‘Family Roots’ – Altruism in Plants

‘Family Roots’– Altruism in Plants

J.B.S. Haldane is a giant of an historical figure. Once being described as having the appearance of an alert walrus (due in part to his thick mustache), he was one of the main architects of the modern synthesis. One story attributed to Haldane (whether correct or not is up for debate) finds him at a bar arguing with a group of colleagues about altruism. He is said to have hastily scribbled some calculations on the back of an envelope, after which he turned to the group to say, “I will lay down my life for two brothers or eight cousins.” So what did he mean with this enigmatic statement? Stephen Jay Gould (1979/2007) put it most simply in his essay on the subject. Suppose you and three of your brothers are out for a walk on a warm twilit evening when, from behind the hedges, a great beast scuffles onto the path in front of you. Its eyes gleam with ill intent, and it’s obvious the creature intends you and your brothers harm. You know that you’re faster at running than your brothers and could easily escape, leaving them to their fate. But you care deeply for them, so instead you create a distraction, allowing your brothers time to get away. You’re not so lucky, however, and you fall prey to the beast, having laid down your life for your kin. We, as a culture, tend to admire this sort of self-sacrifice, but although your courageous act saved the lives of your brothers, it obviously didn’t do you any good.   You didn’t live long enough to have children, so you’re not directly passing on any of your genes. How could this sort of behavior ever be naturally selected for?

There’s a reason why the other characters in the story are family members. Each of your brothers contains half (more of less) of the genes you have. When they have children of their own, their offspring will receive half of their genes, meaning the children will have a fourth of the genes you had when you were alive. Let’s say that each of your brothers goes on to have two children, for a total of six. That means that 150% of your genes end up being passed on to the next generation because of your one selfless act. According to this theory, the more related an individual is to you, the more likely you are to be altruistic toward them (at least in times of crisis), a process known as kin selection; hence the meaning behind Haldane’s cryptic statement and the underlying biological basis for the statement that blood is thicker than water.

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Altruism in animals has been well studied, and there are entire books devoted to the researchers who described the original biological theory back in the 1960s, including William Hamilton, John Maynard Smith, and George Price, who eventually took his own life after struggling throughout it to find to find a basis for the kindness he so eloquently derived in his mathematical formulae. Since the theory was initially postulated, instances of altruism have been found in several groups of animals. For example, vampire bats fly back to their caves at night to share the blood they’ve eaten with others who’ve had unsuccessful hunts (Wilkinson, 1988). Several types of mammals, including gazelles and monkeys, give loud calls, warning of predators nearby when it makes the individual who gave the alarm more likely to be caught. Ravens call to others of their species upon finding a carcass instead of keeping it to themselves, even though that means they might receive a smaller portion (Heinrich, 1989). Dolphins display several types of altruistic behaviors, including healthy dolphins swimming beneath sick individuals in order to keep them near the surface to breath (Connor and Norris, 1982). In ants, sterile workers live their entire lives caring for the queen and the nest. In fact, the Hymenoptera is riddled with this sort of enigmatic social behavior that seems to defy natural selection. It seems that no one, however, thought of looking for altruism in plants, likely because plants lack the sort of conspicuous behavior that can be easily observed and quantified. But in 2007, a group of researchers attempted to do just that in a small, unassuming plant that grows along the sandy shores of the Great Lakes in North America, Cakile edentula, or the Sea Rocket.

In order for plants to exhibit kin selection, they must first have some method with which they can identify their close relatives. This might seem like a difficult task for plants, which don’t have the senses we’re familiar with using in order to perceive our surroundings, such as touch, taste, and sight. But researchers are beginning to realize that plants have a remarkable array of senses that allow them to react to their environment. They know if they have neighboring plants based on the ratio of light wavelengths (Davis and Simmons, 1994), if a herbivore is munching on their leaves (Karban et al., 2000), and what types of pollen have attached to their stigmas, just to name a few. Plants can also send and receive signals through their roots, from plant to plant, or even to and from fungi (Moscatiello et al., 2010), and it appears that these signals can allow them to determine whether or not they’re related to the plants around them. When researchers Susan Dudley and Amanda File grew seeds of Cakile edentula from the same parent in a pot, the resulting plants allocated less fine root growth than did seeds from differing parents (less related). This suggests that plants can not only recognize their siblings, presumably via root exudates, but that they also make a concerted effort not to compete with them, since the more fine root mass a plant has, the more nutrients and water is has the ability to absorb. If these plants are sending signals through their roots, however, no one yet knows what they are.

Now, obviously plants can’t be consciously kind to one another, and this type of reduced antagonism among kin might actually benefit all involved, since competition in an environment might mean that a limiting nutrient might get spread in too small an amount among all individuals, possibly suppressing reproduction, in which case no one wins (Callaway and Mahall, 2007). But research into this question has literally just begun, and since altruism has played such an important role in the natural history of mammals, possibly even contributing to the formation of human civilization, it seems reasonable to expect its widespread occurrence in plants.

Citations

1Callaway, R. M. and Mahall, B. E. (2007). Plant ecology: Family roots. Nature, 448: 145-147.

Connor, R. C., and Norris, K. S. (1982). Are Dolphins Reciprocal Altruists?. The American Naturalist, 119: 358–374.

Davis, M. H., and Simmons, S. R. (1994). Far‐red light reflected from neighbouring vegetation promotes shoot elongation and accelerates flowering in spring barley plants. Plant, Cell and Environment, 17: 829-836.

Gould, S. J. (1977). So cleverly kind an animal. In Ever since Darwin: Reflections in natural history (pp. 260-267). London, Norton. (Original work published 1979).

Heinrich, B. (1989). Ravens in winter. New York: Summit Books.

Karban, R., Baldwin, I. T., Baxter, K. J., Laue, G., and Felton, G. W. (2000). Communication between plants: induced resistance in wild tobacco plants following clipping of neighboring sagebrush. Oecologia, 125: 66-71.

Mahall, B. E., and Callaway, R. M. (1992). Root communication mechanisms and intracommunity distributions of two Mojave Desert shrubs. Ecology, 73: 2145-2151.

Moscatiello, R., Squartini, A., Mariani, P., and Navazio, L. (2010). Flavonoid‐induced calcium signalling in Rhizobium leguminosarum bv. viciae. New Phytologist, 188: 814-823.

Wilkinson, G. S. (1988). Reciprocal altruism in bats and other mammals. Ethology and Sociobiology, 9: 85-100.

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