Starvation-induced cell fusion and heterokaryosis frequently escape imperfect allorecognition systems in an asexual fungal pathogen

Abstract

Background Asexual fungi include important pathogens of plants and other organisms, and their effective management requires understanding of their evolutionary dynamics. Genetic recombination is critical for adaptability and could be achieved via heterokaryosis — the co-existence of genetically different nuclei in a cell resulting from fusion of non-self spores or hyphae — and the parasexual cycle in the absence of sexual reproduction. Fusion between different strains and establishment of viable heterokaryons are believed to be rare due to non-self recognition systems. Here, we investigate the extent and mechanisms of cell fusion and heterokaryosis in the important asexual plant pathogen Verticillium dahliae. Results We used live-cell imaging and genetic complementation assays of tagged V. dahliae strains to analyze the extent of non-self vegetative fusion, heterokaryotic cell fate, and nuclear behavior. An efficient CRISPR/Cas9-mediated system was developed to investigate the involvement of autophagy in heterokaryosis. Under starvation, non-self fusion of germinating spores occurs frequently regardless of the previously assessed vegetative compatibility of the partners. Supposedly “incompatible” fusions often establish viable heterokaryotic cells and mosaic mycelia, where nuclei can engage in fusion or transfer of genetic material. The molecular machinery of autophagy has a protective function against the destruction of “incompatible” heterokaryons. Conclusions We demonstrate an imperfect function of somatic incompatibility systems in V. dahliae. These systems frequently tolerate the establishment of heterokaryons and potentially the initiation of the parasexual cycle even between strains that were previously regarded as “incompatible.”

Components of TOR and MAP kinase signaling control chemotropism and pathogenicity in the fungal pathogen Verticillium dahliae

“…To confirm the role of these genes in TOR signaling of V. dahliae , we tested sensitivity of Δ gtr1 and Δ tsc2 to factors that induce autophagy and compared their phenotypes to that of the Δ atg1 mutant which is impaired in autophagy (Vangalis et al, 2021b). The ability of conidia to germinate in the absence of nitrogen sources was significantly reduced in all the deletion mutants compared to the wild-type strain (Fig.…”

“…In addition to their role in chemotropism, gtr1 and tsc2 were required for normal morphogenesis, physiology, and stress response, indicating a conserved role of the TOR signaling pathway in fungal development (Yu et al, 2014). The conidial germination defect of the Δtsc2 mutant could be attributed to its inability to undergo autophagy, a process that was shown to be important for conidial germination (Kikuma et al, 2006;Vangalis et al, 2021b).…”

“…In addition to their role in chemotropism, gtr1 and tsc2 were required for normal morphogenesis, physiology, and stress response, indicating a conserved role of the TOR signaling pathway in fungal development (Yu et al, 2014). The conidial germination defect of the Δ tsc2 mutant could be attributed to its inability to undergo autophagy, a process that was shown to be important for conidial germination (Kikuma et al, 2006; Vangalis et al, 2021b). Besides the contributions of Gtr1 and Tsc2 to fungal chemotropism towards root signals and other developmental processes, both components were found to be required for full virulence of V. dahliae on eggplants, further supporting a conserved role of TOR signaling in virulence of fungal plant pathogens (Li et al, 2019; Yu et al, 2014).…”

Monocyte–Macrophage Lineage Cell Fusion

“…Studies also showed that hematopoietic stem cell bias is regulated by the distinct niche they occupy in bone marrow [ 7 ]. Based on the bias, myeloid-biased (My-Bi), balanced (Ba), lymphoid-biased (Ly-Bi), platelet-biased (Pl-Bi), and quiescent Peroxisome Proliferator-Activated Receptor γ positive (PPARγ+) osteoclast-biased (Os-Bi) progenitors have been discovered ( Figure 1 ; [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]).…”

A Nuclei-Based Conceptual Model of (Eco)evolutionary Dynamics in Fungal Heterokaryons

“…It was previously thought that heterokaryon formation is rare in nature due to heterokaryon incompatibility (HI), i.e., regulated cell death occurring at the place of fusion between the two hyphae of incompatible mycelia ( Glass and Dementhon, 2006 ; Daskalov et al, 2017 ; Gonçalves et al, 2020 ; Rico-Ramírez et al, 2022 ). Whilst this is not always the case ( Heller et al, 2018 ; Daskalov et al, 2019 ), evidence is accumulating that in spore germination phase, HI mechanisms may be relaxed or absent under different environmental conditions ( Roca et al, 2005 ; Ishikawa et al, 2012 ; Mehta and Baghela, 2021 ; Vangalis et al, 2021 ). Fusion of two mature colonies of incompatible strains of Colletotrichum lindemuthianum induces cell death, while conidial anastomosis tube (CAT) fusion generates a stable mycelium, capable of producing uninucleate conidia phenotypically different from the parents ( Ishikawa et al, 2012 ).…”