A grueling road trip led to an extraordinary experiment at UConn's Rankin Seawater lab that discovered how inverted chromosomal segments help Atlantic silversides adapt
John S. Rankin Laboratory at Avery Point. (Sean Flynn/UConn Photo)
When a species lives in two distinct types of habitats, individuals with traits better suited to each habitat will thrive and reproduce, naturally selecting descendants with those traits. But what about mobile, aquatic species that live across a broad 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 from Cornell and the University of Connecticut finds that chromosomal inversions - which occur when a chunk of chromosome containing tens to thousands of genes breaks off, flips 180 degrees and reattaches to the same chromosome - play a central role in shaping these advantageous adaptations.
"Each chromosomal inversion locks together a large set of genes, effectively forming a genetic switch with two states (flipped or not flipped). What's surprising here is that multiple 'switches' can combine to generate smooth, continuous variation, not just on-or-off differences," says Cornell associate professor Nina Overgaard Therkildsen.
The study focused on Atlantic silversides, a small fish species that lives all along the Atlantic coastline 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's lab collaborated with Hannes Baumann, associate professor at UConn, and David Conover, emeritus professor at the University of Oregon.
The researchers devised a plan to create conditions that would not happen in nature, says Baumann,
"In this case, mate fish from far apart places, that would normally have a hard time meeting in real life," says Baumann.
This required careful planning, and over 48 hours of driving, in a car packed with equipment to catch and safely transport the fish over hundreds of miles through dense east coast traffic.
The team set off one May morning and drove 18 hours from Avery Point to Jekyll Island, Georgia, where they caught the fish from the southern cohort. They then immediately began the journey back north to catch fish from the northern cohort.
This was just the first series of hurdles the team had to contend with. Over the next 10 months, Baumann and his team cross-bred the fish, raised their offspring under different temperatures to imitate conditions along the Atlantic coast, and then bred those fish again. Easier said than done, says Baumann, while remembering the many precarious months of laborious fish rearing,
"Lots of things could have gone wrong, but in the end, with luck, we succeeded," says Baumann.
The researchers then measured nine important traits, such as growth rate and swimming performance. The fish then underwent extensive genetic study by Therkildsen's lab.
"The work is stunning in its complexity and comprehensiveness," says Baumann. "Silversides, like many species, have several massive inversions on multiple chromosomes. The novelty of our study is that we show that these inversions contain vital genetic information for genes that determine growth, metabolism, vertebral number and lipid content."
When fish from different regions mate, their offspring inherit a mix of genes from both parents. Chromosomal inversions, the study found, lock together groups of favorable genetic mutations, preserving beneficial gene combinations in spite of ongoing genetic mixing within the species. Without inversions, this mixing would break apart the gene combinations that work well together for survival in either cold or warm water, producing hybrid offspring poorly suited to either environment, Therkildsen says. These chromosomal inversions were most significant in influencing Atlantic silversides' growth rates and number of vertebrae.
"The large effects of inversions on critical adaptive traits suggest they play a fundamental role in maintaining local adaptation," Therkildsen says. "More broadly, traits like growth are usually thought to be shaped by thousands of tiny genetic changes. Our results suggest that in this species, selection can instead act on a small number of powerful genetic switches. That difference could shape how quickly-and how predictably-populations respond as oceans warm and seasons shift."
The research was supported by the National Science Foundation.
