Figure 1. Nuclear content of an individual spore isolated from an arbuscular mycorrhizal fungus. The spores and cells of arbuscular mycorrhizal fungi are, to the best our knowledge, constantly multinucleate. At the extreme, individual spores of species such as Gigaspora gigantea can carry over 20 thousand nuclei within large spores. When the entire belowground mycelium is considered, a single individual may carry hundreds of thousands of nuclei within one large cell (a coenocyte). The image was obtained using confocal microscopy. The final images are colour coded along z axis for depth recognition. White and red colours are closer to the observer while blue colours are the furthest. Nuclei are visible due to staining with SYTO 13 green fluorescent nucleic acid dye. Each image is the result of approximately 300 z stacks (0.35 μm intervals). Vasilis Kokkoris
Genetic and genomic interactions between the symbiotic partners The identification of AMF strains carrying multiple nuclei deriving from two parental (and presumably homokaryotic) strains provides strong evidence that the genetics of these symbionts follows patterns seen in other fungi. Specifically, in many fungi compatible ‘monokaryotic’ strains can exchange nuclei to produce a dikaryotic progeny carrying two nuclei per cell; that is a ‘dikaryotic cell’. Eventually, though not always, these genetically different nuclei fuse and undergo meiosis (sex) to produce a recombined progeny, and there is evidence that analogous mechanisms also occur in AMF. In non-AMF fungal dikaryons (two nuclei per cell), the co-existence of two parental genomes was shown to benefit fungal strains in many ways. Dikaryotic strains can have an advantage over monokaryotic relatives, including a superiority in terms of fitness and function by producing unique proteins or increased enzymatic activity. Similar distinctions may also affect AMF, with recent work showing that AMF heterokaryons are significantly different from AMF homokaryons in terms of ecological traits and function. For example, AMF heterokaryons tend to produce a different number of spores and generate hyphal networks of larger size, highlighting the different roles that each nuclear organisation may play in terrestrial ecosystems. In AMF heterokaryons, the relative abundance of two parental nuclei varies depending on the strain. In some strains parental nuclei are found, on average, at approximately the same rate across the mycelium, indicating that the fungus
potentially involved in mating, that is, a mating type (MAT) locus, was reportedly found in AMF, adding support to the notion that these prominent symbionts are not purely asexual organisms. Genome analyses have also revealed important aspects of their lifestyle. In particular, genome sequences from all known families showed that AMF lack genes involved in the production of fatty acids and sugars, meaning that they are completely dependent on their plant hosts for basic nutrition (and survival). Investigations also revealed that AMF genomes are relatively large compared with those of relatives in the fungal kingdom, ranging from 50 Megabases to well over one Gigabase. This large size strongly correlates with their repetitive nature, with large AMF genomes containing many transposable elements and redundant gene families (some gene families carry well over 2,000 members), while smaller ones like the early branching lineage Paraglomus are significantly more streamlined. Another notable aspect of AMF is that their genomes can vary dramatically in content among closely related strains. For example, multiple strains claimed to belong to the model species R. irregularis can at the extreme vary by up to 50% in gene and repeat content. While this suggests that these strains may actually represent distinct species (or a species complex), the high among-strains genome variability identified in AMF also underpins its role in allowing these organisms to rapidly tackle environmental changes and colonise different plant hosts. It may also explain why they are so ubiquitous across diverse ecosystems globally.
97 Microbiology Today October 2022 | microbiologysociety.org
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