Creature Feature. Two is company

What do you think about when you travel through the vast expanse of a desert? The long passage of time, maybe, when you see how rocks have steadily been worn down to become an ocean of sand. Bleakness, perhaps, when you wonder how much of a struggle it must be for a plant or an animal to persist in a place so dry, so hot, so cold. Trepidation, when you consider how to shield yourself from the prickliness of seemingly every surface in this environment. But travelling through the desert some days ago, I came to realise that feeling small, sad, and scared is but one response to this landscape, and not a necessary one. Having learnt about the desert-dwelling Joshua tree and the moths that pollinate its flowers, the desert now makes me think about dependence.

Spiky clumps positioned precariously at the ends of contorted branches, the Joshua tree cuts an astonishing figure against blue skies and beige rocks. Every few years, hefty clusters of white flowers bloom amidst the spikes, simultaneously festive and unsettling. And yet, like every other living organism, this tree’s unusual form is shaped by a combination of chance and natural selection. But in the case of the Joshua tree, one particular creature — the yucca moth — has an outsized role in how this tree came to be.

The Joshua tree is one member of a group of plants called yuccas that are found in Central and North America. Throughout their range, yucca plants are partnered with species of yucca moths, and the two are indispensable partners to one another. Female yucca moths have evolved specialised mouthparts that they use to carefully collect, store, and transfer pollen from one yucca flower to another, thus fertilising the yuccas and enabling their seeds to form. Before enacting this transfer, however, the moth also extends her ovipositor into a yucca flower, laying her eggs within it. In a few days, these hatch to release caterpillars, which rapidly consume the seeds growing around them. Having eaten their fill, they head towards the ground to build cocoons, become pupae, and eventually transform into adults. But not every seed is eaten by the caterpillars, and so this mutually supportive interaction gives rise to the next generations of both the yucca and the yucca moth.

The dependence of the yuccas and the yucca moths on each other is a finely balanced affair. If the yucca moth has too short an ovipositor, she will not be able to lay her eggs in the best part of the flower. Too long an ovipositor, and she will damage the flower’s ovules, the eggs that turn into seeds after they’ve been fertilised with pollen. Once damaged, the flower will drop, and neither yucca seeds nor yucca moth caterpillars will grow. This tight relationship is best observed in the closely related western and eastern species of Joshua trees, which are pollinated by two different but closely related species of yucca moth. The western form of the Joshua tree has longer flowers, and the moths that pollinate them have correspondingly longer ovipositors.

Given that yuccas and yucca moths have relied on one another for 40 million years, and given that new species of both the plants and the insects have arisen in this time, it is worth wondering if these species have arisen together. In other words, if a population of yucca plants splits into two distinct species, do the moths pollinating them simultaneously also split into two species? In the case of the Joshua trees described above, the answer seems to be yes. But look a bit more closely, and the story gets complicated. On the one hand, it is true that the flowers of the two Joshua tree species and the ovipositors of the two moth species are more different from each other than are any other parts of the plants or moths, suggesting that the close linkage between flower and ovipositor is what drove these populations to become new species. On the other hand, estimations of when these species formed suggest that the tree species diverged from one another a few million years before the moth species did. So what spurred the split of the Joshua tree into two species? Did it begin with the evolution of differences in the shape of flowers, or was that a later development? What happened to the trees’ close relationship with the moths when this split began? How quickly did the moths and, specifically, their ovipositors evolve following the split of the trees?

These questions remain unanswered. Because evolutionary biology is a science that builds an evidence-based picture of the past, and because sometimes we don’t have perfect or perfectly interpretable records of plants’ and animals’ histories, these questions remain unanswerable for a while yet, or forever. Looking more broadly at other yuccas, we see that the associations between these plants and their pollinators are messy at best. New species of yuccas and yucca moths form together, sometimes. Pollinators switch from one species of plant to a distantly related one, sometimes. So it seems that this dependence can be complex and counter-intuitive. Nevertheless, these creatures’ persistence means that their interdependence has not been fickle — like desert sand, it both shifts and endures.

Ambika Kamath

Ambika Kamath is a behavioural ecologist, currently based at the University of California, Santa Barbara; Email: ambikamath@gmail.com

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