Volume 41, Number 2,
June 1995:
The Yucca Plant and
the Yucca Moth

Text-only version

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ABOUT THIS ISSUE
- about KSN
- about the authors

IN THIS ISSUE
- introduction
- mutualism
- coevolution
- a "transparent" system
- yucca moth pollination
- male and female yucca moths
- mark and recapture
- C.V. Riley
- the yucca plant
- yucca flowers
- yucca products
- yucca pods and larval moths
- bailing out of the pods
- old pods
- what we do not know
- what prevents a cheater?
- how did the yucca and yucca moth relationship evolve?
- solving problems
- for additional information

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The Yucca Plant and the Yucca Moth
by Marylee Ramsay and John Richard Schrock

HOW DID THE YUCCA AND YUCCA MOTH RELATIONSHIP EVOLVE?

The first fossil flowering plants appear in rocks formed 90-120 million years ago. Flowers improved a plant's ability to reproduce with variation. There were no pollinating bees and wasps, moths or butterflies prior to flowering plants, and the first pollinators were beetles. The gradual improvement in flowers' ability to attract insects, and the modification of insects to better pollinate flowers resulted in an evolutionary explosion in flowering plants and pollinating insects. North America, at that time, was a major site of flower evolution and the coevolution of the yucca and yucca moth began in southwestern North America.

The beneficial relationship has probably evolved recently in geological time. The primitive yucca plants most likely relied on wind to distribute pollen, fertilize flowers and produce seed. Today, only the moth can do this job.

There are close relatives of the yucca moth that mine the vegetative tissues of the plant; they are yucca "pests" and provide no benefits to the plants. The ancestors of the yucca moth almost certainly began as harmful feeders on yucca tissues but converted to feeding on seeds and eventually took over the pollination duties.

One possible scenario follows. The ancestral yuccas were plagued with small moth caterpillars that fed inside plants shoots. As with modern moths, there is some variation in each generation, and a few eggs are laid beyond the stems on blades and flower parts. Eggs laid in fertilized flowers discovered an untapped developing supply of seeds rich in protein, and their young survived in high numbers and reinforced this population of flower-inhabiting larval moths. The variant larval moths that ate seeds added a burden to the plant, but moths that moved from flower to flower also carried pollen with more accuracy than casting pollen to the wind. Such a tradeoff, perhaps only slightly in the plant's favor at first, became even greater as moth variants became more skillful at transfer of pollen, especially by selection for palps and behavior to comb the yucca pollen from anthers. Meanwhile, the yucca could save much energy by forming pollen that is gummy rather than fine and wind dispersed. To evolutionary biologists, confirming this sequence remains an exciting problem.