evolved a unique (and yet undescribed) cellular mechanism to tightly control their relative abundance. However, in other strains one parental nucleotype always dominates within the cytoplasm. In extreme cases, one parental genotype represents up to 80% of all nuclei present within individual spores, indicating that the nuclei may compete against each other for ‘dominance’ within the same fungal cells. Remarkably, the nuclear dynamics of AMF heterokaryons vary depending on the plant host. In symbiosis with carrots ( Daucus carota ) most of the nuclei from one specific parent were dominant within some AMF strains, yet changing the plant host to chicory ( Cichorium intybus ) resulted in dramatic shifts in nuclear dynamics leading to the other parental
genotype suddenly dominating in the cytoplasm. Overall, these results suggest that the plant host identity can influence the genetics of the fungal symbionts, thus revealing a previously unknown layer of genetic complexity and dynamism within the intimate interactions that occur between the partners of this prominent terrestrial symbiosis. The molecular mechanisms involved in changing AMF genetics are unknown, but it is intriguing to speculate that these are dictated by the plant host’s needs. Regardless, these rapid nuclear shifts most likely affect the transcriptional content of AMF heterokaryons and, consequently, the phenotypic traits of both symbiotic partners. The notion that plant hosts can control the expression output of their AMF symbiont is also supported by recent chromatin-
Coloured scanning electron micrograph of mycorrhiza. Eye of Science/Science Photo Library
98 Microbiology Today October 2022 | microbiologysociety.org
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