When Two Species Meet: A Potential Beetle-Yeast Facultative Mutualism.

Facultative mutualisms, such as some insect-yeast interactions, can be established between partners that only interact in certain stages of their life cycles. While exploring yeasts associated with Brazilian beetles, we found a particular Cyberlindnera yeast associated with Spermophthorus apuleiae. This yeast was found within the beetles' galleries, which are excavated in fruits of Libidibia ferrea, a native Brazilian tree. We isolated a total of 15 yeast and yeast-like species associated with the fruits and beetles, mostly from the orders Phaffomycetales and Serinales, and explored their role in yeast-beetle interactions. Cyberlindnera is the most recurrent yeast, found in 90% of the fruit samples infested with beetles and representing 79% of the total isolates. Results from bioassays support the interaction between Cyberlindnera and Spermophthorus apuleiae. We hypothesise that reciprocal benefits may underlie this association: beetle gallery excavation boosts the density of Cyberlindnera in fruits, while the yeast produces attractive volatiles to beetles. These volatiles are usually associated with signalling towards sugar resources that enhance dispersal, an idea that warrants further exploration. We consider the Cyberlindnera-beetle association a potential model for studying the ecology and evolution of non-obligate (facultative) mutualisms.