From whales and wildflowers to humans and house cats, reproduction is the through-line for the survival of all living things. Microorganisms, including disease-causing fungi, are no exception, but unlike plants and animals, whose reproductive cycles can be observed directly, the way that many microbial fungi reproduce has largely remained a mystery — until now.
In a new study, published Dec. 4 in the journal PLOS Genetics, McMaster scientists have described for the first time how a species of pathogenic fungi reproduces in the wild.
According to researchers, it had long been assumed that fungi primarily reproduced asexually, with a single parent organism producing offspring that are genetic clones of itself. And while that can be true, the new study revealed strong evidence of prevalent sexual reproduction in fungi, too.
“It was always thought that wild fungi most typically reproduced by cell division, and not by mating,” says Jianping Xu, a professor of biology and principal investigator on the new study.
“We thought that they mostly just replicate and divide, and replicate and divide, and replicate and divide in perpetuity. But our new findings show that that’s not always the case.”
The researchers studied a population of disease-causing fungi called Cryptococcus deneoformans, which Xu says is a particularly dangerous species that can cause serious and often fatal infections in immunocompromised individuals, like cancer patients and those living with HIV.
For this reason, the World Health Organization designated Cryptococcus a pathogen of priority concern.
Xu’s team, with collaborators at Duke University, systematically analyzed the genomes of 24 different wild strains of Cryptococcus retrieved from harsh desert environments in Saudi Arabia. In doing so, they discovered clear signs of “recombination” — a mixing of DNA that can only happen through sexual reproduction.
Sex at the microbial scale is a highly coordinated chemical process, says Xu, who notes that it involves several different steps and stages before new fungi are born.
“First, one fungal cell will express pheromones to attract compatible partners,” he explains. “Once those chemical signals are detected by another nearby fungal cell, both fungi undergo a morphological change and eventually fuse together. This allows their genetic information to mix, creating a new cell with genetic information from both parental cells — and this new, fused cell then undergoes meiosis and produces genetically distinct spores that disperse and land elsewhere, and eventually grow into entirely new fungi, each containing about half of the genetic materials from each parent cell.”
Of note, Xu says this process was widespread in the Saudi Arabian population of this species even in areas where the fungal populations were exclusively the same sex, which has implications for how we understand fungal evolution and adaptability.
“Sexual reproduction in fungi — whether by opposite or same-sex mating — reshuffles their genetic code,” he says. “This can broaden genetic diversity, potentially helping fungi adapt to new environments and develop new traits that influence things like virulence and drug resistance.”
Xu, a member of the Michael G. DeGroote Institute for Infectious Disease Research, says that while Cryptococcus and other fungi have been known to reproduce this way in the past, until now, it had only ever been observed in relatively controlled settings. But, in the unpredictable, resource-limited, and competitive environments of the natural world, scientists believed that fungi would turn to a more self-reliant form of reproduction.
“The prevalence of recombination in the samples we studied suggests that sexual reproduction likely gives wild fungi a survival advantage,” Xu says. “Because the process of mating allows this species’ colonies to grow as hyphal filaments and extend outward, it enables them to explore their environment in ways that they could not through cell division, nor through their simple yeast form. It might be that, in harsh environments like the Saudi Arabian desert, this complex sexual process also allows them to search for and discover critical nutrients in their surroundings.”
The findings, researchers say, deepen our overall understanding of the Cryptococcus lifecycle, and offer new insights about how we can potentially inhibit its reproduction to stop or slow the diseases that it causes.
“Reproduction is fundamental to the survival of all organisms, including fungi,” says Xu. “Understanding how fungi reproduce in nature has broad significance not only to basic biology, but also to applied fields such as infection prevention and control.”