Abstract:Cell-to-cell fusion is a fundamental biological process across the tree of life. In filamentous fungi, somatic fusion (or anastomosis) is required for the normal development of their syncytial hyphal networks, and it can initiate non-sexual genetic exchange processes, such as horizontal genetic transfer and the parasexual cycle. Although these could be important drivers of the evolution of asexual fungi, this remains a largely unexplored possibility due to the lack of suitable resources for their study in thes… Show more
“…We also used resistance cassette-specific primers for PCR screening of 14 of these heterokaryons, which confirmed their heterokaryotic nature. Given the complete suppression of conidial fusion on PDA and the previously demonstrated possibility of conidial fusion in a xylem sap-simulating medium [ 37 ], our results suggest that the isolated “compatible” and “incompatible” heterokaryons arose from fusion between strains in planta . Fig.…”
Section: Resultsmentioning
confidence: 56%
“…Starvation induces significantly CAT-mediated fusion of V. dahliae conidia/germlings [ 37 ], and this motivated us to investigate the possibility of conidial fusion between strains of different VCGs under such conditions. Seven representative strains (Additional file 1 : Table S1 [ 36 – 38 ]; Fig.…”
Section: Resultsmentioning
confidence: 99%
“…We recently discovered that starvation of cells induces significantly CAT-mediated fusion of V. dahliae conidia/germlings [ 37 ]. Here, we show that this happens regardless of their VCG classification, even between species, and that inter-VCG fusions often form viable heterokaryotic cells and colonies.…”
Section: Discussionmentioning
confidence: 99%
“…Our previous finding that heterokaryosis in V. dahliae is occasionally possible even between strains of different VCGs (thus being considered incompatible) [ 36 ] raises the question whether parasexuality in asexual fungi may be more important than previously recognized. We recently established fusion between spores via conidial anastomosis tubes (CATs) as a convenient system for addressing this question [ 37 ].…”
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.”
“…We also used resistance cassette-specific primers for PCR screening of 14 of these heterokaryons, which confirmed their heterokaryotic nature. Given the complete suppression of conidial fusion on PDA and the previously demonstrated possibility of conidial fusion in a xylem sap-simulating medium [ 37 ], our results suggest that the isolated “compatible” and “incompatible” heterokaryons arose from fusion between strains in planta . Fig.…”
Section: Resultsmentioning
confidence: 56%
“…Starvation induces significantly CAT-mediated fusion of V. dahliae conidia/germlings [ 37 ], and this motivated us to investigate the possibility of conidial fusion between strains of different VCGs under such conditions. Seven representative strains (Additional file 1 : Table S1 [ 36 – 38 ]; Fig.…”
Section: Resultsmentioning
confidence: 99%
“…We recently discovered that starvation of cells induces significantly CAT-mediated fusion of V. dahliae conidia/germlings [ 37 ]. Here, we show that this happens regardless of their VCG classification, even between species, and that inter-VCG fusions often form viable heterokaryotic cells and colonies.…”
Section: Discussionmentioning
confidence: 99%
“…Our previous finding that heterokaryosis in V. dahliae is occasionally possible even between strains of different VCGs (thus being considered incompatible) [ 36 ] raises the question whether parasexuality in asexual fungi may be more important than previously recognized. We recently established fusion between spores via conidial anastomosis tubes (CATs) as a convenient system for addressing this question [ 37 ].…”
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.”
“…Reactive oxygen species (ROS) are known to play an important role in redox signaling pathways, including cell differentiation, development, and cytoskeleton remodeling [ 48 ]. NADPH-oxidases (NOX) are responsible for the production of superoxide (ROS) by oxidizing NADPH and reducing molecular oxygen, which is believed to be involved during communication, cell fusion, and CAT fusion in several fungi, e.g., N. crassa, F. oxysporum, and V. dahliae [ 48 , 49 , 50 , 51 ]. It has been shown that NADPH oxidase BcNoxA or Nox regulator BcNoxR gene deletion mutant of Grey mold Botrytis cinerea were defective in CAT fusion [ 52 ].…”
The conidia of a hemibiotrophic fungus, Colletotrichum gloeosporioides, can conventionally form a germ tube (GT) and develop into a fungal colony. Under certain conditions, they tend to get connected through a conidial anastomosis tube (CAT) to share the nutrients. CAT fusion is believed to be responsible for the generation of genetic variations in few asexual fungi, which appears problematic for effective fungal disease management. The physiological and molecular requirements underlying the GT formation versus CAT fusion remained underexplored. In the present study, we have deciphered the physiological prerequisites for GT formation versus CAT fusion in C. gloeosporioides. GT formation occurred at a high frequency in the presence of nutrients, while CAT fusion was found to be higher in the absence of nutrients. Younger conidia were found to form GT efficiently, while older conidia preferentially formed CAT. Whole transcriptome analysis of GT and CAT revealed highly differential gene expression profiles, wherein 11,050 and 9786 genes were differentially expressed during GT formation and CAT fusion, respectively. A total of 1567 effector candidates were identified; out of them, 102 and 100 were uniquely expressed during GT formation and CAT fusion, respectively. Genes coding for cell wall degrading enzymes, germination, hyphal growth, host-fungus interaction, and virulence were highly upregulated during GT formation. Meanwhile, genes involved in stress response, cell wall remodeling, membrane transport, cytoskeleton, cell cycle, and cell rescue were highly upregulated during CAT fusion. To conclude, the GT formation and CAT fusion were found to be mutually exclusive processes, requiring differential physiological conditions and sets of DEGs in C. gloeosporioides. This study will help in understanding the basic CAT biology in emerging fungal model species of the genus Colletotrichum.
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