Genetic traits
In this section we define genetic measurements that have been proven or have the potential to reflect differences in life history strategies as “genetic traits” and we discuss how they can fit in a trait-based framework. Among these traits are the genetic organization of AM fungal strains, the spore nuclear content, the genome size, and the GC content of the genome. Recent findings demonstrated that AM fungal strains belonging to one species carry thousands of nuclei in their coenocytic mycelia that either belong to one (i.e., homokaryotic strains) or two nuclear genotypes (i.e., dikaryotic strains (Ropars et al. , 2016) with each of these genotypes having unique structure, genetic content and epigenetics (Sperschneider et al. , 2023). Interestingly, the relative abundance of the coexisting genotypes in the dikaryotic strains appears to be deterministic and their regulation to be responsive to biotic (e.g., plant host identity) (Kokkoris et al. , 2021) and abiotic factors (e.g., pH, temperature, nutrient content) (Cornellet al. , 2022). Carrying two genomes instead of one may reflect differences in life histories strategies if the same is shown in multiple species (Serghi et al. , 2021). Particularly, the homokaryotic strains have higher germination rates and faster germination compared to the low germination rate of the dikaryotic strains. In contrast, dikaryotic strains grow faster and produce larger and more interconnected ERM compared to the homokaryotic ones. The nuclear organization can affect the mycorrhizal response of their plant hosts. Specifically, and in contrast to expectations that two genomes might result in more mutualistic interactions, dikaryotic strains were inferior mutualists compared to the homokaryons when interacting with multiple potato cultivars (a highly mycorrhizal dependent crop) in greenhouse conditions (Terry et al. , 2023). While we recognize that nuclear organization may be an important function trait, until mono vs dikaryons are found in more AM fungal species it might be premature to suggest this trait should be included in a program for standardization of trait measurement across all AM fungal taxa.
The nuclear content of the spores also seems to be associated with particular life history traits although further experimental evidence is needed. The range of nuclei present in spores correlates with spore size, ranging from 35000 nuclei for spores of Gigaspora decipienswhich have an average diameter of 400um, to 130 nuclei for the spores ofGlomus cerebriforme with an average diameter of 80um (Kokkoriset al. , 2020). These huge differences in nuclear content could be associated to spore viability and germination, and overall colonization ability after dispersal. For example, multiple re-germination events have been observed for Gigasporaceae spores when no host is encountered initially (Sward, 1981), a trait that does not appear in Glomussp. spores which usually demonstrate a more ruderal behavior. It has been hypothesized that the numerous nuclei, could serve as resource reserve via nucleophagy when facing starvation, a phenomenon previously observed in fungi (Shoji et al. , 2010; Kokkoris et al. , 2020). Despite the number of genotypes and the number of nuclei present in the AM fungal networks and spores, the overall genome size might influence the reproductive rate, the environmental adaptability and in the overall resource economy of a species/strain. Although not very common for fungi, linkage of genome size to life history affiliations is not a novel concept. (Grime & Mowforth, 1982), linked plant genome size to climate growth conditions. (Veselý et al. , 2012) linked larger plant genome sizes to early flowering events and preference for humid conditions, and (Bhadra et al. , 2023) linked genome size to multiple functional traits related to plant morphology, physiology, performance and survival. Our knowledge on the variation of genome size in AM fungi is limited due to the low number of sequenced genomes. Regardless, we know that the variation is extreme, with larger species (Gigasporaceae) having genomes that reach 740 Mb and smaller species (e.g., Rhizophagus clarus ) 116Mb (Kokkoris et al. , 2020). It is important to keep expanding our datasets with such information to identify links between genome size and the morphological, physiological, and phenological traits of AM fungi. Finally, a particular genetic trait, the guanine plus cytosine (G+C) content of genomic DNA which have the potential to reflect ecological niche or pathogenicity in fungi (Yoder & Turgeon, 2001). Once again with limited data due only few complete genomes available substantial variation in G+C content exists in AM fungi (range from 25 to 36 (mol%) (Malar C et al. , 2022). These differences could potentially reflect differences observed in mycorrhizal response and host preference.