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Photo: USDA/bugwood.org |
A recent article in Science magazine1 describes the mechanism by which plants of the genus Striga, including the pretty, lavender-flowered witchweed, live parasitically off other plants.
The host plants, including crops like cotton, corn, and sorghum, release tiny amounts of chemicals called strigolactones into the soil as they grow. For the host, these chemicals are both a growth hormone stimulating the root system and also lures to attract and stimulate fungi, mycorhizzae, that provide nutrients from the soil like nitrogen and carbon. Any Striga seeds which lie dormant nearby also sense these chemicals, usually at lower concentrations than the host plant and fungi can detect. The strigolactones stimulate the witchweed seeds to germinate and then send tendrils to penetrate the host’s root system, sucking out nutrients before they reach the host itself and so blighting the crop. Each witchweed plant then produces up to 100,000 tiny seeds that disperse into the soil and wait, sometimes for decades, until another host is planted.
If that mechanism is not the perfect definition of botanical vampirism, I don’t know of one that would fit better.
Striga is a problem in Africa, where the plant apparently evolved in relation to sorghum, but it also affects crops in Europe and the United States, although to a lesser extent. The article in Science describes how researchers are studying the genes for strigolactone receptors in an effort to control infestations of witchweed and similar parasites.
As I read the article, it occurred to me that a 19th-century naturalist might have learned of such a relationship between host and parasite and then meditated on the grandeur and intricacy of God’s creation. My own reaction was similar but deflected. I meditate on the power of evolution represented by the DNA molecule and how persistent, random mutations can lead one plant to detect the growth hormones of another and turn them to its own uses. The difference, the deflection, is that I cannot think of a god or any intelligently motivated designer who would think up such a horror as witchweed, digger wasps—which lay their eggs in the larvae of other insects—and similar cases of parasitism and still consider them part of a benevolent world. A sensible god wouldn’t make witchweed, because it adds no glory or benefit or abundance to the world. A random force like evolution would make witchweed for the simple reason that it can.
This isn’t symbiosis or communalism or any of those beneficial relationships we learned about in high-school biology. There, for example, the bacteria that live in my gut consume undigested carbohydrates in the food I eat and benefit from the warm, moist environment I provide, while I benefit from their chemical processes that manufacture some of the vitamins I need, support my immune system, and protect me from more harmful bacteria. In contrast, the witchweed does nothing for the corn or sorghum plant but instead robs it of the nutrients its roots work to obtain from the soil and diverts them to the witchweed’s own use.
This is like the parasitism of the bark beetles, corn borers, and other pests which burrow into a plant and feed on its substance. We humans can understand this kind of attack, however, because it’s not too different from a logger cutting down a living tree for lumber or a farmer stripping the ears off a corn plant for food and then bailing the stalks for animal forage or burning them for fuel. In the wilds of nature, every animal feeds on either plants or another animal species, and the plants feed on the soil. This seems right and natural. The only time we humans get upset about this arrangement is when the bark beetle kills trees we plan to take for lumber ourselves or ones we cherish for their beauty and their shade, or when the corn borer robs us of a crop we plan to eat.
But I find the parasitism of the witchweed chilling. It’s not a member of the animal kingdom, like a hungry beetle or a caterpillar that will one day grow into a pretty little moth. This is one plant feeding on another, and not like a fungus growing on the underside of an already dead log. This is Dracula sucking the blood of a sleeping maiden.
In most cases of parasitism, we can eventually hope to achieve a state of balance. If the corn borer worm kills off all the corn in this field and the next, then the moth has no place to lay her eggs and the local population of the species Ostrinia nubilalis dies out. If the Ebola virus, which hijacks a cell’s genetic machinery to make copies of itself, is so aggressive that it bursts the cell membrane and then destroys so many cells that the human host dies, that particular strain of the virus dies out unless it can infect another host within hours or a few days, depending upon conditions. Most successful parasites either throttle back their aggression so that the host lives—think of the common cold, which manages to infect us again and again with new strains—or the parasite maintains a life cycle slightly less prodigious in terms of physical size or population than the host’s, so that new victims are always available.
Witchweed avoids this limitation by sowing seeds that can lie dormant for decades. If it kills off all of its hosts in the field, it can wait patiently until a new host arrives tens of generations later. This is the vampire that can sleep in its crypt for, comparatively, centuries.
I challenge any incipient gods out there to conceive of a set of tricks like this: mimicking the chemical receptors used by another plant, converting those chemical growth signals into both a trigger for germination and a tracking device to guide the parasite’s root structure sideways toward the host, and finally adapting its needs so that it can feed on nutrients manufactured by another species. Now couple all that with the ability to produce tiny seeds that can lie dormant for decades. It’s a lot to accomplish through random mutation and genetic drift.
Considering the complexity of the witchweed’s development, one might almost be tempted to think that an inventively inclined god must have had a hand in it. But as a convinced evolutionist, I know that for every Striga alive and functioning in the world today, millions of generations existed before it that had none, or one, or only some of these characteristics. The genus Striga had all of history since the first seed-bearing plants developed, about 390 million years ago, to learn its tricks. Some of the characteristics may have been immediately useful, as soon as the genetic mutation allowed them to occur. Others may have lain dormant inside the plant’s genome, like the seed waiting in the soil, until this mutated protein became useful in combination with those others to create one of the witchweed’s survival tricks.
Evolution is a long, slow process. It is not an act of creation but rather of accretion, of putting together genetic puzzle pieces, and adaptation, of leveraging the resulting proteins when accident finds a fit with the natural surroundings. If that fit works and contributes to the organism’s survival, then the genetic configuration and its protein products are preserved and become the stem for more random revision and development. The only rule concerns what can or cannot survive, given the current environment and its available opportunities.2
Working on evolutionary principles, the universe presents us with more possibilities than any sentient intelligence, no matter how powerful, can ever imagine. The universe is full of surprises. And that gives me hope, even if some of the surprises turn out to be nasty ones, like the vampiric witchweed.
1. See “How crop-killing witchweed senses its victims” by Elizabeth Pennisi, Science, October 9, 2015.
2. For more along these lines, see Evolution and Intelligent Design from February 24, 2015.