Category: DNA

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Watermelon: Out of Africa

Summer here in Rome tastes of watermelon. So, as the temperature outside hit the upper 30s today, it was great to sit in air-conditioned splendour in the office this lunchtime, eat a slice of cocomero and read a paper on the origin of the crop in the latest GRACE, which has just come out. Fenny Dane and Jiarong Liu at Auburn have looked in detail at chloroplast DNA from material collected all over Africa in an effort to reconstruct the history of both the familiar fruit (Citrullus lanatus var. lanatus) and the related tsanna or citron melon, which is a different botanical variety (var. citroides) of the same species. It turns out that the split of var. lanatus and var. citroides from a common ancestor (C. ecirrhosus, maybe) is ancient. The citron melon split off independently in the area of Swaziland and South Africa, while the wild precursor of the cultivated watermelon has its roots, as it were, on the other side of the continent, in Namibia. The picture below (courtesy of GBIF) shows why watermelon does ok in the Italian summer heat. Its natural habitat is pretty much desert (the record is for an accession in the US National Plant Germplasm System).

melon.jpg

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More on cowpea breeding

Hot on the heels of a recent nibble on breeding cowpeas for Striga resistance comes a paper in GRACE on the diversity being exploited by cowpea breeding programmes in the US and Africa. It turns out that these programmes are using non-overlapping sets of genetic material and that therefore

US and Asian breeding programs could increase genetic variability in their programs substantially by incorporating germplasm from West Africa, while national programs in West Africa should consider introgression of Asian germplasm and germplasm from other parts of Africa into their programs to ensure long-term gains from selection.

That’s what we mean when we talk about global interdependence in plant genetic resources, I guess. And that’s why the International Treaty was negotiated: to facilitate the exchanges of germplasm necessary to broaden plant breeding programmes worldwide.

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Is the kouprey a species?

The kouprey is a very elusive wild bovid that is said to roam the Southeast Asian jungle. It was only discovered by outsiders at the beginning of the last century, but it has seldom been seen since and there are concerns that it may, in fact, already be extinct. (It is currently listed on the IUCN Red List of Endangered Species). As if that wasn’t bad enough, there is now a vigorous debate about whether the kouprey is — or was — in fact, a separate species at all, or merely some type of feral hybrid.

A report published some months ago in the Journal of Zoology showed that a comparison of mitochondrial DNA obtained from several banteng revealed some homology — similarity — with a previously published DNA sequence from a kouprey. ((Galbreath et al., 2006. J. Zool. 270:561)) The banteng is another bovid that has to some degree been domesticated, but can also still be found in the wild in several Southeast Asian countries.  Based on their analysis, the authors concluded that the kouprey is more than likely just a hybrid originating from crosses between zebu cattle and the banteng. Now, some might argue that it is pretty bold to reach such a profound conclusion based on a very small sample size, and not surprisingly some have indeed so argued. They have pointed out that anatomical and even DNA evidence from their own studies did not support stripping the kouprey of its species status. ((Grigson, 2007. J Zool. 271:239; Hassanin and Robiquet, 2007. J. Zool. 271:246)) An alternative explanation might simply be that there has been introgression of DNA from one species into the other by occasional matings. And there is ample evidence that this has happened in other species. Cattle DNA is commonly found in the American bison, for example, because of past attempts by cattle breeders to generate what they thought would be more viable bison-cattle hybrids. ((Rasmussen et al., 2005. Mol. Reprod. Dev. 70:228))

What is clearly needed to settle the matter is additional analysis of nuclear DNA obtained from more than one kouprey. But if the animal has, indeed, already disappeared, that may prove to be difficult. ((Contributed by Michael Kubisch))