Fungal Endophytes as a Driving Force in Land Plant Evolution: Evidence from the Fossil Record

Abstract

Plant–fungal interactions occur at multiple levels and help to shape plant communities and the ecosystems they comprise. Such interactions may involve competition, antagonism, and varying degrees of mutualism. Just as herbivores can impact the structure and dynamics of a plant community by overall decreasing fitness of interacting species, fungi bring about similar outcomes in the ecosystem (Dighton 2003). In addition, microorganisms, including fungi, drive the bio-geochemical cycles of nature as the principal decomposers of organic matter in the biosphere (e.g., Setala et al. 1998; Fang et al. 2005; Taylor et al. 2009a). Fungi also impact the ecosystem in negative ways, ranging from parasites of plants and animals to pathogens and disease causative agents of these organisms. Perhaps the most notable of plant–fungal interactions involve mutualistic relationships between certain fungi and the roots (or other parts) of land plants ranging from a bryophytic grade of evolution to angiosperms. This intimate association, termed a mycorrhiza, is the most prevalent symbiosis on Earth, and is estimated to occur in more than 80% of living land plants (e.g., Cairney 2000; Selosse et al. 2006). It demonstrates the coevolution of the two partners. In fact, this type of mutualistic symbiosis between an alga and fungal partners may have been the necessary prerequisite to the establishment of plant life in the terrestrial realm (Raven 1977; Selosse and Le Tacon 1998). The purpose of this chapter is to demonstrate various fungal (in the broad sense, including members of the Oomycota (Peronosporomycetes) and Hyphochytridiomycota) endophytes, and land plant–fungal endophyte interactions from the fossil record. While fungal endophytes can be documented throughout the Phanerozoic, it is the Early Devonian and Carboniferous fossils that have been critically examined systematically. As a result, there are a number of welldocumented examples for endophytic fungal associations with land plants from these periods. We have selected examples that demonstrate endophytic occurrences of fungi in shoot axes