Avoiding genetic rescue not justified on genetic grounds

12 03 2015
Genetics to the rescue!

Genetics to the rescue!

I had the pleasure today of reading a new paper by one of the greatest living conservation geneticists, Dick Frankham. As some of CB readers might remember, I’ve also published some papers with Dick over the last few years, with the most recent challenging the very basis for the IUCN Red List category thresholds (i.e., in general, they’re too small).

Dick’s latest paper in Molecular Ecology is a meta-analysis designed to test whether there are any genetic grounds for NOT attempting genetic rescue for inbreeding-depressed populations. I suppose a few definitions are in order here. Genetic rescue is the process, either natural or facilitated, where inbred populations (i.e., in a conservation sense, those comprising too many individuals bonking their close relatives because the population in question is small) receive genes from another population such that their overall genetic diversity increases. In the context of conservation genetics, ‘inbreeding depression‘ simply means reduced biological fitness (fertility, survival, longevity, etc.) resulting from parents being too closely related.

Seems like an important thing to avoid, so why not attempt to facilitate gene flow among populations such that those with inbreeding depression can be ‘rescued’? In applied conservation, there are many reasons given for not attempting genetic rescue:

  1. Outbreeding depression/upsetting local genetic adaptation/local purity and provenance
  2. Limited quantitative information on the expected consequences of outcrossing
  3. Lack of clear guidelines
  4. High costs
  5. Risks of disease, pest and parasite spread
  6. Disrupting social systems in some animals
  7. Moving biological material across political jurisdictions
  8. Regulatory barriers

The first three are primarily genetic issues, and are the ones on which Dick focussed (although he did address the others and in most circumstances, argued that they are not necessarily good reasons for not doing genetic rescue). Outbreeding depression is a clincher here — Dick and colleagues have previously outlined a way to avoid depressing fitness in populations by crossing them: one should (i) only cross the same species (obviously), (ii) identify that the two populations have no fixed chromosomal differences, (iii) ensure that the two populations are adapted to similar environments and (iv) estimate that there has been gene flow between them at least within the last 500 years.

So that aside, the main impediment is number 2: What are the consequences of outcrossing (genetic rescue)?

Compiling data from 156 cases across 77 taxa (18 invertebrates, 15 vertebrates and 44 plants) he found that a whopping 93% resulted in fitness benefits (either improved population persistence, growth rate, fertility, survival, proportion of normal offspring, sperm quality, fertilisation success or a composite of all of the above) to the outcrossed population, with only 9 cases showing (mild) deleterious effects (and only 1 with definitive and mild outbreeding depression).

Further, the mean effect size (the improvement in composite fitness) was 58%, but much greater (148%) in stressful compared to benign (45%) environments. And let’s face it, most inbred populations of conservation concern live in pretty stressful environments.

From a conservation perspective, there’s little doubt therefore that genetic rescue is almost always a good thing, and it can really reduce the chance of extinction from inbreeding depression. Now there’s little excuse not to try!

CJA Bradshaw


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2 04 2016

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