“Fred took a likin’ to Alison.” It was a mnemonic device a high school biology teacher taught me to remember that a fungus plus an alga made a lichen, and I never forgot it.

That phrase now seems to come with qualifications. In a study published Thursday in Science, scientists found that another fungus, called basidiomycete yeast, may be a third symbiotic partner in many lichens.

The findings are very surprising, said François Lutzoni, a professor of biology and a lichen expert at Duke University who was not involved in the study. “They add another layer of complexity to lichen symbiosis.”

Lichens come in various sizes, shapes and color, but you can commonly find them as leafy tufts or crusty patches adorning bark and rocks. The prevailing definition of a lichen is that it arises from a symbiosis between a fungus and a photosynthesizing alga or bacteria. The alga or bacteria provides food through photosynthesis. In return, the fungus provides protective structures, which also gather moisture, nutrients and an anchor to the environment.