Evolution and Diversification

In this chapter, we discuss the current knowledge of the evolution and diversification history of mycoheterotrophic plants, including aspects about the origin of mycoheterotrophy, and common evolutionary trends in mycoheterotrophic lineages. While phylogenetic reconstruction of mycoheterotrophic plant relationships is hampered by the rarity of the taxa and high rates of DNA substitutions, current evidence shows that all mycoheterotrophic plant lineages arose from autotrophic mycorrhizal ancestors. Mycoheterotrophic interactions evolved independently in nearly all of the major land plant lineages and let to at least 46 independent origins of full mycoheterotrophy. In some groups there is evidence that full mycoheterotrophy is preceded by partial and initial mycoheterotrophy. Clades of fully mycoheterotrophic species are in some cases ancient, which demonstrates that full mycoheterotrophs can persist and diversify over a considerable amount of evolutionary time. However, even ancient lineages of full mycoheterotrophs contain relatively few species which suggests that speciation rates are low, extinction rates are high, or a combination of both processes. Shifts to full mycoheterotrophy are often accompanied by parallel evolutionary trends, including the reduction of vegetative parts, reduction of seed size and seed complexity, shifts to more shaded habitats, high rates of molecular evolution, and increased mycorrhizal specificity.