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.