Arbuscular mycorrhizal fungi (AMF) form symbioses with the majority of land plants, improving the nutrition and productivity of plants. the fungi are coenocytic, grow clonally, and no sexual stage in their life cycle is known. Recent evidence sugggest that AMF are hetrokaryotes, i.e., that genetically different nuclei coexist within a single cytoplasm. In this chapter, we present the last studies that have investigated the ecological and evolutionary consequences of an AMF heterokyryotic system on both sides of the symbiosis: Fungi and plants. We first present two important mechanisms, namely segregation and genetic exchange, two processes directly related to the heterokaryotic state od AMF. We then present the consequences of segregation and genetic exchange on AMF/plant interactions. Finally, we discuss the role of heterokyryosis in the enhancement of AMF adaptability and hoe AMF have maintained gnetically different nuclei.