Unique Relationship Between Joshua Tree and Moth Documented for First Time
By Edward Ricciuti
Rubbing elbows with UFO buffs snooping about the perimeter of nearby Area 51, scientists have set up shop in the Nevada desert near the town of Rachel, “UFO Capital of the World,” seeking to solve a mystery surrounding life right here on Earth: How did it become so incredibly diverse?
Camped out in the Mojave Desert’s Tikaboo Valley, along State Highway 375, the so-called “Extraterrestrial Highway,” these scientists are spying on covert activities of a non-descript gray moth that pollinates and lays eggs in the largest of all yucca plants, the Joshua tree. The lifestyle of the seemingly insignificant moth has profound ramifications for understanding the evolutionary processes that have made biodiversity blossom.
For more than a decade, Christopher L. Smith, Ph.D., and colleagues have probed the life-or-death link between insect and plant, which are so tightly bound together that neither can exist without the other. The moth (Tegeticula antithetica) is the tree’s exclusive pollinator, and the tree (Yucca jaegeriana) is the moth’s only host, producing the seeds eaten by its larvae. Their partnership is a classic example of a phenomenon called coevolution, a reciprocal adaptation between two or more species and a continual circle of evolutionary feedback believed to be an engine driving biological diversity. Charles Darwin first theorized about the role of coevolution and scientists like Smith still search for whether it indeed improves the fitness of organisms and leads to new species.
Smith, an evolutionary ecologist at Willamette University, is senior author of a paper describing research that is another step toward understanding the nuts-and-bolts mechanics of coevolution by the moths and their host trees. The research was published in March in the Annals of the Entomological Society of America. Several other species of yucca have a coevolutionary relationship with specific months but none more finely tuned that that of the Joshua tree.
The paper reports on efforts to figure out if the respective anatomy of the reproductive machinery in moth and tree have an evolutionary correlation. If so, which had initial impact on the other? In other words, did the design of the bolt shape that of the nut, or vice versa?
Trademark plant of the Mojave, with a lifespan lasting hundreds of years, the Joshua tree prefers uplands, seldom below 1,000 feet in elevation and sometimes up to six times that. The lowlands of Death Valley region geographically separate two groups of Joshua trees: the tall, truly tree-like western and the bushy eastern. Until 2003, scientists assumed one species of moth, Tegeticula synthetica, partnered with both trees. It turned out that the eastern tree had its own genetically distinct moth species, the six-millimeter long, look-alike T. antithetica, and that a few millimeters difference could cause changes in moth behavior that, while seemingly miniscule, could impact the evolution at a species level. Recently, some scientists have proposed that the trees so closely adapt to their own moth species they, too, are species in their own right.
A strip bordering both sides of Highway 375 is the preeminent of only two places where both moths and both trees meet and mingle, creating a unique, ready-made laboratory for scientists. Smith’s study focused specifically on how, during spring bloom, the female T. anthithetica moth transfers pollen from one Joshua tree flower to another and lays its translucent, white eggs near the flower’s fertilized ovules. The only other known observations of such moth behavior are notes and drawings made in 1893 of the other Joshua tree moth, nine-millimeter-long T. synthetica.
The Joshua tree is among those plants that rely on insects to reproduce because its pollen is too heavy for wind dispersal. Typically, pollinators such as bees pick up their cargo at the anther, the tip of a flower’s male part (stamen), as they search for nectar. They unload pollen at the top of stigma, a knob atop the bottle-shaped female part (pistil). Below the stigma, a tube called the style leads to the bulbous ovary, containing the flower’s ovules, at the base.
There is nothing accidental about the way these moths deliberately pollinate their host plants. Nectar, moreover, has nothing to do with it because the tree produces none and the adult moths do not feed. Instead of a tongue for sipping sweets, female T. anthithetica moths have tentacles extending from their mouthparts, used to scrape off and ball up pollen from an anther into a golden ball. The moth tucks it under her chin, carries it to the stigma of another flower, and distributes it there with her tentacles.
When ready to lay, the moth pierces the style just below the stigma with its stiletto-like ovipositor, which places eggs inside the style’s central canal, near the ovules. The plant reciprocates by providing room inside the flower for the month’s larval caterpillars to form and, on hatching, board when the ovules develop into seeds.
Smith explored fine points of the process, including measuring the exact placement of the moth’s eggs, comparing the information with that contained in the 1893 account of T. synthetica. Through a slit they cut through the style of the developing, cream-colored eastern Joshua tree flower, the researchers spied on T. anthithetica, observing and filming the moth in action for the first time.
“It was not known before where moth places its ovipositor and where the eggs are deposited,” says Smith.
Earlier research shows that the length of the style—slim long-necked in the western tree, squat and short-necked in the eastern—matches that of the respective moth’s ovipositor. The arrangement seems logical. Inserted near the tip of the eastern tree’s style, T. anthithetica’s hard, pointed ovipositor may be just long enough to place eggs near the ovules but not close enough to damage them. Measurements described in the paper show that when eastern moth tries to lay eggs in western tree, as happens in the Tikaboo Valley, its eggs are farther away from the ovules than when deposited in its preferred host. Smith and fellow researchers hope to determine if spacing virtually imperceptible without a microscope might be too far way for a hatchling larvae to reach its food. When the western moth lays eggs in the flower of the eastern tree, its longer ovipositor could possibly damage the ovules, conceivably causing flowers to drop off the tree. It is probable, but not provable, that the mismatch is a reason the moths reproduce successfully only when they visit their preferred tree.
Both moths perch atop the pistil when depositing eggs and pollinating, but their movements as they go about business differ, at least partly due to contrasts in the two flowers, which are still developing and almost closed when the moths visit. The eastern tree’s flowers are elongated, with long, narrow petals tightly wrapped around the pistil, leaving little wiggle room inside. The researchers found that T. anthithetica stays topside on the pistil. The broad petals of the rounded western flower arch over the pistil, providing enough space for T. synthetica to maneuver around and up and down the pistil. Cramped, T. anthithetica keeps its tentacles parallel to the style while packing pollen into the stigma. With more elbow room, T. synthetica holds them perpendicularly and draws them back and forth.
Smith and his associates recorded that when the female T. anthithetica arrives at a flower, it enters the small opening in the tip head first, crawls out and backs in again. For an hour, the moth alternates between pollinating and laying eggs. The moth aligns the rear of its abdomen with the stigma when depositing eggs. T. synthetica moves up and down the pistil between pollinating and egg laying.
Over millennia, Smith suggests, evolution of moth and tree may be in what he calls “an arms race” that caused the two Joshua trees to diverge, rather than the more obvious explanation of geographical isolation by the lowland barrier of the Death Valley region. The ancestral Joshua tree, he opines, might have had a short style. Trees with somewhat longer styles made it more difficult for caterpillars to reach seeds, providing protection of sorts and improving the tree’s reproductive success. Meanwhile, keeping up, moths that happened to have longer ovipositors could cope with the increased style length.
Until all of the theories can be proven by hard, cold science, it all remains speculation, even if probably on target. The evidence may be waiting in the desert near Area 51.
Annals of the Entomological Society of America
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.