Angel Rivera-Colón, a former graduate student now postdoc in the lab of Julian Catchen (CIS/GNDP), an associate professor in the department of Evolution, Ecology, and Behavior at the University of Illinois Urbana-Champaign, explores the genetic mechanisms underlying this anomaly in Antarctic Notothenioid fish.

Antarctic notothenioids, or cryonotothenioids, have evolved to live in freezing waters around Antarctica, where most fish would otherwise freeze solid if exposed to such cold temperatures. However, cryonotothenioid fish are able to survive in these waters due to antifreeze glycoproteins they produce in their cells. The AGFPs bind to any ice crystals that form, preventing them from growing and the cells from freezing.

Antarctic icefishes, a family within cryonotothenioids, are even more specialized to live in the icy waters. Icefishes also are the only vertebrate that has adapted to live without hemoglobin in their blood cells, causing their cells and tissues to be translucent/white in color. Hemoglobin is a protein in blood cells that helps increase oxygen uptake and results in the red coloration of cells. Normally animals need hemoglobin to get enough oxygen, but in the cold, oxygen-rich waters around Antarctica, icefishes have developed morphological changes, such as bigger hearts for pumping blood, that they no longer need hemoglobin to get enough oxygen.

Despite this extreme specialization, one species of icefish called Champsocephalus esox, or the pike icefish, has escaped Antarctica and now lives in warmer, less oxygenated, South American waters. “The movement of this species to warmer waters posed an interesting evolutionary mystery that I wanted to try to solve,” Rivera-Colón said. “If you’re specialized to only live in very cold environments, how do you survive and adapt to this new warmer environment?”

To understand how the genome of the fish changed as it migrated into warmer waters, Rivera-Colón compared the genetics of the pike icefish to that of an Antarctic species of icefish, C. gunnari. The team took tissue samples collected by collaborators and fishermen from southern Chile, South Georgia, and the Sandwich Islands to sequence the genomes.

“This is the first time we’ve looked at a genome of a notothenioid species that escaped Antarctica into this new temperate environment. A big part of that is because the pike icefish is very rare and elusive, so the help of these fishermen as well as collaborators for gathering samples was indispensable.” Rivera-Colón said. The researchers used continuous long read sequencing to generate a chromosome-level genome for each fish species.