Thursday, 14 May 2009

Outbreeding Depression in Arabian Oryx





As a reminder, if you need any definitions for terms, check out the link to my glossary in the righthand bar!

I've previously mentioned Outbreeding Depression, and given you a bit of a mechanism for it, in a previous post. However, outside the ibex example, there's not many good illustrations of the power of outbreeding depression in animals. Most of the good examples exist in plants, which are considerably easier to study.

I'd like to discuss one of the other examples of outbreeding depression, which was stumbled upon when conservation managers tried to save the Arabian Oryx. The advent of repeating rifles, motorized transport, and access lead quite a few Arabian rural users to dramatically over-harvest the Arabian Oryx, in addition to several species of Gazelle, Ibex, and the Arabian Tahr. All of these species were rendered endangered, and the Oryx was made completely extinct in the wild. Phoenix Zoo in Arizona instituted rescue breeding of Arabian Oryx to great success, and reintroductions began in 1980s. This is where the study of Marshall and Spalton (2000) really kicks in.

There's invariably some level of juvenile mortality, especially in that are reintroduced to the wild, and as part of on-going research to fine tune the reintroduction of the Oryx, they collected tissue and horn material post-mortum; additionally, they collected blood from live animals whenever possible. They went through and extracted DNA from the materials, and genotyped them at 6 microsatellite loci. Remember that microsatellites are meaningless repeat units in DNA that drift in frequency `randomly.`

Marshall and Spalton computed a few correlates of inbreeding and outbreeding - Heterozygosity (which I've previously defined/explained) and d^2, which is a measure of outbreeding, and it's the difference in size of alleles within an animal (measured in repeat units), squared. So if one animal has a 200 long allele at a loci that can grow or shrink by 2 units at a time, and a second allele that is 204, it is 204-200=4/2 and then 2^2=4. Sorry about the math! They also looked at protein content, and a few other factors

I'll spare you the statistical model they applied to see the effect that inbreeding and outbreeding had on mortality, but they found that analysed alone, neither seemed to be a major factor. That is to say, they couldn't detect it. In a low sample of 57, this isn't terribly surprising. But when they considered both together, they found both were significant factors.

Wait, how can you be both inbred and outbred? Well, to answer that, we need to go back to the re-introductions. The Oryx reintroduced into the Oman refuge, you see that they came from very different populations, and 5 of them. The unrelated populations can lead to intragenomic issues, when one part of their DNA is telling them to do one thing, and another part of their DNA is telling them to do another - they're not co-adapted. Alternatively, they could have traits from animals that might be really good in Saudi Arabia, but are terrible for Oman.

To add inbreeding on top of that, all you need to do is take these already out-bred individuals and have them mate with close relatives. Broken bits of DNA will accumulate as they always do, while they continue to have problems with the rest of their working DNA complement.

There's a few lessons we can learn from the Arabian Oryx, relating to over-harvest, land management, and so forth. Those are probably the issues that are going to doom the Arabian Oryx (if they're ignored), but let's just ignore them for the moment and focus on conservation implications. First, we need to take greater care to maintain better lines of animals. This is difficult, as we simultaniously seek to keep fairly diverse, non-inbred animals. Finally, Introductions need to be large enough that inbreeding is statistically unlikely.

(Photo credit: Wikipedia)
Cite: Marshal T.C. & Spalton J.A. 2000. Simultaneous inbreeding and outbreeding depression in reintroduced Arabian oryx. Animal conservation, 3, 241-248

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