Text by Lauri Laanisto

Spreading contingency on intimate relationships

The world of lichens seemed a pretty clear when I was a bachelor student of biology in late 90s. Pretty and clear, to be more precise. It´s difficult to like these strange creatures. Of course, we only studied species that could be identified with the eye or a little magnifying glass. All the species were in one book, and that was it.

A few years later things begun to get confusing. Suddenly appeared the term mycobiont and started dominating in lichen talk. There was no more symbiontic lichen – the holobiontic nature of this being was disentangled into two unequal parts. Fungus was the real deal, and algae (photobiont) was suddenly an outcast, marginal nobody hiding somewhere on the edge of the mycobiontic thallus. It was because sequencing came along. How do you identify the genetics of a symbiontic organism. You have scrap all but one of the partners.

The new system took a while, but we all got used to it. Identification in the field was based on the whole organism, and databases mainly used the genetics of the mycobiont.

But then came another blow by Spribille et al. Apparently the lichen was not so simple. There´s not just the fungus and algae – there is also some kind of shady yeast living in the lichen. Permanently. And very species-specifically, meaning that the the same myco-, photo-, and yeast biont species always lived together. Lichen became a conservative threesome, where all the partners are faithful and commited to a single thallus.

Our study, however, makes things more complicated. Sorry! Our analysis showed that mycobiont and photobiont are indeed basically always coupling up in the same combinations of species. But the yeast. That one is volatile. Sometimes it was one species living together with the same combination of myco- and photobiont, sometimes other. They were also taxonomically more variable than previously thought. We could not really put our finger to their distribution drivers. Our samples, collected by Kristiina, were from Estonia and Switzerland, showed completely different species (or OTUs, to be more precise) pool for these two countries. But why…

JH Lawton famously called community ecology collecting stamps. The same seems to be the case with lichens. It´s a symbiotic contingency. Once ecologists become interested in something, the contingency virus will spread and corrupt the pretty and clear systems established long time ago. This is why we cannot have nice things. Just ecology…

Citation: Mark, K., Laanisto, L., Bueno, C. G., Niinemets, Ü., Keller, C., & Scheidegger, C. (2020). Contrasting co‐occurrence patterns of photobiont and cystobasidiomycete yeast associated with common epiphytic lichen species. New Phytologist, https://doi.org/10.1111/nph.16475 (link to full text)

Abstract:

The popular dual definition of lichen symbiosis is under question with recent findings of additional microbial partners living within the lichen body. Here we compare the distribution and co‐occurrence patterns of lichen photobiont and recently described secondary fungus (Cyphobasidiales yeast) to evaluate their dependency on lichen host fungus (mycobiont).

We sequenced the nuclear internal transcribed spacer (ITS) strands for mycobiont, photobiont, and yeast from six widespread northern hemisphere epiphytic lichen species collected from 25 sites in Switzerland and Estonia. Interaction network analyses and multivariate analyses were conducted on operational taxonomic units based on ITS sequence data.

Our study demonstrates the frequent presence of cystobasidiomycete yeasts in studied lichens and shows that they are much less mycobiont‐specific than the photobionts. Individuals of different lichen species growing on the same tree trunk consistently hosted the same or closely related mycobiont‐specific Trebouxia lineage over geographic distances while the cystobasidiomycete yeasts were unevenly distributed over the study area – contrasting communities were found between Estonia and Switzerland.

These results contradict previous findings of high mycobiont species specificity of Cyphobasidiales yeast at large geographic scales. Our results suggest that the yeast might not be as intimately associated with the symbiosis as is the photobiont.