Number of arbuscular fungal species sporulating in trap cultures propagated by INVAM.	Species composition of arbuscular fungal communities at the genus level in roots of four-month-old sudangrass (Sorghum sudanense) in trap pot cultures (15-cm pots) of inocula from four separate geographic locations.  KE = Kenya, MN = Minnesota, VA = Virginia, VZ = Venezuela).
	Genus	KE104	MN409	VA105	VZ103
	Acaulospora	2	2	1	2
	Entrophospora	0	0	1	0
	Glomus	4	2	1	3
	Gigaspora	0	2	1	1
	Scutellospora	1	2	2	2

Irrespective of the complexity of interactions within a fungal community, a large body of evidence indicates the phytobiont is the ultimate arbiter of the success or failure of the symbiosis at the scale of the plant community and above. Moreover, coevolution of the mycobiont with most, if not all, ancestral plant lineages indicates that genetic and regulatory mechanisms are likely to differ greatly among plant lineages regardless of the community structure of the mycobiont. Probably the most conserved processes are those regulating recognition events and formation of arbuscule and hyphal interfaces in root cells. Blockage of mycorrhizal colonization from epidermal and hypodermal cell regions appears to be universal (Gianinazzi-Pearson et al., 1996). Another general phenomenon is a reduction in mycorrhizal development as soil (and plant) P levels increase (Smith and Read, 1997). In this case, the causal mechanism diverges among plant lineages, from inhibition of entry point formation in leek (Amijee et al., 1989) to inhibition of infection unit growth and secondary colonization in cucumber (Bruce et al., 1994).

Mycorrhizal phenotypes of different host-fungus combinations are known to vary considerably, which again can be attributed to divergence of coevolving mycobiont and phytobiont lineages. As an example, legume mutants which do not form bacterial nitrogen-fixing nodules differ considerably in their response to mycorrhizal development. Hyphal ingress into roots is blocked by both Pisum and Medicago mutants, but appressoria formation is not affected in the former and significantly distorted in the latter (Bradbury et al., 1993). In contrast, nod– mutants of Glycine do not inhibit mycorrhizal colonization (Wyss et al., 1990). Phytobiont species which have evolved into nonhosts also exhibit divergent mechanisms of resistance to the symbiosis that appear to involve different mechanisms from those elicted by plant pathogens (Giovannetti et al., 1995). The presence of physico-chemical active defense mechanisms in incompatible (myc –1) Pisum mutants suggests that plant-controlled defense genes are suppressed during mycorrhizal development. In only one report to date, a species in Chenopodiaceae produces localized necrosis in response to limited ingress by arbuscular fungi (Allen et al., 1989). In contrast, a number of Brassica species appear to effect some control over preinfection events in soil by failing to exude molecules that normally stimulate hyphal proliferation from spores and then show no indication of any active defense response at the root surface (Glenn et al., 1988). Inhibition of colonization and blockage of arbuscule formation in Pisum roots grafted to nonhost Lupinus indicates other mobile mechanisms of controls at different stages of mycorrhizal development (Gianinazzi, 1991). Such variation in mechanisms among lineages and at different stages of mycorrhizal development are an expected outcome of such long-term historical interactions.

Despite powerful phytobiont controls, the mycobiont still has considerable room to diversify in properties that do not impact negatively on long-term population dynamics of the phytobiont. While variation in fungal morphology in constrained by simplicity of design, hyphal architecture and behavior can change dramatically in response to phytobiont and environmental variables (Brundrett, 1991; Brundrett and Kendrick, 1990). Patterns and abundance of sporulation also are highly plastic, even under similar environmental conditions (Morton, unpublished). Contemporary differential responses among mycobiont genotypes have been measured in efficiency of P uptake independent of degree of mycorrhizal development (Smith et al., 1994), efficiency of carbon and P exchange and translocation correlated with changes in fungal colonization (Pearson and Jakobsen, 1993a), and activation of root nutrient uptake (Pearson and Jakobsen, 1993b). Unfortunately, these data do not indicate whether the traits measured are heritable and thus phylogenetically significant. With the likelihood of heterogeneous nuclei in single cells (Lloyd-MacGilp et al., 1996) that have considerable mobility and are somatically transmitted, internal selection pressures could be intense and lead to diverse phenotypes that interact in complex ways with the phycobiont in response to local variables. Coevolution of both mycobiont and phytobiont are still occurring, albeit at a rate and magnitude that is poorly understood. To partition historic from local processes, more effort is needed to determine the conservativeness of processes being measured when host or fungus genotypes vary or when environmental conditions are altered.