When individual Dictyostelium cells sense they are running out of food, they cooperatively coagulate to form a slug. This mobile slug searches out more suitable conditions, ultimately raising itself up on a “stalk” formed by some of the selfless siblings. Atop the stalk, Dictyostelium spores form and are released, dispersing some lucky, individual cells that will flourish while their comrades wither in the sedentary stalk.
So-called cheater cells in the slug have a genetic mutation that allows them to ignore the stalk assembly signal--seemingly giving them the upper hand to become spores. However, the old adage “cheaters never win” holds true in this case because the mutation that allowed cheating, subsequently makes the cheater unable to effectively compete with other cells that will become spores. Watch the fate of “cheater cells” in this animation of the Dictyostelium life cycle. Credit: Zina Deretsky, National Science Foundation |
It is often assumed that much of biology can be explained by a need to produce offspring.
This concept is rooted in the theory that organisms are preprogrammed to reproduce and carry their DNA forward to future generations. So how does altruism, that is, the concept of self-sacrifice or a greater concern for the survival of others, persist throughout time? A Rice University research team funded by NSF’s FIBR program is piecing this puzzle together by studying a lowly, single-celled amoeba.
The slime mold, Dictyostelium discoideum, is normally found on the forest floor nestled among the leaves. When individual Dictyostelium cells detect they are running out of food, they cooperatively coagulate to form a slug. This mobile slug searches out more suitable conditions, ultimately raising itself up on a “stalk” formed by some of the selfless siblings. Atop the stalk, Dictyostelium spores form and are released, dispersing some lucky, individual cells free to go about life while their comrades wither in the sedentary stalk.
United We Stand, Divided We Fall
It seems logical. To realize its egocentric goal of self-propagation, a cell should prefer to become a spore, allowing others to self-sacrifice in the stalk. Some try to cheat the natural order, and do just that. Studying these so-called “cheaters” with the latest molecular biology techniques allows the Rice research team to probe the genetics underlying social behaviors and the evolutionary persistence of altruism.
Analyzing the fate of one type of cheater mutant shows that early in the slug stage, this cheater can position itself to become a spore because it is able to ignore a stalk assembly signal. This seems good for the cheater, and its ambition of being a spore should be realized. However, the old adage “cheaters never win” holds true in this case as the mutation that resulted in cheating subsequently renders the cheater unable to effectively compete with other cells that live by the rules in their quest to become spores.
The Needs of the Many Outweigh the Needs of the Few
This interdisciplinary team of scientists has also discovered what seems to be a new class of Dictyostelium cells that literally “throw themselves on the grenade” for the sake of the clan. These noble cells circulate within the slug soaking up toxins, only to be sloughed off the back of the slug with no hope of survival.
Such preset behaviors of Dictyostelium, proving detrimental to some individuals, unquestionably contribute to the survival of that species. Though such mechanisms for altruism in slime molds may not permeate the culture of other organisms, this FIBR project will undoubtedly provide starting points for future related research on other organisms.
