Researchers reveal in PLOS Genetics on March 31 that it’s possible that certain common mushrooms have more than 17,000 sexes. In addition to demonstrating the growing capability of genome sequencing, this work may aid our understanding of the development of sexual reproduction.
Researchers used state-of-the-art genetic approaches to establish the great diversity of sexes in Trichaptum mushrooms, confirming what scientists have assumed for a long time: that certain species of fungus contain thousands or perhaps tens of thousands distinct biological sexes.
Joseph Heitman, a geneticist and infectious disease researcher at Duke University who wasn’t involved in the work but has previously researched fungus mating techniques, said that the pace of improvements in DNA sequencing are really mind blowing. Even five years ago, this type of investigation would have been time consuming and expensive.
The genus Trichaptum contains the woody, plate-shaped mushrooms known as shelf fungus, which are often found in temperate regions of the Northern Hemisphere. Fungal biologist and University of Oslo collaborator Inger Skrede said that she believe they’re fairly attractive, but not particularly flashy. Of course, the mushrooms weren’t picked because of how good they look. Since members of this genus had already been investigated by scientists and were simple to cultivate in the lab, an international team of investigators chose to focus their attention on it.
The researchers were able to acquire 180 specimens representing three species of Trichaptum. Spores were collected from each type of fungus and then cultured separately on agar plates for many weeks so that their genetic makeup could be studied and potential hybrids could be tested.
Previous research had proposed that two sections of the genome, MATA and MATB, were responsible for regulating sex in these mushrooms; each of these areas included several alleles. Both areas must be distinct from the other’s in order for a couple to be considered compatible.
Because of this variety, sequencing has been more difficult. Since there are so many different alleles, it’s nearly hard to construct primers for accurate, cost-effective targeted sequencing. Given the large number of individuals and the depth of sequencing required to assure accuracy, the cost of sequencing these fungus has made it impractical for researchers to use traditional genome sequencing methods.
However, the reduced price of next-generation sequencing technology has made this formerly impossible task possible. Long-read sequencing, which Skrede and her colleagues were able to utilize to produce high-quality genomes that served as templates for precise assembly of the short reads, is a relatively recent development in the field. The researchers were able to accurately identify which portions of the MATA and MATB genomic regions were crucial for sex identification and quantify the number of significant changes thanks to the combination of approaches. After doing the math, the researchers discovered that among these seemingly ordinary shelf fungus, there are in fact 17,550 possible variations.
To this day, it is unclear why any organism would need such high levels of sexual variation, but study author and University of Oslo geneticist David Peris speculates that it has to do with the immobile lifestyle of the mushrooms, since being genetically distinct in two regions reduces the likelihood that spores released from the same mushroom will successfully combine.
Peris estimates that there is a 98% probability that any two neighbors are sexually compatible with one another because of the abundance of varieties available. This might be beneficial to the long-term survival of the species. Peris argues that when conditions are changing, you want to develop this genetic variety, since it might serve as fuel for adaptation.
The research is such a great illustration of balancing selection — the phenomenon where the evolutionary preferring of rare alleles leads to greater levels of heterozygosity — that evolutionary biologist Duur Aanen of Wageningen University in the Netherlands is considering using it in his population genetics classes.
Peris D, Lu DS, Kinneberg VB, Methlie I-S, Dahl MS, James TY, et al. (2022). Large-scale fungal strain sequencing unravels the molecular diversity in mating loci maintained by long-term balancing selection. PLoS Genet 18(3): e1010097. https://doi.org/10.1371/journal.pgen.1010097