Some copepods, tiny crustaceans that occupy an outsized place in the aquatic food web, can evolve fast enough to survive in the face of rapid climate change, according to new research addressing a long-standing question in the field of genetics.
Just over a millimeter long, the copepod Eurytemora affinis paddle in large numbers in the coastal waters of oceans and estuaries around the world, mostly consumed by juvenile fish, such as salmon, herring, and anchovies.
“It’s a dominant coastal species, serving as very abundant and nutritious fish food,” says Carol Eunmi Lee, a professor in the Department of Integrative Biology at the University of Wisconsin-Madison and lead author of a new study on copepods published in the journal . nature of communication. “But they are vulnerable to climate change. »
The salinity of the ocean, explains Lee, changes rapidly to measure the depth of the ice and the changing rainfall regimes: «Ces copepodes sont une espèce d’eau salée que doit maintenant s’adapter à une eau beaucoup plus douce dans leur environment. »
Many copepods (and countless other animals) have evolved in salt water. As their environment changes, they will have to adapt to maintain their body chemistry…or die.
“Salinity is a very strong environmental pressure on aquatic habitats,” says David Stern, lead author of the study and a former postdoctoral researcher in Lee’s lab, who now works at the National Center for Biodefense Analysis and Countermeasures.
Lee, Stern, and the rest of the research team studied how certain copepods reacted to this pressure. They maintained a population of Eurytemora affinis from the Baltic Sea in his laboratory: small crustaceans that swim in waters as salty as their range and reproduce for several generations.
The researchers then divided the copepods into 14 groups of a few thousand each. Four control groups lived the experience in the environment such as the Baltic. The other 10 groups were exposed to falling salt levels, mimicking the kind of pressure caused by climate change. Each had its water reduced to a lower salinity with each new generation (about three weeks for this copepod) for a total of ten generations.
The researchers then sequenced the genomes of each copepod lineage at the beginning of their experiment and again after six generations and 10 generations, tracking evolutionary changes through their genomes. The strongest signals of natural selection, where changes were larger and more common in groups stressed by lower salinity, were in parts of the genome thought to be important in ion regulation, such as sodium transporters.
“In salt water there are many ions, such as sodium, which are essential for survival. But when you get to fresh water, those ions are valuable,” says Lee. “So copepods have to absorb them from the environment and stick to them, and the ability to do that depends on these ion transporters that we’ve found through natural selection. »
At the end of the experiment, the researchers found that copepods with certain ion transporter genetic combinations were repeatedly more likely to survive through successive generations, even if water salinity decreased. In fact, the same genetic variants, or alleles, found in copepods that survived lower salinity in the laboratory are also common in cooler parts of the Baltic Sea.
“With the number of genes we have that code for features in our copepods, there’s no way to see how much parallelism we’ve made unless something is driving it,” says Stern.
The evolutionary experiment is new evidence for a genetic mechanism called positive epistasis, in which the positive effect of a genetic variant is magnified when it works in combination with other key genes. It’s a theory that legendary UW-Madison genetics professor Sewall Wright and others championed nearly a century ago as a counterpoint to additive evolution, the idea that the effect of each gene is equally weighted and that the effects of many genes add in a linear fashion. Fashion.
“Computer simulations of evolution under our experimental conditions predict that additive evolution would have given us much greater variation among our 10 lineages,” adds Stern. “We haven’t seen that kind of variation. »
Epistasis had not been proven for lack of experimental tools, but vast amounts of genomic data from modern sequencing and computer simulations have made it possible to show positive epistasis at work in parallel evolution and to describe the power of genetics to study climate change. Stern, Lee and colleagues show in the new study that positive epistasis can drive parallel evolution of groups of animals by repeatedly favoring sets of alleles through natural selection.
“This copepod gives us an idea of what it takes, an idea of the necessary conditions, that allow a population to rapidly evolve in response to climate change,” says Lee. “It also shows how important evolution is to understanding our changing planet and how, or even if, populations and ecosystems will survive. »