The secret sex of cheese

Mmmm...that is some sexy, sexy mold. (Image via Wikimedia)

Mmmm…that is some sexy, sexy mold. (Image via Wikimedia)

Are you grossed out by blue cheese? (I’m not, but I know many who are). Does that blue-green marbling of delicious fungus kind of make you gag? Well, this little factoid probably won’t help: there may be sex going on in that cheese.

Until pretty recently, a big chunk of fungal species were thought to reproduce without sex–until people really started to look. It turns out, there’s a lot more sex going on in the fungal world (on the down-low) than people thought. And that includes fungi that are used to make delicious blue cheese. Jeanne Ropars and colleagues in France, the home of Roquefort cheese, looked at the genomes of the mold species used in this particular cheese to see what kind of funny business was going on in their snack of choice. They found much more diversity than could be explained by asexual reproduction. And even more telling, the genes used by fungi to find mating partners have been kept intact and functional by evolution, meaning there’s probably some sex going on. The results were published November 21 in the online journal PLoS One (see link below).

Besides providing people with a lovely conversation starter at wine and cheese parties, why did these scientists want to know about the sex lives of their cheeses? Well, whether you like it or not, people love moldy cheese. It’s a huge industry; as pointed out by the authors of this study, France alone produces 56,865 tons of blue cheese every year. But with an asexually producing mold, there isn’t much room for innovation. Producing new strains of mold (in this case, Penicillium roqueforti) relies on luck. Cheese makers have to wait for random mutations to occur and then select for strains that have qualities they like.

Know what’s faster, easier, and more likely to result in new and improved strains? Sex! This paper offers the possibility that Penicillium roqueforti, the fungal species used in most blue-veined cheeses, can use sex for reproduction. They definitely haven’t proved that, but they do present several lines of evidence in support of sex.

First, all the genes needed for sexy-time are there. The researchers determined this by sequencing the whole genome–all the DNA–of one strain of this species. The genome isn’t available yet, but I assume their getting it ready for publication separately. In addition to this one strain, they also collected strains of the mold from all over the world and compared their genes. Many strains were provided by cheese producers, others were strains from non-cheese environments, and a bunch were isolated directly from cheese.

The genes needed for sex include all the machinery used for meiosis and recombination and, importantly, the mating-type gene MAT. MAT genes are responsible for making sex pheromones to attract a partner of the opposite mating type.

Each individual clone of P. roqueforti had only one of the two possible mating types in its genome. This is usually the case with fungi that reproduce sexually and have two mating types. You need an individual with mating type 1 to meet up with another individual of mating type 2 in order to induce sex. And guess what? In some of the cheeses they investigated, molds of both mating types were present. In the same cheese. So whether sex was actually happening is still a mystery, but the opportunity was definitely there. Mmmm… Unfortunately for the world of cheese production, the conditions needed to get this fungus in the mood for sex are still completely unknown.

Image by Rachel Black, via Wikimedia.

Anyone else getting hungry? (Image by Rachel Black, via Wikimedia)

Next, there were plenty of clues left by evolution that sex is either happening right now in your cheese, or that it had been happening until fairly recently. The aforementioned sexy-time genes were evolving by purifying selection. This is a kind of evolution that keeps things from changing (I know, sounds like the opposite of evolution). Genes that are really important for life (or sex, and really is there a difference?) shouldn’t change. But mutations will happen, so evolution has to get in there and remove anyone carrying those mutations from the population. Basically, get mutations in really important genes, those genes no longer work, and you die. If there was no need for sexy-time genes anymore, then there would be no problem with mutations building up.

There were also footprints of sex in the genome. When sex occurs, DNA gets reshuffled. The researchers found evidence of this kind of reshuffling in some of the chunks of DNA they looked at, meaning that sex had to occur at some point in the recent past.

So, bottom line, I don’t know if there is sex going on in the cheese you eat. So far, no one has actually seen this mold having sex. But it could be. It could be doing it right now. Who knows what kind of awesome super-cheese could be evolving, right under your nose?

Bon appetit!

Ropars J, Dupont J, Fontanillas E, Rodríguez de la Vega RC, Malagnac F, Coton M, Giraud T, & López-Villavicencio M (2012). Sex in Cheese: Evidence for Sexuality in the Fungus Penicillium roqueforti. PloS one, 7 (11) PMID: 23185400

15 thoughts on “The secret sex of cheese

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  6. Having taught classes on mold in homes for many years, I would say most people aren’t aware of the positive side of fungi, including cheeses. Fungi could use a good flack.

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