Most terrestrial plants distributed worldwide establish at least one of seven types of mutualistic symbiotic mycorrhizal associations in their roots (Brundrett, 1991; Smith and Read, 1997). The most widespread of these is an endomycorrhizal symbiosis between soil-borne fungi [the mycobiont] presently classified in the phylum Zygomycota, order Glomales (Morton and Benny, 1990) and approximately 80% of terrestrial plant species [the phytobiont] (Trappe, 1987). This symbiosis has been demonstrated to be mutualistic in measurable ways. The most obvious benefit to the mycobiont is a ready supply of carbon within a relatively secure niche in root cortical cells. Jakobsen and Rosendahl (1990) indicate the mycobiont can capture as much as 20% of the fixed carbon in the phytobiont, mostly by converting glucose to trehalose (Shachar-Hill et al., 1995). The symbiosis is termed "arbuscular" in this chapter because the mycobiont produces specialized arbuscules in root cortical cells that are involved in bidirectional exchange of carbon, phosphorus, and other physiologically important molecules. These arbuscules are considered the key structural evolutionary innovation because they are a conserved feature in all lineages of Glomales (Morton, 1990a). The phytobiont depends to varying degrees on the mycobiont for hyphal uptake and translocation of essential cations in soil which otherwise might not be available from root uptake alone (reviewed in Smith and Read, 1997). The magnitude of dependency often is measured in terms of plant growth responses, which usually are inversely related to plant nutrient (mostly phosphorus) content.

The arbuscular (AM) symbiosis has received considerable attention by botanists, ecologists, and agriculturalists in recent years because of dramatic responses to plant growth, vigor, and reproductive potential under largely controlled (and therefore somewhat artificial) conditions (Safir, 1987; Smith and Read, 1997). Similar responses in natural settings tend to be more inconsistent or absent, suggesting that growth benefit is only one factor that contributes to the long-term stability of the AM symbiosis. A multitude of other interactions also must be taking place which result in a positive outcome for both symbionts at the scale of the community and above. Clues as to the nature of these interactions come from measurement of present-day processes, but they rarely are interpreted in an evolutionary context. Genealogical properties of both the mycobiont and phytobiont lineages also play an important role, with causal mechanisms varying with different combinations of host and fungal genotypes operating in specific environments. These historical processes undoubtedly are significant, and so the purpose of this chapter is to elaborate on current hypotheses concerning evolution of the arbuscular mycorrhizal endosymbiosis and link historical patterns and processes to those occurring at the present time.