Description

Many have claimed that fungi can have thousands of sexes, but this confuses two different systems in biology: mating types and sexes. Let's explore the difference and why it's important.

Sources

Cepelewicz, J. (2018). Why nature prefers couples, even for yeast. Scientific American.

Fraser, J., Heitman, J. (2003). Fungal mating-type loci. Current Biology, 13(20).

Gaines, J. (2022). This fungus has more than 17,000 sexes. The Scientist.

Geng, S., De Hoff, P., Umen, J. (2014). Evolution of sexes from an ancestral mating-type specification pathway. PLOS Biology, 13(1).

Goymann, W., Brumm, H., Kappeler, P. (2022). Biological sex is binary, even though there is a rainbow of sex roles. BioEssays, 45(3).

Kirk, D. (2006). Oogamy–inventing the sexes. Current Biology, 16(24).

Lehtonen, J. (2017). Gamete Size. Encyclopedia of Evolutionary Psychological Science.

Lehtonen, J., Kokko, H., Parker, GA. (2016). What do isogamous organisms teach us about sex and the two sexes? PTBS, 371(1706), 2.

Lehtonen, J., Parker, G. (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction, 20(12).

Peris, D., Sun Lu, D., Kinneberg, V., et al. (2022). Large-scale fungal strain sequencing unravels the molecular diversity in mating loci maintained by long-term balancing selection. PLoS Genetics, 18(3).

Perrin, N. (2011). What uses are mating types? The ‘developmental switch’ model. Evolution, 66(4), 947-956.

Scharer, L. (2017). The varied ways of being male and female. Molecular Reproduction & Development, 84.

Scharping, N. (2019). Why this fungus has over 20,000 sexes. Discover Magazine.

Transcript

Fungi are used for many things, from food, to medicines, and even as pest control for crops. They can also be quite beautiful to look at. This fascination has extended into the biology of sex, where some people claim fungi can have thousands of sexes! Even some pop science magazines have made such claims (Gaines 2022; Scharping 2019). But these examples actually involve something called mating types, a more ancient sexual system that evolved before sexes even existed. So what are mating types, what are sexes, and what’s the difference?

Sexes are defined by a difference in gamete size, where one sex (the female) provides the larger gamete (the egg) that holds most or all the cellular resources for the future zygote, and the other sex (the male) provides the smaller gamete (the sperm) that holds almost no cellular resources (Lehtonen 2017). While both gamete types each hold half the genetic material of the parent, their unequal contribution of cellular resources is why they are different sizes. This system is called anisogamy (aniso means unequal, and gamy means union). It is almost universal across the plant and animal kingdoms.

It is true the two sexes can be composed in a variety of ways across species. Some species have both sexes in one individual (such as corn in plants, or the slug in animals). Other species separate the sexes, where individuals are either male or female (such as the ginkgo tree in plants; or mammals in the animal kingdom). No species, however, has been observed to have more than two sexes (Lehtonen & Parker 2014; Scharer 2017; Goymann, Brumm, Kappeler 2022).

As for the fungi that people claim have thousands of sexes, they use a more ancient system where gametes are roughly similar in size. Because of this, the reproductive contribution is also similar: the two parents provide roughly equivalent cellular resources to the zygote (Lehtonen, Kokko, & Parker 2016). In such a system, there are no sexes – no males and no females. This is called isogamy (or equal union). Isogamy uses mating types, not sexes.

Mating types are genetic mechanisms that regulate compatibility between fusing gametes (Perrin 2011), common among unicellular organisms, some types of algae, and fungi (Kirk 2006). The compatibility between differing mating types is decided through different versions of the same gene, called alleles [Geng, De Hoff, & Umen 2014]. If you have two gametes each with the same allele (+ and +, for example, or – and –), then they will usually not form offspring, but two gametes with different alleles (+ and -) will. Think of it like magnets, where opposite poles attract while the same poles repel. Gametes with different mating types are compatible while those with the same type are not (Fraser & Heitman 2003). Such mating types help reduce inbreeding and facilitate genetic diversity in microorganisms.

While there are often only two mating types in organisms like fungi, some fungal species can have thousands, practically eliminating the chances of inbreeding. This occurs when there are multiple locations on the genome where compatibility is decided (called mating type loci), and each locus can have many possible alleles. Multiply all the possible combinations and you can quickly have tens of thousands of different options for mating. And it is in these examples, such as the wood-rotting fungus hilariously dubbed Schizophyllum commune, where you will see pop science magazines incorrectly claiming 20,000 sexes. Schizophyllum commune has four gene compatibility groups. Group A-alpha has 9 gene variants, A-beta has 32, B-alpha has 9, and B-beta has 9. Multiply the possible combinations together (9 x 32 x 9 x 9) and you get 23,328 mating types (Cepelewicz 2018). But don’t get too excited. These are just mating types, not sexes, providing this fungal species with thousands of options for fusing their similar-sized gametes.

Mating types and sexes, therefore, are not the same and must be kept distinct. As evolutionary ecologist Nicolas Perrin writes, “‘Mating types refer to incompatibilities between otherwise phenotypically similar partners (isogamy), whereas ‘sexes’ imply some anisogamy, that is, an asymmetry (in terms of size or behavior) between the interacting gametes or partners (reaching an extreme in the oogamy of plants and animals)” (Perrin 2011).

There’s one more curious reason why mating types and sexes should not be conflated. Some organisms utilize both sexes and mating types. For example, many species of the Neurospora fungi produce male and female gametes, yet the male gametes can only fertilize the female gametes from a different mating type (Perrin 2011), revealing that sexes and mating types are distinct phenomena.

While mating types are not evidence there are more than two sexes, they do show what sexual reproduction was like before there were males and females, providing us with an ancient look into our evolutionary origins.

For an inside look into how biologists think the two sexes evolved, check out our video, “Origins of Two Sexes.”

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