Codfishes survived essential gene loss by functionally co-opting freshwater ovulatory pathways to marine egg hydration.

Codfishes (Gadiformes) lost aqp1ab-type genes yet still produce highly hydrated pelagic eggs, with short Aqp4 isoforms recruited to oocyte hydration.

• Why it matters: Essential gene loss that would normally threaten embryonic survival is common, but the specific mechanisms enabling persistence after losing key hydration genes are unclear.
• What they did: The study confirmed conserved osmotic-drive mechanisms, then used comparative aquaporin transcript analyses and zebrafish plus Atlantic cod experiments (immunolocalization, functional assays, inhibition) to test Aqp4 versus aqp1ab roles.
• The result: Short Aqp4 isoforms are required for egg ovulation in more ancestral freshwater teleost lineages and are linked to codfish oocyte hydration, while older and modern marine pelagic-egg lineages still use aqp1ab genes.

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