Too many chromosomes

Quaking aspen is my favorite tree. In the autumn its leaves change color to a warm yellow, making a perfect contrast against its white and black bark. At sunset the forest glows with the light filtering through the stands of this species.

summit clone

I never stop learning new things about it, either. On a recent hike on Mt. Bigelow in Arizona, I came across two aspen saplings, both growing in a sunny meadow that was heavily disturbed by a recent wildfire. The two trunks were only a meter apart, but one had leaves the size of a coin, and the other, leaves the size of a dinner plate (photo credit: Kristine de Leon).

aspen

The high level of trait variation in aspen is something that has always fascinated me, and has been the subject of a few past papers. But this dimorphism was beyond what I had seen before – the closest example I could recall was a clone growing in the shadow of a cabin in Colorado, where the shade leaves were large and the sun leaves were small. But not this big and not this small.

Then I remembered a fact that my friend (and fellow ecologist) Burke Greer once mentioned to me. Aspen is sometimes triploid. This means it can have three copies of each chromosome instead of two, probably an outcome of reproduction between normal haploid gametes with abnormal diploid gametes that failed to separate during meiosis.

Why should that matter? Well, three copies of each chromosome mean a bigger genome, and a bigger genome requires a bigger cell to fit it in. All else being equal, that could cause bigger leaves and larger growth. The relationship between triploidy, leaf size, and trunk size was long-ago recognized in a related European species Populus tremula, and subsequently also found in our North American species Populus tremuloides (e.g. Ynge Melander’s 1938 report from Sweden).

This variation in traits between diploid and triploid individuals presumably has ecological consequences as well: variation in growth rates, water usage, thermal tolerance, and so on. A recent study by Mock et al. was able to show that triploidy is widespread in North American aspens, but more common in colder areas where (presumably) sexual reproduction may be more difficult. Not, in other words, Arizona.

This led me to wonder: if I had a diploid aspen in one hand, and a triploid in the other, with very different performance characteristics, what were they doing growing in the same place? I don’t know. Two answers suggest themselves: first, that one of the two clones may soon die, ending a brief evolutionary experiment; or second, that they actually aren’t diploid and triploid at all. I didn’t have the cytotyping resources on hand to be sure, and didn’t think to bring back leaf samples on my pleasure walk.

So – the mystery will have to wait until the next time someone more curious and better equipped walks that ridgeline!

3 Comments Add yours

  1. monomiao says:

    But why suggested one of the two clones may have to die soon….. Couldn’t the evolutionary experiment continue? Will you revisit them next year? 🙂

      1. monomiao says:

        I look forward to reading another post next year!

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