Frontiers in desert tentacle sex

In the course of writing a piece this week for KCET on the possibly upcoming Joshua tree mating season, I found a paper entitled “Asymmetric hybridization and gene flow between Joshua trees reflect differences in pollinator host specificity,” published in the most recent issue of Molecular Ecology*. You know the feeling you get when your area of expertise gets upended and you have to fit a new, fun piece of information into your worldview? This paper did that for me.

There are a couple of things  you need to know about Joshua trees to make sense of the paper.  First is, as I said in the KCET piece this week, the intimate relationship between the tree and its insect pollinator:

 In years where the Joshua trees have gotten enough winter water and a good freeze, some of the leaf buds at the ends of those ungainly branches will turn into flower buds. Between late February and April, those flower buds will open into big, tight clusters of cream-colored flowers.

At about the same time, legions of little dusky yucca moths will emerge from the ground and flit around the blossoms. The moths mate, and then the females get to work pollinating the Joshua tree’s flowers. It’s a remarkable process: the females have specially evolved tentacles near their mouths, unique in the insect world, with which they collect pollen and pack it into little balls. They then move from flower to flower, take a bit of pollen off the ball and wedge it into the floral ovary, thus fertilizing the flower.

After pollinating each blossom, the female moth then lays a single egg inside the ovary. As the fruit develops, that egg hatches out and the moth’s larva starts eating the developing seeds. By the time the Joshua tree’s dry, husky fruit is ripe that larva will have eaten about a fifth of the seeds in it, whereupon it emerges from the husk, falls to the base of the tree, and then burrows into the ground to pupate.

The Joshua tree can’t reproduce without the yucca moth. The moth can’t reproduce without the tree.

An interesting point I mostly left out of the KCET piece: this behavior on the part of the moth is unusual in the insect world. Most insect pollinators transfer pollen more or less inadvertently: they accidentally collect pollen on their bodies as they visit a flower for one reason or another, and then they accidentally leave some of it in the next flower they visit. Yucca moths’ behavior, by contrast, is volitional — assuming we can ascribe volition to an organism with the approximate mentative power of a keychain LED torch. The female yucca moth carefully wraps pollen into a ball with her maxillary tentacles, then just as carefully takes pieces of that ball of pollen and wedges them into clefts in various flowers’ ovaries.

The other thing you need to know is that there are two kinds of Joshua tree. Current taxonomical opinion has it that these two forms are best described as subspecies. The Joshua tree’s Latinate binomial is Yucca brevifolia; the larger subspecies, which grows in the western part of the Mojave near Los Angeles (and also right outside my window here) is Yucca brevifolia brevifolia, and the smaller-statured subspecies that grows to the east in the Colorado River drainage is Yucca brevifolia jaegeriana, named after pioneering desert botanist Edmund Jaeger. “Brevifolia” means “short-leaved,” and so you might expect the western subspecies — the “short-leaved short-leaved yucca” — to have shorter leaves than “Jaeger’s short-leaved yucca.” But Yucca brevifolia brevifolia actually has leaves twice to three times the length of those of its sister subspecies.

Like I said, it’s roughly the consensus that the eastern and western Joshua tree populations are subspecies, but in 2007 Lee Lenz of the Rancho Santa Ana Botanic Garden in Claremont, California suggested that the two populations be declared full species in their own right: Yucca brevifolia for the western trees and Yucca jaegeriana for the eastern.

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