Survival is a fierce and relentless business. And the strategies for survival are many.
Parasitism – in which one organism benefits at the expense of another – is one of the most ancient strategies for survival. Across the plains, deserts and mountains of West Texas, organisms take advantage of this tactic.
One example is the parasitic plant known as dodder. Studying this native plant carries us into a world of complex adaptations and invisible chemical cues.
It will invade shrubs, herbaceous plants and trees. And on agricultural and horticultural crops – alfalfa, tomatoes, flax, trumpet vine – it can be devastating. With leaves like tiny scales, it appears as a mass of tangled, hairy vines. For these reasons, it's been called “strangleweed,” “devil's guts” and “witch's hair.”
There are more than one hundred species of dodder – most of which are part of the genus Cuscuta. Several species are native to West Texas.
Photosynthesis – the conversion of light into chemical energy – is the fundamental process for most plants. Dodder plants are virtually incapable of photosynthesis.
Michael Nickell, museum scientist at the Sibley Nature Center, said this makes parasitism obligatory for the dodder plant.
“The cells of the dodder plant have very little if any chlorophyll,” Nickell said, “so they don't have the ability to really undergo photosynthesis. They're dependent on finding a host plant.”
Dodder is a flowering plant that produces large quantities of seeds. Those seeds can lie dormant on the ground for years. When the conditions are right, the seed germinates, and establishes a root system.
Now the clock is ticking. Without a host, the plant will die in a week or less.
Dodder has a fine-tuned ability to locate a host. Picking up on chemical cues released by nearby plants, the dodder will grow rapidly in the direction of a likely host.
“All living things have a chemistry about them,” Nickell said. “The dodder are going to have receptors to the chemicals that these host plants are giving off. So they're going to grow in the direction of these host plants. It's very complicated chemistry.”
Research shows that dodder can tell the difference between host plants. In a 2006 study, dodder seedlings placed between tomato plants and wheat plants grew towards the tomatoes. Those plants had more to offer in the way of nutrients and minerals – and the dodder could sense that.
When it reaches its destination, the dodder begins to twine and wrap itself around the host. Then it begins its parasitic work.
“The dodder produces these other root-like structures that are called hastoria,” Nickell said, “which will penetrate and dig into the vascular tissues of the plant. It will be getting carbohydrates and minerals and water coming from a new host.”
The dodder abandons its own roots. The host is compromised on multiple fronts.
“Not only it is being directly sapped,” Nickell said, “but [the dodder] is also making it more vulnerable to other types of infections, like bacterial and viral infections. The dodder can grow over multiple plants. The dodder has the ability to transmit the viruses of one particular plant into another plant.”
The host may emit chemicals designed to fight off the invader. But an encounter with dodder is often terminal.
Nickell compared dodder to another native parasite – mistletoe. In contrast to a perennial parasite like mistletoe, many species of dodder are annual, and are “parasitoids.”
“With a parasite, you're obtaining multiple meals from a host,” Nickell said. “You want to keep your host alive – that's your bread and butter. But with a parasitoid, it's more dire consequences for the host. It's going to be one host, and that host is going to sacrifice its life unwillingly for that parasitoid.”
Nickell said that parasitism like the dodder's is a ubiquitous feature of the natural world.
“It goes back a long ways,” he said. “It's actually one of the more common ways in which organisms make a living. There are a tremendous number of parasitic relationships. It's not the exception.”
It's a rough business. And from alfalfa fields to creosote flats, parasitic dodder has staked its claim on the West Texas landscape.