Fungal Discrimination

Meet Neurospora crassa, otherwise known as red bread mold. These delectable life forms— fungi, actually—interact with each other by fusing together. Sometimes fungal fusion is beneficial—such as symbiotic relationships with plants—and sometimes it’s destructive—think of plant or animal diseases. The thing is, this particular interaction is very selective. N. crassa only fuses with its own kind—genetically identical asexual spores, also called germlings. This behavior, known as “kind discrimination,” isn’t unusual in the microbial world, and protects the survival of these fungi out in the open world. But how do the fungi germlings communicate with each other to recognize their own kind?

“These genetically identical cells undergo a dialogue, alternately ‘listening’ and ‘speaking,’ which is essential for chemotropic interactions,” says UC Berkeley’s Louise Glass, lead author of a new paper in PLoS Biology about N. crassa’s communication.

But not all fungi ‘sound’ the same, says co-author Jens Heller, also of UC Berkeley. “It seems like all strains speak the same basic fungal language, but due to different dialects, some strains cannot understand each other, and therefore are unable to establish communication necessary for cell fusion,” he says.

The team found that the different ‘dialects’ are all located in a specific set of highly variable genes (called “determinant of communication” or doc genes). Slight genetic differences at these doc genes are all the fungi need to determine their own kind. These doc genes are known as green-beard genes—genes that stick out to the identical cells. The name was coined by Richard Dawkins to describe a model for the evolution of kind discrimination. According to this system, organisms must acquire three things—an arbitrary peculiarity (the “green-beard”), the ability to detect the green-beard on others, and the tendency to treat such green-bearded individuals preferentially.

Recognizing the doc genes in other N. crassa allows the fungi a type of long-distance kind recognition, which results in fusion, the team reports. “It was fascinating and surprising for us to see how well this kind discrimination system actually works,” says Heller.

“Since we know that programmed cell death can result from fusion of incompatible partners in N. crassa, choosing the right partner at a distance can be important,” says Glass. It also could be helpful for scientists to understand how fungi communicate and cooperate for destructive purposes, she says. “We have only scratched the surface on communication and interactions of these enigmatic organisms.”

Image: Roland Gromes