Arbuscular mycorrhizal fungi (AMF) are obligate plant symbionts that colonize the roots of the majority of vascular land plants. These fungi have a unique nuclear organization, in which thousands of nuclei co-exist among an unsegmented fungal body. In individual strains these nuclei can all be genetically similar (homokaryotic) or be derived from two distinct parents (dikaryotic). In other fungal groups the presence of two distinct nuclei in one cell (fungal dikaryons) can change their fitness, function, and symbiotic relationship; begging the question, what impact does the presence of two parental genotypes have on the arbuscular mycorrhizal symbiosis? I am investigating this by measuring the mycorrhizal response (MR) of potato cultivars with different degrees of domestication using representative AMF homokaryons (4) and AMF dikaryons (4). I found that the genetic organization (dikaryotic vs homokaryotic) and domestication status of the host (modern vs old) are both significant factors in the mycorrhizal response of host plants. Specifically, biomass is significantly greater when inoculated with homokaryotic AMF compared to dikaryotic AMF. Dikaryotic strains have low arbuscule colonization in modern cultivars and higher in old, although there are not significant differences in other fungal responses between homokaryotic and dikaryotic AMF. Furthermore, nutrient uptake (N and P) is greater in old cultivars than modern cultivars, although the root:shoot ratio is lower in old cultivars. Analyses of single spores using digital droplet PCR (ddPCR) confirm that nucleotype ratio of dikaryotic spores shifts depending on the host identity. This research provides novel insights into the role of AMF genetic organization in the mycorrhizal symbiosis in greenhouse conditions. In particular, this work shows that the presence of two distinct nucleotypes results in the fungi being more readily adaptable to the host leading to a more stable MR and a potentially selfish strategy, when in symbiosis with potato cultivars.

Arbuscular mycorrhizal (AM) symbiosis, a widespread plant-fungal relationship, is based on reciprocal resource exchange. The functioning of this fragile relationship balances on the scale from mutualism to parasitism, depending on the specific context. The thesis aims to interlink the functioning of AM symbiosis both with the composition of AM fungal communities and with different abiotic conditions. The thesis is divided into a methodological and a factual part and consists of three publications and one manuscript. All experiments were conducted in greenhouse conditions with medic (Medicago sp.) as host plant. Host plants were inoculated with single AM fungal species in Paper I and II, and with a synthetic AM fungal community of five species in Paper III and IV. The host plant identity, the amount of phosphorus (P) in substrate and the type of substrate played an important role for the achievement of mutualistic AM symbiosis, as demonstrated in Paper I. Paper II showed that mitochondrial and nuclear qPCR markers can be used alternatively for the quantification of particular AM fungal species. However, intraradical fungal biomass was better related to copy numbers of nuclear DNA than of mitochondrial DNA. The functioning of AM symbiosis was modulated by the availability of P, light and water,...