Microbial nepotism
The father helps the son to get a job in his company. The evil stepmother treats the stepchildren worse than her own. But should even protozoa prefer their relatives?
Nepotism is a feature of almost every human culture, and the tendency towards it seems to be ingrained. Even in many animal species, blood relatives stick together and stand up for one another. Especially when there is danger, older family members often sacrifice themselves for the younger ones if necessary. But for this, the relatives must first recognize each other. And that usually requires eyes, ears, a nose or at least the sense of touch - and ideally something like a brain that processes the sensory impressions and concludes: looks like grandma, smells like grandma - is grandma!
Do you really need the sensory organs and a complex nervous system to recognize relatives? The latest findings from a research group at Rice University in Houston now cast doubt on this.
The protozoan Dictyostelium purpureum, which belongs to the cellular slime molds, has been of interest to biologists for a long time due to its unusual way of life: On good days, the amoeba, which feeds on bacteria, lives alone in the forest floor and reproduces asexually by dividing. If there is a lack of food, thousands join together to form a gelatinous, mobile band that crawls around like a slug. The cell assembly then turns into a fruiting body, in which some of the cells develop into spores, while the other forms a stalk and capsule structure for them and then dies. The stalk increases the likelihood that the spore pods will snag on passing animals, which may carry them to more nutrient-rich areas. Given this behavior, the slime mold is also known as the social amoeba.
But Dictyostelium purpureum is not the same as Dictyostelium purpureum: There are several clones that have subtle genetic differences between them, while the amoebas within a clone are genetically exactly the same. The scientists working with Natasha Mehdiabadi now bred D. purpureum amoebas collected from various places in the forest in the laboratory, which probably all belonged to different clones. To tell the clones apart later, they tagged one in twos with a fluorescent dye, and then starved the two together.
As expected, the starving amoebas coalesced into fruiting bodies. However, in 12 of 14 experiments, the clones initially mixed but subsequently formed separate fruiting bodies.
Even when the biologists assembled so few amoebae that the cells from one clone alone were barely enough to produce fruiting bodies, they refused to cooperate. However, the separation was not quite as strict as in previous attempts.
Apparently the amoebas prefer to join with their own kind to form a fruiting body. This ensures that all spores belong to the same clone from which the cells are sacrificed to form the stalk. The protozoa are therefore careful not to lose their lives for strangers. A closely related species, Dictyostelium discoideum, certainly does. Here one clone often outperforms the other by producing more spores but contributing less to the stalk than the other.
Even without eyes, nose or ears - even without a mind - Dictyostelium purpureum recognizes Grandma, Grandpa and even Aunt Trude. How the single-celled beings manage to identify and bond with their kin while genetically rejecting cells that are only slightly different remains to be understood.
