Since 1970, wildlife populations globally have plummeted by about 73%. One driver is habitat fragmentation that isolates small populations, leading to inbreeding and fewer healthy offspring. California’s endangered Pacific pocket mouse, limited to three small populations, is a case in point. A new study finds that mixing individuals from these populations in a breeding program improved survival and reproduction, outweighing concerns about genetic differences.
“Examples of genetic rescue in the conservation realm are still few and far between, so any empirical evidence is a major contribution,” Sarah Fitzpatrick, an associate professor of integrative biology with Michigan State University, U.S., not involved with the study, told Mongabay by email.
The Pacific pocket mouse (Perognathus longimembris pacificus), so-named for the external cheek “pockets” it uses to carry food, once ranged from Los Angeles, U.S., to the Mexico border. However, urban development fragmented much of its habitat — the mouse was believed extinct for 20 years until researchers rediscovered three small populations, all suffering from inbreeding and reduced fitness.
“Genetic rescue” can help in such cases, where unrelated individuals from different populations are mixed to boost genetic diversity. But the approach risks “outbreeding depression,” where the mice are so genetically different, they’re unable to reproduce healthy offspring.
One of the isolated populations of the Pacific pocket mice has developed 58 chromosomes, while the other two have 56, raising concerns about whether they can successfully reproduce together.
Conservation guidelines recommend avoiding genetic rescue if populations have been isolated for more than 500 years, live in distinct environments or have chromosomal differences, as is the case for the pocket mice. But researchers, led by Aryn Wilder, a conservation genetics scientist with the San Diego Zoo Wildlife Alliance, tested if those rules were too rigid based on empirical data from a conservation breeding program that started in 2012 with mice from all three wild populations.
“So, we already had information available on how well individuals that had parents from different populations survived and how well they reproduced,” Wilder told Mongabay in a video call.
Researchers found that mixing individuals increased genetic diversity and produced more healthy offspring across all mouse pairings. Mice that benefited from genetic rescue but had mismatched chromosomes had slightly fewer healthy young than mice with the genetic boost and the same number of chromosomes but they still outperformed the highly inbred mice.
“The benefit of crossing those populations, of mixing them, outweighed this cost of having the different number of chromosomes,” Wilder said.
Researchers have also reintroduced mice from the breeding program into areas where the species has disappeared to see if the lab-tested results hold true in the wild.
Wilder said their findings can guide the conservation of other endangered species.
“Even when you have populations that seem like they would be prime candidates for outbreeding depression, that doesn’t necessarily spell doom for using genetic rescue as a strategy.”
Banner image:A Pacific pocket mouse. Image via Animalia.bio (CC BY-SA 3.0).
