© M. Dawson
Patrick McGoohan in his role as the less-than-sentimental King Edward ‘Longshanks’ in the 1995 production of ‘Braveheart’ said it best in his references to the invocation of ius primæ noctis:
If we can’t get them out, we’ll breed them out
What a charmer.
Dabbling in molecular ecology myself over the past few years with some gel-jockey types (e.g., Dick Frankham [author of Introduction to Conservation Genetics], Melanie Lancaster, Paul Sunnucks, Yuji Isagi inter alios), I’m quite fascinated by the application of good molecular techniques in conservation biology. So when I came across the paper by Fitzpatrick and colleagues entitled Rapid spread of invasive genes into a threatened native species in PNAS, I was quite pleased.
When people usually think about invasive species, they tend to think ‘predator eating naïve native prey’ or ‘weed outcompeting native plant’. These are all big problems (e.g., think feral cats in Australia or knapweed in the USA), but what people probably don’t think about is the insidious concept of ‘genomic extinction’. This is essentially a congener invasive species breeding with a native one, thus ‘diluting’ the native’s genome until it no longer resembles its former self. A veritable case of ‘breeding them out’.
Who cares if at least some of the original genome remains? Some would argue that ‘biodiversity’ should be measured in terms of genetic diversity, not just species richness (I tend to agree), so any loss of genes is a loss of biodiversity. Perhaps more practically, hybridisation can lead to reduced fitness, like we observed in hybridised fur seals on Macquarie Island.
Fitzpatrick and colleagues measured the introgression of alleles from the deliberately introduced barred tiger salamander (Ambystoma tigrinum mavortium) into threatened California tiger salamanders (A. californiense) out from the initial introduction site. While most invasive alleles neatly stopped appearing in sampled salamanders not far from the introduction site, three invasive alleles persisted up to 100 km from the introduction site. Not only was the distance remarkable for such a small, non-dispersing beastie, the rate of introgression was much faster than would be expected by chance (60 years), suggesting selection rather than passive genetic drift. Almost none of the native alleles persisted in the face of the three super-aggressive invasive alleles.
The authors claim that the effects on native salamander fitness are complex and it would probably be premature to claim that the introgression is contributing to their threatened status, but they do raise an important management conundrum. If species identification rests on the characterisation of a specific genome, then none of the native salamanders would qualify for protection under the USA’s Endangered Species Act. They believe then that so-called ‘genetic purity’ is an impractical conservation goal, but it can be used to shield remaining ‘mostly native’ populations from further introgression.
Fitzpatrick, B., Johnson, J., Kump, D., Smith, J., Voss, S., & Shaffer, H. (2010). Rapid spread of invasive genes into a threatened native species Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0911802107
Lancaster, M., Bradshaw, C.J.A., Goldsworthy, S.D., & Sunnucks, P. (2007). Lower reproductive success in hybrid fur seal males indicates fitness costs to hybridization Molecular Ecology, 16 (15), 3187-3197 DOI: 10.1111/j.1365-294X.2007.03339.x