“Share and share alike”, so goes the old adage about equally distributing goods, even at one’s own expense. Darwin’s idea of natural selection explained evolutionary change by posing that traits spread in the population due to the benefits these traits confer on their bearers. Individuals with certain characteristics produce more offspring relative to those individuals with other characters and as a result the population as a whole takes on a different appearance. However, Darwin’s idea couldn’t explain those traits that provide benefits to others at the expense of their bearers. Sharing with others at one’s own expense is called altruism and in biology altruism remained a puzzle for over a century after Darwin.
Then come the 1960’s and William Hamilton. Hamilton said that a rare gene underlying some altruistic behavior could spread despite the cost to the bearer. But how? How could a gene that results in fewer offspring for its bearer spread in the population? Lucky for Hamilton he had an understanding of genetic inheritance that was not available to Darwin. If behavior is directed towards those individuals in the population who also harbor the same genes for altruism then the trait will spread in the population through the recipients of altruistic behavior. Rare genes for altruism would more likely spread if there were a readily observable marker of the same altruistic gene in others. Richard Dawkins popularized this idea in the 1970’s calling it the ‘green beard effect’. In a hypothetical example Dawkins imagined that genes for altruism would result in a marker, such as a green beard, along with the altruistic behavior. With this clear marker of the altruistic gene in others helpers could direct their behavior only towards those that shared the gene for altruism.
Amazingly there is good evidence for the ‘green beard effect’ in nature in several organisms, from slime molds and fire ants. Even more astonishing, there are examples where a single gene is responsible for ‘green beard’ altruism. Scott Smukalla, Marina Caldara, and Nathalie Pochet of Harvard University and their colleagues report in the latest issue of the journal Cell that they have found a ‘green beard’ gene in the budding or brewer’s yeast, Saccharomyces cerevisiae.Yeast is not only the critical component in the making alcoholic beverages but it is also a classic model system in the study of the eukaryotic cell. Yeasts are single celled organisms but wild strains of Saccharomyces cerevisiae in times of stress will aggregate into multicellular mats often called biofilms. These aggregates of cells can protect cells from antibiotics, heat and cold stress, ethanol, and other toxins. The coming together of single yeast cells into a multicellular group is called flocculation and the aggregations are known as flocs. Occurring in wild yeast in response to stress, flocculation allows the population to ride out tough times.
Typical of many organisms grown under the resource-rich and stress-free conditions of the laboratory, times are seldom that tough and many years of culture in the lab have lead to the loss of flocculation in laboratory strains. Comparing a wild, flocculent strain called EM93 with a laboratory strain, S288C, incapable of forming flocs, Smukalla and colleagues found that flocculation fell under the control of a single variable gene called FLO1. This was confirmed by activating the expression of FLO1 in normally non-flocculent S288C cells. Expression of FLO1 resulted in flocculation exactly like that observed in wild yeast. FLO1 expression creates cell membrane proteins that allow cells to recognize and adhere to other yeast cells expressing the FLO1 gene.
FLO1 in Saccharomyces cerevisiae acts like the ‘green beard’ gene predicted by Hamilton as it allows yeast cells to detect others also expressing FLO1 and form multicellular aggregates and thus provide group protection against environmental toxins. But, remember altruism by definition involves a cost to the altruist. Where is the cost? When grown under toxin free conditions and ideal temperatures yeast expressing the FLO1 gene suffer a 4-fold reduction in population growth relative to yeast cultures that do not express the FLO1 gene.
A mixed culture of FLO1 expressing and non-FLO1-expressing cells grown under conditions that lead to flocs results in flocs containing primarily FLO1 expressing cells and free cells that do not express the FLO1 gene. FLO1 is therefore a true ‘green beard’ gene as it promotes the altruistic, social trait (flocculation) and at the same time excludes participation of those cells not expressing the social trait. Requiring FLO1 for cell adhesion eliminates the spread of selfish cheaters, yeast cells that forego the cost of expressing FLO1 while times are good but also reap the benefits of flocs when times are tough.
Research on the evolution of social, altruistic traits like flocculation can shed light on one of the most important transitions in the history of life, the evolution of multicellular organisms from single celled organisms. Saccharomyces cerevisiae, social amoebae, slime molds and many social bacteria move between a single celled and a multicellular lifestyle. Like Saccharomyces cerevisiae, multicellular forms in other microorganisms are often in response to stressful environments. Very early in our own evolution the colonization of harsh environments by our single celled ancestors likely promoted the same altruistic behavior seen in many modern microorganisms today.
S SMUKALLA, M CALDARA, N POCHET, A BEAUVAIS, S GUADAGNINI, C YAN, M VINCES, A JANSEN, M PREVOST, J LATGE (2008). FLO1 Is a Variable Green Beard Gene that Drives Biofilm-like Cooperation in Budding Yeast Cell, 135 (4), 726-737 DOI: 10.1016/j.cell.2008.09.037