A 70-million-year-old genetic rescue allowed codfishes to escape extinction and shape the North Atlantic

A 70-million-year-old genetic rescue allowed codfishes to escape extinction and shape the North Atlantic Lisa Lock Scientific Editor Robert Egan Senior Editor A new study has revealed how the repurposing of an ancestral gene saved the embryos of cod's ancestors from extinction, enabling them to form the large populations that, millions of years later, would change the course of human history. The codfishes and their relatives in the order Gadiformes lost genes essential for egg buoyancy and the survival of their embryos, yet survived by adaptively recruiting mechanisms that aid egg ovulation in freshwater fishes. The team included researchers from the Institut de Ciències del Mar (ICM-CSIC) and Institute of Biotechnology and Biomedicine of the Universitat Autònoma de Barcelona (IBB-UAB), Spain, and the University of Bergen (UoB), Norway. The study is published in the Proceedings of the National Academy of Sciences. Key role of hydration and buoyancy in marine fish eggs The scientists have studied how codfish eggs float in the water column, where ocean currents transport them across considerable distances. These and other studies have revealed that the origins of many marine fish populations are closely connected to the physical and biological processes of the ocean itself. The eggs float because, unlike the eggs of freshwater fishes, marine fish eggs become engorged with >90% water during the final stages of egg development, thus making them less dense than seawater. It is like inflating a life vest just before release. Without this massive influx of water, the eggs act like stones and sink out of the water column to nonsurvivable depths. Consequently, any mechanism that affects this process on a population level is an existential threat to the lineage, as it would not be able to complete its life cycle. Such a mechanism was discovered in codfishes. The mechanism in question involves the loss of water channel (aquaporin) genes (aqp1ab1 and aqp1ab2) that mediate the process of egg hydration in all other marine fishes that produce floating eggs. Aquaporins are proteins that are regulated to enter or exit membranes in order to accelerate or reduce water transport in cells, respectively. For water to move through an aquaporin inserted in a membrane, the concentration of dissolved solutes needs to be higher on one side of the membrane. Fish oocytes achieve this by breaking down yolk proteins to their building blocks—free amino acids. This generates the concentration gradient required for water entry into the oocyte, a feature that the scientists found was conserved in codfishes. Surprisingly, however, codfish eggs such as those of Atlantic cod have the highest water contents (93%–97%) of any species, allowing them to float even in low-salinity environments such as the Baltic Sea. With the aqp1ab-type water channel genes that mediate the process in other marine fishes absent, it was a mystery how they achieved this extraordinary feat. Even more paradoxically, codfishes repeatedly hydrate their eggs many times over a spawning season at a rate approximately 20 times that of natural diffusion. The huge volumes of eggs released form the basis of a sustainable population, which in the Northwest Atlantic collapsed in catastrophic fashion due to overfishing and mismanagement. Understanding how codfish eggs do things differently and survive in the oceans was therefore a key goal of the scientists. Functional gene co-option as the solution To resolve the apparent paradox, the scientists looked for other genes in distantly related freshwater and marine fishes that could have been recruited to the hydration process during egg formation in the cod. This uncovered a different aqp4-type water channel on the same chromosome from which the original aqp1ab-type water channels were lost. The studies showed that the aqp4 channel is utilized by primitive freshwater species to aid ovulation, a process in which mature oocytes are released from ovarian follicles to become unfertilized eggs. In these fishes, aqp4 is inserted in the outer oocyte membrane during meiotic maturation to allow a small increase in water content, which causes the oocyte to swell slightly and the surrounding follicle cells to rupture for the release of the newly formed egg. The role of aqp4 in this process was confirmed using a specific inhibitor of the channel, which prevents ovulation. However, they noted that, unlike the channels in other marine species, the related Atlantic cod Aqp4 channel is also localized in the outer membrane of the oocyte, and that its inhibition prevents the massive hydration of the oocyte during meiotic maturation. Consequently, it was revealed that codfishes escaped extinction by functionally co-opting (repurposing) the ovulatory pathways of ancestral freshwater fishes for the hydration of their marine eggs. Based on observations that close relatives of the Gadiformes still retain intact aqp1ab-type genes, this event seems to have occurred 60–84 million years ago. "Finding that codfishes lost the standard water channels used by almost all other marine fish was a complete mystery," says Dr. Joan Cerdà, a researcher at ICM-CSIC and IBB-UAB. "What fascinated us was discovering how evolution hijacked an entirely different channel—originally used just for egg ovulation in freshwater ancestors—and repurposed it into the mechanism of oocyte hydration to assure egg buoyancy. It shows how remarkably flexible genetic toolkits can be when facing existential evolutionary dead ends." Publication details Ángel Rey-Alfonso et al., Codfishes survived essential gene loss by functionally co-opting freshwater ovulatory pathways to marine egg hydration, Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2626713123 Journal information: Proceedings of the National Academy of Sciences Provided by Autonomous University of Barcelona