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Friday, April 10, 2009

The humble origins of yet another obsession

This is the first in a series of entries about my fascination with a spider found on the Link Trail. If you're not interested in population genetics, give this one a miss.

Up until July I'd never heard of Enoplognatha ovata. Now I'm not only reading research articles on the little suckers, but getting ready to contact some of the folks who wrote those research articles so that I can better understand the population genetics involved. Oh well, anything for a weird life.

Back in July I saw a lot of spiders on the section of trail south of Freber (aka Hodge) Road. At the time I thought that most of them were the same species. After taking a look at the macro shots, I saw the differences in coloration and laughed at my silliness. But after I spent some more time looking at the shots and trying to identify them in BugGuide, I was startled to find that I was right the first time: they were the same species!

Since the female E. ovata shelters her egg sac in a rolled-up leaf, her species only lives near foliage that it can curl up just so. Those kinds of leaves grow on the blackberry bushes along the Link Trail - and in the hedgerows of Nidderdale, a town on the far side of England in Yorkshire. As it turns out, researchers as far off as Nidderdale and as nearby as the Syracuse University research station in Lafayette, New York have been puzzling over E. ovata for decades. And the reason they're so interested is the same reason I thought I was seeing different species.

E. ovata has a striking color polymorphism, which is a pretty way of saying that a spider can have one of several distinct color patterns that are determined by the color genes it got from its parents. Most of the spider's body is a creamy white, yellow, or greenish color. The Lineata morph has no additional color. Redimita individuals have a pair red stripes running down their backs, and in the Ovata morph there's no gap between the stripes: the entire back of the abdomen, i.e. the dorsal opisthosoma (god, I love saying "dorsal opisthosoma") is red. The individual in the lower right corner of the composite image above is Redimita. The other two are Lineata. I didn't see any Ovata, and I'm not likely to: none of the researchers found any in this part of New York State.

Now here's the hook: Although the gene for Ovata is dominant over that for Redimita, and both of the red morphs are dominant over Lineata, percentages of the three morphs in natural populations are the inverse of what one might expect. Most surveys show somewhere around 70% Lineata, 25% Redimita, and 5% Ovata. And these proportions are remarkably stable within almost every local community sampled. This piqued my curiosity. It did the same to a lot of researchers. In the posts to follow I'll be telling you about their findings.

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