The role of mycorrhizal fungi in driving ecotype formation in mycoheterotrophic plants

In his seminal paper, Göte Turesson proposed the term ecotype as the ‘ecological unit to cover the product arising as a result of the genotypical response of an ecospecies to a particular habitat'. Turesson further outlined that to fully understand the ecology of a species, a knowledge of its most important ecotypes is needed. Whereas Turesson's original idea of an ecotype mainly referred to the response of a species to abiotic conditions, there is mounting evidence that ecotypes can also originate as a response to biotic conditions. Hence, to understand the ecology of a species, one should also understand the distribution and variation in biotic interactions. For plants, one such biotic interaction involves mycorrhizal fungi. Particularly in mycoheterotrophic plants, i.e. plants that rely on mycorrhizal fungi for their carbon supply, adaptation to and specialization on mycorrhizal fungi have the potential to drive ecotype formation and speciation. In this paper, we provide evidence that populations of mycoheterotrophic plants inhabiting contrasting habitats commonly encounter divergent mycorrhizal fungal communities leading to geographic mosaics of mycorrhizal interactions. Adaptation to local fungal communities, in turn, can induce (partial) reproductive isolation and contribute to ecotype and ultimately species formation as a result of decreased fitness of immigrant genotypes (immigrant inviability). In the most extreme case, loss of photosynthesis and mycorrhizal switching have led to reproductive isolation and the development of novel species that have become fully mycoheterotrophic. Such shifts have occurred repeatedly during the evolutionary history of land plants. We conclude that mycorrhizal fungi not only contribute to the early divergence of populations of mycoheterotrophic plants, but also play a crucial role in the further diversification and origination of this unique set of species that rely on mycorrhizal fungi for obtaining carbon and completing their life cycle.