Do homokaryotic and dikaryotic Rhizophagus irregularis strains exhibit divergent adaptive strategies in response to contrasting soil P conditions?

Arbuscular mycorrhizal (AM) fungal strains of Rhizophagus irregularis can occur in two genetic groups: homokarya, in which nuclei are genetically similar and dikarya, in which they harbor two genetically different nuclear populations. Data show trait conservatism in these groups, but their adaptability to environmental changes remains unclear. We tested if dikaryotic strains can potentially adapt faster to changes in soil phosphorus (P) conditions than homokaryotic strains. First, tested the growth responses of Allium ampeloprasum L. to each of four homokaryotic and four dikaryotic strains under contrasting soil P levels (conditioning phase); second, we examined if the conditioned strains showed adaptive characteristics (e.g., became more mutualistic) when grown under matching P conditions across both experimental phases, compared to mismatched conditions (adaptive potential phase). In the conditioning phase, dikarya were better mutualists (i.e., host biomass promoters) and formed more vesicles than homokarya when P was high. In the adaptive potential phase, regardless of nuclear organization and soil P treatment, strains conditioned under low P became better mutualists than those conditioned in high P. However, dikarya were not more adaptable to soil P than homokarya with the host. Instead, strains of any genetic category became better mutualists after experiencing low rather than high P soil conditions. This study suggests that a single generation of R. irregularis soil P conditioning may be sufficient to change the strength of mutualism. As such, soil P fertility optimization and pre-conditioning are relevant to consider in mycorrhizal applications, regardless of the AM fungal genetic background.