Cheese Fungus Sexuality

Cheese, Blue Cheese, Fungus, Fungal Sex

A recent investigation of the fungus responsible for blue cheese starters indicates that this organism is capable or was recently capable of sexual reproduction. Such a discovery has exciting potential implications for cheese makers who might want to take advantage of this sexual reproduction to create improved fungal varieties for the creation of new cheese flavors.

The fungus Penicillium roqueforti (also known as “blue mold”) is used to ripen cheeses and confer the cheese’s characteristic flavor, texture, and appearance. Not only blue cheese, but also Gorgonzola and Stilton cheeses rely upon the action of P. roqueforti for these purposes. The fungal spores first settle on the outside of the cheese, forming the characteristic crust. Once established, the fungus begins to grow inward towards the heart of the ripening cheese, softening it and giving it a creamy consistency and its characteristic odor.

Each year France alone uses P. roqueforti to produce 56,865 tons of blue veined cheese. Astonishingly, the different strains of fungus used to produce this tonnage are genetically quite similar. This is because P. roqueforti reproduces asexually. Unlike with sexual reproduction, asexual reproduction does not involve the combination and re-shuffling genetic information from two genetically dissimilar organisms. Rather asexual reproduction means that cells of the fungus divide to produce more genetically identical cells. Asexual reproduction is completed either when part of the fungus is broken off to form two separate organisms, or when asexual spores are released.

The advantages of asexual reproduction are the speed with which the organism can grow and the how cheese-makers can be assured that spores from the same starter will all produce nearly identical results. However the lack of variation might also be seen as a disadvantage because it does not allow for much genetic variation. A lack of genetic variation means that one’s ability to create new, improved strains of P. roqueforti is limited to selecting for minor fungal variants that arise out of random mutation events.

However in a recent study it was discovered that perhaps the cheese-making fungus is indeed capable of sexual reproduction or at least was capable of sexual reproduction in the recent past. Several key points of evidence suggest this to be the case. First, researchers discovered that some populations of P. roqueforti showed a surprising amount of genetic variation. Of 16 possible allele combinations studied, 11 were found. A strictly asexually reproducing fungus would be unlikely to show such a high level of genetic diversity.

In addition, researchers found evidence for different mating types in P. roqueforti. Similar to male and female animals, fungi can exhibit two different kinds of sexual mating types—referred to as MAT1-1 and MAT1-2. The genes for conferring these mating types were found to still be intact. More than that, populations of P. roqueforti fungus were found to have a one to one ratio of the different mating types. Such an even balance between the fungal sexes indicates sexual reproduction. If populations only reproduced asexually, over time one of the two sexes tends to be completely eliminated from the population.

Finally researchers found evidence of repeat induced point mutations (RIP) within the P. roqueforti genome. These mutations indicate recent sexual reproduction events. Repeat induced point mutations have never been observed to occur without sex. However it is entirely possible that these mutations only indicate a recent past of sexual reproduction and that P. roqueforti has since lost this ability.

All of this evidence only indirectly points to the possibility of sexual reproduction in P. roqueforti. Nonetheless, the prospect that this fungus can reproduce sexually and thus be bred for desired characteristics ought to excite both cheese-makers and cheese-enthusiasts.

By Sarah Takushi

Sources

PloS ONE

New England Cheesemaking Supply Company

Cultures for Health

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