish from very distant places mate, which would normally be difficult to find in real life,” says Baumann.

This required careful planning and more than 48 hours of drivingin a car full of gear to safely catch and transport the fish hundreds of miles through heavy East Coast traffic.

The team set out one morning in May and drove 18 hours from Avery Point to Jekyll Island, Georgia, where they caught the southern cohort fish. They then immediately began the journey back north to fish from the northern cohort.

This was just the first series of obstacles the team had to face. Over the next 10 months, Baumann and his team bred the fish, raised their offspring under different temperatures to mimic conditions along the Atlantic coast, and then raised those fish again. Easier said than done, says Baumann, as he recalls the many precarious months of laborious fish farming,

“A lot of things could have gone wrong, but in the end, hopefully, we got it done,” says Baumann.

The researchers then measured nine important traits, such as growth rate and swimming performance. The fish then underwent extensive genetic study in Therkildsen’s laboratory.

“The work surprises with its complexity and breadth,” says Baumann. “Ssidevers, like many species, have several massive inversions on multiple chromosomes. The novelty of our study is that we show that these inversions contain genetic information vital for genes that determine growth, metabolism, vertebrae number and lipid content.”

When fish from different regions mate, their offspring inherit a mix of genes from both parents. The study found that chromosomal inversions link clusters of favorable genetic mutations, preserving beneficial gene combinations despite continued genetic mixing within the species. Without inversions, this mixing would break up combinations of genes that work well together to survive in cold or warm water, producing hybrid offspring poorly adapted to both environments, Therkildsen says. These chromosomal inversions were most significant in influencing the growth rates and number of vertebrae of Atlantic silversides.

“The large effects of investments on critical adaptive traits suggest that they play a critical role in maintaining local adaptation,” says Therkildsen. “More broadly, traits like growth are generally thought to be shaped by thousands of small genetic changes. Our results suggest that in this species, selection can act on a small number of powerful genetic switches. That difference could determine how quickly populations respond as the oceans warm and the seasons change.”

The research was supported by the National Science Foundation.