News | 29 March 2023

Researchers discover how fish ancestors managed to conquer the oceans


A study involving the ICM-CSIC has identified a unique set of genes that allows marine teleosts fishes to hydrate their eggs so that they float and disperse for survival in the oceans.

Although fish have been swimming in the sea for hundreds of millions of years, this has not always been the case / Joaquim Garrabou (ICM-CSIC).
Although fish have been swimming in the sea for hundreds of millions of years, this has not always been the case / Joaquim Garrabou (ICM-CSIC).

Scientists from Spain, Italy and Norway have provided new insights into how teleosts, that represent 96% of modern marine fishes, evolved hydration mechanisms that cause their eggs to float in a new study published in Molecular Biology and Evolution.

Concretely, the study, led by researchers from the Institute of Agrifood Research and Technology (IRTA) and the Institute of Biotechnology and Biomedicine of the Universitat Autònoma de Barcelona (IBB-UAB) in collaboration with the Institut de Ciències del Mar (ICM-CSIC), uncovers a cluster of water channel genes that only exists in teleost fishes. The genes in question allow water to flow across cell membranes and are specifically expressed in the outer membranes of the maturing egg.

Duplicate genes with the same function

An unusual aspect of the genes identified by the researchers is that they are closely related duplicates that perform the same function. Typically, when closely related gene duplicates arise, one may acquire a new function or is lost due to overlapping redundancy.

«In the present case, the new genes perform the same function in the same membrane of the eggs», explains Joan Cerdà (IRTA)

By screening hundreds of teleost genomes, the study reveals that virtually all teleost species that produce floating eggs exclusively retain at least one of the genes, with a third of them retaining both genes. By contrast, almost half of the species that produce eggs that sink have lost both genes, and nearly all species that incubate their eggs internally such as sea horses have also lost both genes.

To uncover how floating eggs utilize the genes, the authors employed a broad variety of experimental techniques coupled with state-of-the art DNA sequencing technologies to show that the proteins resulting from each gene evolved specific mechanisms that control their insertion in the outer membranes of the eggs.

«When such mechanisms are activated each channel binds to a previously unknown type of protein that keeps the channels in the membrane» says Roderick Nigel Finn, who adds that hese new proteins discovered by the researchers are also only found in teleost fishes.

A surprising feature is that in modern marine teleosts, a second activation mechanism causes one of the channels to be moved to a different part of the outer egg membrane. In this way, both channels continue to perform the same function and avoid competing for the same space in the membrane. The result is that water flow into the maturing egg is accelerated. The process is terminated when the novel binding proteins are released causing the two channels to leave the outer egg membrane. This locks the acquired water inside the egg.

The hydration mechanism is so efficient that it endows the eggs with more than 90% water. When released into the marine environment and fertilized, the eggs float as passive passengers in the oceanic currents and are thus carried off to new horizons.