What the research is about
Eels migrate between rivers and oceans, surviving by carefully regulating water balance and ion levels inside their bodies. One of the key players in this process is a protein called aquaporin. Aquaporins form tiny tunnel-like structures in cell membranes that allow water and other substances to pass through.
Aquaporins come in several types, one of which is Aquaporin 10 (Aqp10). Most fish possess two Aqp10 genes: Aqp10.1, which can transport a wide range of substances, and Aqp10.2, which transports a more limited range of molecules.
During evolution, however, the ancestors of eels completely lost Aqp10.1 and retained only Aqp10.2. For example, close relatives such as conger eels have just a single copy of Aqp10.2 remaining in their genomes.
Surprisingly, Japanese eels and European eels have retained multiple copies of Aqp10.2. Why have these species kept several seemingly similar genes when one appears to be enough? To answer this question, a research team led by Assistant Professor Ayumi Nagashima at Institute of Science Tokyo (Science Tokyo) investigated the functions of the multiple aquaporin genes found in eels.
Why this matters
The researchers compared each of the Aqp10.2 genes that had been duplicated and retained in eel genomes. They focused on the European eel, which possesses three different Aqp10.2 genes, and examined the properties of each one.
The results revealed that among these three genes, two that had been produced through gene duplication could efficiently transport urea and boric acid. In other words, these duplicated genes had not simply increased in number; they had evolved new functions that allowed them to transport a broader range of substances.
What changes had occurred in the proteins produced by these genes?
A closer look showed that a single, critical amino acid had changed. At a key position in the protein, the amino acid tyrosine had been replaced by glycine. Because glycine is smaller than tyrosine, this tiny change widened the passage through the aquaporin channel, allowing not only water and glycerol but also urea and boric acid to pass through.
This study demonstrates the precise molecular process by which a function once lost in the ancestors of eels was regained through the duplication of a remaining gene and subsequent amino acid changes. The researchers also showed that this functional shift could be traced back to the replacement of just one amino acid.
What's next
This study provides a clear example of how gene duplication and random mutations can work together to create and maintain new biological functions.
The findings also suggest that it may be possible to predict which substances an aquaporin can transport simply by examining specific amino acid sequences within the protein.
In the future, this work may help researchers uncover, at the molecular level, how eels achieve the remarkable physiological regulation that allows them to move between freshwater and seawater environments. The findings may also contribute to broader studies of environmental adaptation in fish and advances in fisheries science.
Comment from the researcher
A gene that has been completely lost from a genome cannot simply be restored, as if turning back the hands of a clock. However, genomes are not static blueprints; they transform through the continuous birth and death of genes.
Our study of eels reveals this process at the molecular level. After one gene was lost, another surviving gene duplicated and eventually acquired a new role through a change in just a single amino acid.
When we compare the genomes of diverse organisms, we are likely to find many similar evolutionary stories. I hope readers will take a moment to reflect on the ongoing cycle of gene birth and loss that has shaped the incredible diversity of life we see today.
(Ayumi Nagashima, Assistant Professor, School of Life Science and Technology, Institute of Science Tokyo)
Dive deeper
Explore more research in Science for All
Science for All showcases cutting-edge research at Science Tokyo and highlights the ideas, people, and possibilities shaping the future.
