Texas A&M, Uppsala Study Uncovers How Herring Adapted To Baltic Sea Light Environment
The evolutionary process that occurs when a species colonizes a new environment provides an opportunity to explore the mechanisms underlying genetic adaptation, which is essential knowledge for understanding evolution and the maintenance of biodiversity.
An international team of scientists led by researchers from Texas A&M University and Uppsala University reports that a single amino acid change in the light-sensing rhodopsin protein played a critical role when herring adapted to the red-shifted light environment in the Baltic Sea.
The study, published today in the journal PNAS (doi.org/10.1073/pnas.1908332116), reports that, remarkably, about one-third of all fish living in brackish or freshwater carry the same change.
The Atlantic and Baltic herring are excellent models for evolutionary studies for two reasons, according to Dr. Leif Andersson, a professor in the Texas A&M College of Veterinary Medicine & Biomedical Sciences’ (CVM) Department of Veterinary Integrative Biosciences (VIBS) and Uppsala University, who led the study.
“Firstly, their enormous population sizes allow us to study the effects of natural selection without the disturbing stochastic changes in the frequency of gene variants that happens in small populations,” he said. “Secondly, the colonization of the brackish Baltic Sea by herring within the last 10,000 years—following the most recent glaciation—provides an opportunity to study what happens when a species adapts to a new environment.”
“We have examined the entire genome in many populations of Atlantic and Baltic herring and find that a single amino acid change in the protein rhodopsin, in which phenylalanine has been replaced by tyrosine, played a critical role during the adaptation to the Baltic Sea,” said Jason Hill, a scientist at Uppsala University and first author on the paper.
This makes a lot of sense, according to the researchers, since rhodopsin is a light-sensitive receptor in the retina and satellite data show that the Baltic Sea has a red-shifted light environment compared with the Atlantic Ocean, because dissolved organic material absorbs blue light.
“A careful genetic analysis of our data shows that the evolutionary process must have been very rapid. We estimate that the rhodopsin gene variant found in Baltic herring increased in frequency to become the most common variant within only a few hundred years,” said scientist Mats Pettersson, from Uppsala University.
The amino acids phenylalanine and tyrosine are structurally very similar and only differs by the presence of a hydroxyl (-OH) moiety in tyrosine, so could this change really be so important?
“In fact, the crystal structure of rhodopsin shows that residue 261 is located in the vicinity of the chromophore retinal where light absorption occurs,” Andersson said.
The presence of tyrosine in Baltic herring rhodopsin makes light absorption red-shifted by about 10 nanometer and can thereby catch more photons in the red-shifted light environment in the Baltic Sea, according to Dr. Patrick Scheerer, at Charité–Universitätsmedizin Berlin, in Germany, another study co-author.
When the scientists analyzed the rhodopsin sequence from more than 2,000 fish, they found that about one-third of all species occurring in brackish or freshwater carry exactly the same genetic change as the Baltic herring, whereas nearly all fish living in marine waters have a rhodopsin gene variant with phenylalanine like the Atlantic herring.
“It is remarkable that we find the same mutation occur independently and at least 20 times across thousands of fish species; this provides a really striking example of convergent evolution at the molecular level,” said Erik Enbody, co-author and post-doctoral fellow at Uppsala University.
“Our hypothesis is that this change in rhodopsin is particularly important during the juvenile stage and that the Baltic herring variant allows fish larvae to better utilize the light environment in the Baltic Sea when searching for food or avoiding predators,” Andersson said.
Their hypothesis is supported by their finding that both Atlantic salmon and brown trout that always spawn in freshwater but may live most of their life in marine water have tyrosine 261 in rhodopsin like a freshwater fish.
In contrast, the European and Japanese eel which both are born in marine waters but live most of their adult lives in freshwater carry phenylalanine 261 like the great majority of marine fish.
###
For more information about the Texas A&M College of Veterinary Medicine & Biomedical Sciences, please visit our website at vetmed.tamu.edu or join us on Facebook, Instagram, and Twitter.
Contact Information: Jennifer Gauntt, Interim Director of CVM Communications, Texas A&M College of Veterinary Medicine & Biomedical Science; jgauntt@cvm.tamu.edu; 979-862-4216
