This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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.
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