Antarctic Fish Shed Genes in the Cold, Revealing Natural Knockouts

In the frigid, thermally stable waters of the Southern Ocean, a group of fish has been quietly rewriting its own genetic instruction manual for millions of years. A new comparative genomics study of Antarctic notothenioid fishes has produced a genome-wide catalog of genes that these cold-adapted species have lost entirely, revealing dozens of so-called natural knockout loci. These are genes that remain intact in related, non-Antarctic relatives but have been inactivated or erased across the Antarctic clade, offering scientists a rare, naturally occurring experiment in vertebrate gene loss. The findings, published in BMC Genomics, suggest that the harsh polar environment did not simply select for new traits; it also permitted, and perhaps encouraged, the wholesale abandonment of ancestral genetic machinery.

The research team, led by Vinita Lamba, Andrew J. Alverson, and Xuan Zhuang of the University of Arkansas, together with Jacob M. Daane of the University of Houston, set out to systematically identify gene-inactivating mutations across the cryonotothenioid radiation, the lineage of notothenioid fishes that diversified after the Southern Ocean cooled to its present near-constant low temperatures. What makes this group especially powerful for such an analysis is the availability of non-Antarctic notothenioid outgroups. Because these relatives live in warmer waters and retain many ancestral genes, researchers can infer which genes were present in the common ancestor and therefore pinpoint which losses occurred specifically after the Antarctic lineage became established.

Methodologically, the study is notable for its rigor. The researchers used whole-genome alignments and orthology-aware comparative analyses spanning eleven cryonotothenioid species and four non-Antarctic outgroups. Orthology-aware analysis means the team carefully tracked which genes correspond to which across species, distinguishing true gene loss from mere annotation gaps or assembly errors. To reduce reference bias, a common pitfall in comparative genomics where results are skewed by the choice of reference genome, the authors applied stringent filters. They also excluded any candidate lost loci that showed detectable transcriptomic support, meaning that if a gene appeared to be missing from the genome but was still being transcribed, it was removed from consideration. This conservative approach was designed to ensure that the final catalog reflects genuine losses of coding potential rather than artifacts of sequencing, assembly, or annotation.

After these filters, the team identified 30 high-confidence single-copy orthologs with loss of coding potential across the sampled cryonotothenioids but intact coding sequences in the outgroups. Single-copy orthologs are particularly informative because they exist as one copy per genome in the ancestor, so their loss cannot be masked by a duplicate gene stepping in to perform the same function. The fact that these losses are shared across the Antarctic clade suggests they occurred near the base of the radiation, early in the group’s polar history, and have since become part of the inherited genomic background on which all subsequent specialization was built.

The functional categories of the lost genes are striking. They include genes associated with lipid and amino acid metabolism, water transport, renal glucose reabsorption, skeletal mineralization, circadian regulation, and tRNA modification pathways. Each category hints at physiological adjustments that may have accompanied life in the Southern Ocean. Lipid metabolism genes, for example, could relate to the demands of maintaining membrane fluidity and energy storage in the cold. Water transport and renal glucose reabsorption genes may reflect the osmoregulatory challenges of a stable, cold marine environment. Circadian regulation losses are especially intriguing given the extreme seasonal light cycles of polar regions, where the sun disappears for months at a time, potentially relaxing the selective pressure on molecular clocks tuned to daily rhythms.

Beyond the clade-wide losses, the study identified 12 additional single-copy orthologs that have been lost across the examined icefishes, the famous white-blooded family of Antarctic fish, while being retained in red-blooded notothenioids and in the outgroups. The icefish family, Channichthyidae, is already celebrated for its extraordinary loss of hemoglobin expression, making it the only vertebrate group whose adults lack functional red blood cells. The new findings extend this pattern of loss into other genes associated with oxygen transport, iron handling, erythroid biology, and vesicular trafficking. Erythroid biology encompasses the development and maintenance of red blood cells, so additional losses in this pathway are consistent with the icefishes’ radical evolutionary departure from oxygen-carrying blood. Vesicular trafficking genes, meanwhile, point to changes in how cells move materials internally, a process that intersects with iron metabolism and membrane biology in ways that may be linked to the icefishes’ unusual blood physiology.

One of the most compelling aspects of the catalog is that many of the lost genes are conserved across vertebrates and are associated with disease-relevant phenotypes in humans or model organisms. In laboratory genetics, a knockout is a deliberately engineered disruption of a gene used to study its function, and knockout organisms often display developmental defects, metabolic disorders, or other pathologies. The Antarctic fishes, by contrast, carry these loss-of-function variants in viable, reproducing wild populations that thrive in one of Earth’s most demanding environments. This makes them a natural comparative system for studying how loss-of-function mutations can persist and even become fixed in a lineage without causing overt harm.

The authors frame these candidate natural knockouts as a source of testable hypotheses about the mechanisms that allow vertebrates to tolerate gene loss. Three broad mechanisms are proposed. Environmental buffering refers to the possibility that the stable, cold Southern Ocean environment reduces the selective cost of losing certain functions; if a gene’s role becomes unnecessary under constant conditions, its loss may be effectively neutral. Paralog compensation occurs when another member of the same gene family takes over the lost gene’s function, cushioning the impact of the knockout. Pathway rewiring describes deeper evolutionary restructuring, in which metabolic or regulatory networks are reorganized so that the lost gene’s contribution is no longer needed. Disentangling which mechanism applies to which locus is now an open empirical question, and the catalog provides the raw material for such investigations.

The study also underscores a broader point about genome evolution: gene loss is a major but often underappreciated mode of evolutionary change. Genomes are frequently discussed in terms of what they gain, through duplication and innovation, but what they lose can be equally consequential. Once a loss event becomes fixed in a population, it permanently alters the inherited gene complement and can constrain or redirect the evolutionary trajectories available to descendant lineages. Shared losses near the base of a radiation, as seen here, effectively set the stage for later specialization, meaning that the icefishes’ dramatic adaptations were built atop a genomic foundation already shaped by earlier deletions and inactivations.

Funded in part by the U.S. National Science Foundation, including a CAREER Award to Xuan Zhuang, the work opens a window onto vertebrate physiology under chronic cold that no laboratory model could easily replicate. As genome assemblies for polar species continue to improve, the natural knockout catalog of the notothenioids is likely to grow, and each entry represents an invitation to ask a deceptively simple question: what happens when a vertebrate loses a gene that humans and model animals cannot live without? For the fishes of the Southern Ocean, the answer has been unfolding for millions of years, written in the absences scattered across their genomes.

Subject of Research: Comparative genomics of gene loss in Antarctic notothenioid fishes

Article Title: Gene loss under constant cold reveals “natural knockout” loci in Antarctic notothenioid fishes

Article References: Lamba, V., Alverson, A. J., Daane, J. M., & Zhuang, X. (2026). Gene loss under constant cold reveals “natural knockout” loci in Antarctic notothenioid fishes. BMC Genomics. https://doi.org/10.1186/s12864-026-13346-x

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13346-x

Keywords: gene loss, Antarctic notothenioids, icefish, comparative genomics, natural knockout, genome evolution, molecular evolution, Southern Ocean, hemoglobin, evolutionary physiology, orthology, cold adaptation