PROVIDENCE—The expanding global human footprint is dividing the world’s flora and fauna into ever-smaller, more isolated populations that could wink out because of inbreeding, disease, or environmental change. For decades, conservationists have proposed revitalizing those holdouts by bringing in new blood from larger populations. But they’ve wondered whether it really works—and how to do it without swamping the genetic identity and unique adaptations of the group at risk. Last month at Evolution 2019 here, researchers described how genomic tools are refining what is known as genetic rescue.
Although zoos have worked to maintain genetic diversity in endangered species by carefully matching individual animals for breeding, the strategy has rarely been tried in nature. Genetic rescue “should be attempted more frequently,” Andrew Whiteley, a conservation genomicist at the University of Montana in Missoula, and his colleagues wrote last week in Trends in Ecology and Evolution. But showing that it works requires tracking multiple generations for years, something few studies have attempted. And researchers have only recently been able to detect what happens on a molecular level. Now, says Sarah Fitzpatrick, an evolutionary biologist at Michigan State University’s (MSU’s) W. K. Kellogg Biological Station in Hickory Corners, “We have genomic tools to study these populations … in ways we never could before.”
Adding new blood to small populations really does help, a long-term experimental evolution study of wild guppies in Trinidad has demonstrated, says Brendan Reid, an MSU conservation biologist who works with Fitzpatrick. Decades ago, researchers seeded the headwaters of two streams in the mountainous country with guppies taken from a distant habitat. In one stream, the displaced fish had to travel a long way and only slowly made their way downstream to a small, isolated population. In the other stream, the fish more quickly joined another isolated group. Every month for 2.5 years, Fitzpatrick and her colleagues caught, marked, and studied all the fish they could find at the isolated groups’ territories before returning the fish to the streams. They tracked the growth, survival, and genetic diversity of the fish over about seven generations.
In both streams, the populations increased 10-fold and genetic diversity doubled. Later generations were more fecund, with many of the most fit offspring being hybrids of the local and introduced fish, Reid reported at the meeting. But the findings also sounded a note of caution. In the second stream, the rapid infusion of new fish almost completely eliminated pure residents—an outcome conservationists usually hope to avoid. That result suggests “a slow trickle of immigration might be preferable,” Fitzpatrick says.
Another genomic study showed some small populations experience natural genetic rescue—and benefit from it. Nancy Chen, a population geneticist at the University of Rochester in New York, and her team study the threatened Florida scrub jay (Aphelocoma coerulescens), whose numbers are down to a few thousand individuals, split among a few hundred sites. For 50 years, researchers have regularly counted and assessed all the jays found at Archbold Biological Station near Lake Placid, Florida. More recently, they’ve collected blood samples from each bird, which enabled Chen and her colleagues to track genetic changes over time.
The team discovered that the population naturally gets a slow infusion of new blood. Typically, birds trickle in from smaller groups a few kilometers away. The newcomers are less genetically diverse than those already there, but because they are from a different population, they help maintain the resident group’s diversity. However, with fewer birds arriving in recent years because of population declines, that diversity is declining, putting the population at risk of dying out. “Gene flow from small populations may be really important,” she concluded at the meeting.
Most biologists have assumed that larger populations are better sources of new blood. But Chris Kyriazis, a graduate student at the University of California, Los Angeles, used computer models to study the impact of deleterious mutations hidden in a source population. Because such mutations tend to be harmful only when both parents pass the mutation to offspring, they are likely to be eliminated from historically small, inbred populations and to persist in larger ones. Kyriazis’s modeling suggests intermediate-size populations, not the biggest ones, could be the best source for genetic rescues, he reported at the meeting and in a preprint posted 21 June on bioRxiv.
Sometimes, genomic results suggest the rescue strategy may backfire. Just 1000 island foxes (Urocyon littoralis) are left on California’s Santa Catalina Island, and 60% of them have a cancer that affects their ears. Paul Hohenlohe, an evolutionary biologist from the University of Idaho in Moscow, had identified many genes that make the foxes susceptible to the cancer and wondered whether they were a candidate for genetic rescue. But he found that the Santa Catalina foxes have a genetic advantage over neighboring populations that might be sources of new blood: They have more variation throughout their genome, including in the cancer genes, he reported at the meeting. Furthermore, the Santa Catalina foxes are better adapted to the island’s hot, arid climate than the other foxes, many of which live on wetter, cooler islands. So, he recommends letting nature take its course and monitoring whether the foxes eventually evolve resistance to the cancer.
These studies are helping invigorate a strategy that many believe is sorely needed. Fitzpatrick says, “The urgency of the problem and the availability of the tools makes it a really exciting time.”