By Sarah Lawhun
Fungi are hidden in plain sight. They’re everywhere; in the soil, the water, and the air. And yet the role of many fungal species in ecosystems remains mysteriously incomplete or entirely unknown.
The parasitic, single-celled fungi called chytrids, for example, crawl under the radar, typically undetected by multicellular organisms such as ourselves.
Chytrid awareness has risen in recent years due to massive amphibian die-offs caused by invasive chytrid species. One species that wipes out frog populations in North America, Batrachochytrium dendrobatidis, known as Bd fungus, has been studied intensely by epidemiologists to help restore the ecosystems it affects.
Lillian Fritz-Laylin, 2018 Whitman Center Fellow and assistant professor at the University of Massachusetts, Amherst, is a cell biologist by trade, but her training in genomic comparisons and evolutionary theory came in useful when she began looking at Bd and other chytrid fungi and asking questions no one had asked about them before.
“I love studying cells because of their morphological diversity,” says Fritz-Laylin. “Whenever I ask other scientists about their work, I often ask them what questions they use to approach their research. I always ask single-celled organisms such as these chytrids, ‘What are you doing?’”
This video shows examples of Bd chytrid cells crawling between two glass coverslips using pseudopods. Credit: J. Cell Biology (2017) doi: 10.1083/jcb.201701074
The quest to understand how Bd infects its host, while an important line of inquiry, can overlook more basic functions of the fungus within its ecosystem, such as what other organisms it interacts with. When Fritz-Laylin began genomic comparisons between Bd and other animal cells that use a special kind of motility, called crawling motility, she found that, unbeknownst to all, Bd could crawl. It had tiny, arm-like cellular projections, called pseudopods, which it uses to drag itself across solid surfaces. Fritz-Laylin and colleagues recently published a paper on pseudopod function in eLife and one on the structures’ genetics and evolution in the Journal of Cell Biology.
The presence of pseudopods opens up entirely new possibilities for how Bd interacts with its environment. Fritz-Laylin’s genomic comparisons of Bd with animal cells and other organisms showed that the genes Bd uses for pseudopod crawling are used by other organisms to engulf particles and feed. These organisms can extend their pseudopods like tentacles and envelop food particles, then absorb them into the cell for digestion.
The potential for Bd to feed on material other than its amphibian host could drastically change how epidemiologists address its outbreaks. If it feeds on something else, it can sit and wait after frog populations have been wiped out. This means reintroduction of frog species to restore an ecosystem might be futile — they would only be infected by the parasitic fungus again.
“I’m really excited to test my idea that chytrids may use pseudopods for both crawling and feeding,” Fritz-Laylin says. “If we knew chytrids could feed off of particulates they engulfed from their environment, our understanding of these food networks would be very different.”
Fritz-Laylin’s basic question of asking the fungus, “What are you doing?” sums up her MBL research this summer. In addition to exploring whether chytrids can also use pseudopods for feeding, she’s discovering which chytrid species can crawl and how closely they’re related to each other and to other single-celled organisms that crawl.
“Globally, there’s thousands of species of chytrids,” says Fritz-Laylin. “Chytrid species are important in many different ecological niches.”
What she finds will help address the not only amphibian die-offs due to parasitic chytrids in freshwater ecosystems, but also their outbreaks in marine ecosystems, where they cause massive diatom (microbe) die-offs. By examining these fungi from a new angle — one that doesn’t ask how they infect hosts, but simply what they’re doing in their ecosystem — Fritz-Laylin is improving our understanding of these covert crawlers.