Imagine swimming in a muddy river with water so murky it was impossible to see. You'd have to depend on smell or hearing to find food while avoiding predators. Some fish in this situation have gained a leg up by evolving an extra sense – they produce electrical fields and use them to sense other animals. Compared to their ancestors, these fish have a new electric organ. Look carefully, and inside that new organ you will find new proteins made by new genes.
Genes are the blueprint for proteins, the heavy lifters of biology. Everything your body does – from moving muscles to digesting food to reading this article – requires the activity of thousands of different proteins working together. But for a body to do something new (produce electricity, for example) new proteins and genes are required. So where do they come from?
One of the most common ways new genes are born are duplication events. Duplications are a kind of mutation, a change in an organism's genes due to random mistakes. Sometimes during the production of eggs or sperm two copies of a gene are accidentally included instead of just one. When that sperm or egg joins another parent's sperm or egg to create an embryo, all of the cells in the new organism will have an extra copy of that gene. It will then pass on that extra copy to all of its own children. Typically, only one working copy of a gene is necessary for survival. That means the extra gene copy is invisible to natural selection, and it slowly accumulates mutations that prevent it from making working proteins. The end result is like the duplication never happened.
Duplications are a kind of mutation, a change in an organism's genes due to random mistakes.
But sometimes the mutations that occur in the extra gene copy don't stop the protein from being made, they just change the protein's function. Natural selection then slowly sculpts the sequence of that gene to make a new protein that does something totally different from the original gene copy. This process is called neofunctionalization, and it is possible because the duplicated gene copy is not necessary for the organism to survive. It is free to change without consequences.
Neofunctionalization is what happened in the case of the electric fish. The ancestor of the majority of fish species had every single one of its genes duplicated 300 million years ago. Almost every living descendant carries two duplicated copies of a particular sodium channel gene, which makes a protein in muscles that is used like a gate to transport sodium. But in some of the descendants, like the Glass Knife Fish and the Elephant Nose Fish, part of the muscle tissue has evolved into an organ that emits an electric field the fish uses to “see†in murky water. In these fish, one copy of the sodium channel gene has been neofunctionalized into a gene that makes a protein expressed in the electric organ.
The sodium channel gene is what is known as a dosage-sensitive gene – the protein it makes must be present at very specific levels relative to other proteins, so a single duplication could have dire consequences for the organism. But if every single gene an organism has is duplicated, then these protein ratios are kept intact and everything is fine. Because all those extra copies are still required to make the correct amount of protein, scientists thought dosage-sensitive genes couldn't be neofunctionalized if every single gene is duplicated. But in the case of the electric fish and their modified sodium channel, neofunctionalization clearly occurred. So how did this happen?
Scientists propose a process called compensatory drift to explain how a dosage-sensitive duplicated gene can gain a new function without altering the amount of the original protein present. After duplication occurs, the two copies of the gene initially produce the exact same amount of proteins, say 5 and 5, making the total amount of proteins 10. Mutations slowly accumulate in the two gene copies that change their individual protein production levels, but the overall amount remains the same. So now the production of one copy is 6 and the other is 4 – but the total protein is still 10. And since that total remains the same, this process is invisible to natural selection and the amount of protein made by the individual gene copies is free to drift around.
Eventually one copy of the gene makes the majority of protein (9), and it compensates for the very low production of the other copy (1). That low production copy is free to neofunctionalize, since it is no longer needed to maintain the correct protein level! A mathematical model shows that compensatory drift could have led to the neofunctionalization of the dosage-sensitive sodium channel gene in electric fishes.
The genes in an organism are constantly shifting from generation to generation; mutation always makes changes, and natural selection works to keep things the same. The birth of new genes requires both forces working together. New genes don't just spring up out of nowhere – they are made from alterations to the fabric of genes as they already exist.
About the Author
 Katie Pieper is a PhD student in the Department of Genetics at the UGA. She studies the molecular evolution of sex chromosomes in fruit flies. In her free time, she enjoys baking delicious desserts and winning at trivia contests. She is also the head tweeter for the Athens Science Café official Twitter account @AthSciCafe. Get in touch with Katie at @kpeeps111 or kpieper@uga.edu. More from Katie Pieper. |
