Common knowledge says that evolution takes a long time, right? It might take thousands of years for us to detect the effect of a selective pressure. But for steelhead trout, evolution has happened much faster. Game fisherman stocked the freshwaters of Lake Michigan with steelhead trout from the saltwater coast of California in the late nineteenth century. Since then, the steelhead have settled into their new Great Lakes home and adapted to freshwater life in just over 100 years.
Janna Willoughby and her colleagues from Purdue University, Michigan State University, and Oregon State University, USA, were interested in how a saltwater fish could make the move to freshwater. How had the genetic code of these fish changed and evolved to allow them to grow, reproduce and live in a completely new environment? To uncover the answer, Willoughby and her colleagues sequenced the entire genome for over 250 steelhead trout, some from the modern California population and others from the Lake Michigan population a century on from their original trans-continental road trip.
The team then scrutinized the two genomes, looking for sequences that varied between the two populations and found three regions that differed dramatically. Two of the three genomic regions play a role in osmoregulation, which is the ability of fish to maintain their internal salt balance; the freshwater fish have to work hard to keep salt ions in their bodies, while the saltwater fish have to work hard to pump excess salt ions out of their bodies. This means that in just 100 years, the transplanted saltwater trout population had experienced dramatic changes in its genetic code, which allowed it to cope better with freshwater living.
The third gene region identified by the team plays a key role in wound healing, indicating that freshwater steelhead likely heal more effectively than their saltwater counterparts. This change might seem strange at first glance, but the ability to heal rapidly should prevent the loss of internal salts following an injury or attack. Additionally, the introduced fish are vulnerable to attacks by parasitic lampreys – residents of the Great Lakes that latch onto fish with razor-sharp teeth – which they would never encounter in their native California coast. The ability to heal quickly from a lamprey attack would be another bonus for freshwater steelhead.
Fascinatingly, the rapid evolution that Willoughby and colleagues have revealed in the steelhead genome occurred despite the limited genetic diversity in the small population of transplanted fish. Only a tiny portion of the California population was moved to Lake Michigan in the late 1800s, creating a population bottleneck that could reduce the population's genetic diversity. Less diversity could potentially restrict their opportunity to evolve in their new home. Yet, this new study excitingly highlights how animals can evolve and adapt rapidly to changing environments and, more promisingly, shows that certain animals, such as the steelhead trout, can succeed even if they started with fewer cards in their genetic deck.