Functional Significance of Anastomosis in Arbuscular Mycorrhizal Networks

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs (Glomeromycota), which live symbiotically in the roots of most land plants and facilitate mineral nutrition of their hosts. Their spores are able to germinate in the absence of host-derived signals, but are unable to complete the life cycle without establishing a functional symbiosis with a host plant. Such behaviour did not represent a selective disadvantage, as a result of diverse survival strategies allowing them to compensate for the lack of host-regulated germination and to overcome their obligate biotrophic state. The ability to form hyphal fusions (anastomoses) between compatible mycelia may represent an important mechanism evolved by AMF to increase their chances of survival, since fungal germlings can plug into pre-existing extraradical mycelial networks, thus gaining immediate access to plant-derived carbon before asymbiotic growth arrest. In fusions between hyphae of the same or different individual germlings of the same isolate, perfect anastomoses occur with the highest frequency and are characterized by protoplasm continuity and complete fusion of hyphal walls. High anastomosis frequencies are also detected between extraradical mycelial networks produced by the same isolate, spreading from plants of different species, genera and families. Pre- and post-fusion incompatibility are often observed in hyphal interactions between asymbiotic and symbiotic mycelium and between genetically different germlings belonging to the same isolate, while pre-fusion incompatible responses, hindering hyphal fusions, occur between germlings of geographically different isolates. The analysis of vegetative compatibility/incompatibility during hyphal fusions represents a valuable tool for genetic studies of AMF, which are recalcitrant to axenic cultivation. Molecular analyses of the progeny of mycelium derived from nonself vegetative fusions of genetically different germlings of R. irregularis showed that genetic exchange occurs, despite low anastomosis frequencies and post-fusion incompatible responses, suggesting that anastomosis between genetically different mycelia may represent a recombination mechanism in the absence of an evident sexual cycle.