Is genetic pollution necessarily a bad thing? Well of course there’s pollution and pollution, and the term is often used to describe what might happen to a crop or its wild relatives when a GM variety of that crop starts to be grown in their proximity. But here I mean the genetic mixing of two previously isolated plant populations. As could happen, for example, when you regenerate two genebank accessions of an outcrossing crop side by side. Or when you cultivate an exotic variety of an agroforestry species near wild stands of the same species. Or when you use seed of a wild species from point A to help restore its numbers at far-away point B. The debate about whether genetic mixing is a good or bad thing for conservation has been going on for a while, of course. It is generally thought to be bad. But by coincidence three papers came across my desk this week which suggest that we should perhaps keep an open mind.
The first paper looks at an annual plant in California, Mimulus laciniatus. ((Sexton, J., Strauss, S., & Rice, K. (2011). Gene flow increases fitness at the warm edge of a species’ range Proceedings of the National Academy of Sciences, 108 (28), 11704-11709 DOI: 10.1073/pnas.1100404108)) The researchers compared the progenies of different kinds of crosses in a common garden experiment to measure their overall fitness. They crossed plants from populations at the centre and at the warm periphery of the distribution of the species in various combinations. The most successful crosses, in terms of lifetime reproductive success, were between plants that came from different edge populations. So in this case, a little bit of pollution is actually a pretty good thing, if it comes from the right place.
The second paper looks at a long-lived woody shrub in Australia, Telopea speciosissima. ((Rossetto M, Thurlby KA, Offord CA, Allen CB, & Weston PH (2011). The impact of distance and a shifting temperature gradient on genetic connectivity across a heterogeneous landscape. BMC evolutionary biology, 11 PMID: 21586178)) There’s a lot of different aspects to the study, but let me focus on just one. The species is found along an altitudinal transect. There is considerable genetic structure along this transect, and a close association between altitude, temperature and flowering time. Altitude influences flowering phenology, differences in which throw up a temporal reproductive barrier between coastal and upland populations, leading to genetic differentiation. But not as much as formerly. The current temperature gradient is much flatter than it used to be at glacial maximum, and this has led to phenological overlap and genetic mixing at intermediate altitudes. What’s happening is a natural version of the centre x edge crosses done artificially in the previous paper. The fitness result? The authors are not sure. Could be good, could be bad. They’re going to set up the experiments to find out.
And finally we have a paper on the use of natural hybrids in forest restoration. ((Frascaria-Lacoste, N., Henry, A., Gérard, P., Bertolino, P., Collin, E., & Fernández-Manjarrés, J. (2011). Should Forest Restoration with Natural Hybrids Be Allowed? Restoration Ecology DOI: 10.1111/j.1526-100X.2011.00804.x)) This really ups the ante. We’re no longer talking about the kinds of intra-specific hybrids found at the admixture front at mid-altitudes in the previous paper, or the progenies of the artificial crosses between populations made in the first. We’re talking about hybrids between related tree species. Genetic pollution squared. The authors point out that “restoration programs rarely use local hybrid individuals if a local species exhibits natural hybridization.” But they think they should.
…our current research based on ecophysiological measures of water-use efficiency under controlled conditions on seedlings from these populations suggest that hybrids deal with drier conditions better than either parental species, a trait that could be important for climate change adaptation.
So, should we be a bit more relaxed about genetic pollution? The debate can only intensify as the need for species and habitat restoration, and perhaps assisted migration, increases with climate change. Perhaps we should start by choosing a less value-laden term for it.
The benefits and disadvantages of the genetic mixing of two previously isolated populations will always depend on what our objectives are and on the status of the populations. If we are considering two viable populations with clear genetic differentiation, the genetic mixing of these two populations will probably only contribute to blur their genetic identity and may lead to loss of genetic diversity and perhaps to some outbreeding depression. For me, that is the starting point and the reason why managers should take careful consideration and have strong and previously contrasted arguments before acting in this direction. Having said this, I think there are several situations in which the genetic mixing of two population may be beneficial for certain puroposes, such as in the rescue of genetically depauperated populations or the implementation of reintroductions. Nevertheless, my feeling is that in some cases these actions are implemented before the populations are properly diagnosed or the circumstances surrounding each population are properly evaluated. This brings the subject to a different matter which is the proper selection of the populations. As indicated in the paper by Sexton et al. (2011) results on lifetime overall reproductive success of the descendants of these crosses greatly depended on the environmental conditions of the populations that were used in the crossings. Once again, there are evidences of conservation actions being taken using germplasm of inappropriate genetic origin where results are far from optimum from a conservation viewpoint. The great amount of economic resources involved in these operations may produce better results if applied to less media appealing in-situ conservation actions oriented to protect existing natural populations.
I agree with Jose that it all depends on the objectives. What are we trying to achieve? You mention regeneration of genebank accession and the possiblility of outcrossing between accession. This genetic pollution is bad because the objective of genebanking is to maintain the genetic integrity of the accession being regenerated. However for in situ conservation, I was previously of the opinion that we should only restore plant population from materials collected from the same area, but now I come to realise that making use of as much diversity as possible is actually not a bad thing and provides more “options” for the population to survive. Let nature decide what genes are good or bad. If a a material is unadapted it will die out any way. GMO is another thing though.
Whether gene flow is a “good” or “bad” thing – like other issues in conservation biology – can only be judged in the social science context. What is it that we as humans value? The hybridization between sea beet and sugarbeet that stimulated the evolution of the weed beet has increased biodiversity in terms of a new lineage that is now establishing in ruderal areas beyond beet fields, but is certainly a “bad” thing for the farmers that have had to cope with it. Hybridization between the red wolf and coyotes has probably kept red wolf alleles from going extinct while swamping the red wolf gene pool. Regarding “assisted migration”, my feeling is that we should be careful about what we wish for! For more thoughts see Ellstrand et al. 2010. “Got Hybridization? A Multidisciplinary Approach for Informing Science Policy” BioScience 60:385-388.