Genetic rescue of small inbred populations by outcrossing
Due to the widespread clearing and degradation of native vegetation, many animal and plant species have fragmented distribution with small isolated populations. These populations are likely to suffer inbreeding problems and/or reduced ability to evolve to cope with environmental change. Without outcrossing to another population within the species (genetic rescue), these populations are likely go extinct. Following a combined analysis of published data sets (a meta-analysis), I concluded that the risks of outcrossing are small, but the benefits great. This finding is likely to change the way we manage threatened species.
There have been only ~ 20 published cases of outcrossing for conservation purposes, whilst there are likely millions of populations that would potentially benefit from outcrossing. But before outcrossing is recommended more widely, it’s important to assess the benefits. How large are the benefits of outcrossing and how consistent are they?
Outcrossing of inbred populations resulted in beneficial effects in 93% of 156 cases. The average increase in combined reproduction and survival following outcrossing was 148% in stressful (wild) environments and 45% in benign (captive) ones. Benefits were similar for invertebrates, vertebrates and plants. The ability to evolve in response to environmental change also benefited from outcrossing. My paper also provides revised guidelines for the management of genetic rescue attempts.
A second impediment to outcrossing is that it can sometimes be deleterious (outbreeding depression), especially when different species are crossed. For example, crossing a horse and a donkey yields sterile mules. However, with my colleagues (including Mark Eldridge from AMRI), I previously devised means to predict when outcrossing will be deleterious, versus beneficial. These and other issues are being brought together in two books that I am writing with Mark and our international colleagues on Genetic Management of Fragmented Animal and Plant Populations, one an advanced textbook and the other a practical guide. Our aim is to shake up the conservation management of threatened species and to improve its effectiveness.
In conclusion, there are no scientific reasons to avoid using outcrossing for genetic rescue of inbred populations, provided potential crosses have a low risk of outbreeding depression. Given the large observed benefits from such outcrossing, it has the potential to prevent many unnecessary population extinctions
Dr Richard (Dick) Frankham
Emeritus Professor at Macquarie University
Research Associate of the Australian Museum Research Institute
Frankham R (2015) Genetic rescue of small inbred populations: meta-analysis reveals large and consistent benefits of gene flow. Molecular Ecology doi: 10.1111/mec.13139