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Starvation-Induced Cell Fusion and Heterokaryosis Frequently Escape

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Starvation-Induced Cell Fusion and Heterokaryosis Frequently Escape Vangalis et al. BMC Biology (2021) 19:169 https://doi.org/10.1186/s12915-021-01101-5 RESEARCH ARTICLE Open Access Starvation-induced cell fusion and heterokaryosis frequently escape imperfect allorecognition systems in an asexual fungal pathogen Vasileios Vangalis1 , Ilya Likhotkin2, Michael Knop2,3 , Milton A. Typas1 and Ioannis A. Papaioannou2* 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.” Keywords: Autophagy, Conidial anastomosis tubes (CATs), CRISPR/Cas9, Heterokaryosis, Heterokaryon incompatibility, Parasexual cycle, Vegetative compatibility, Verticillium dahliae * Correspondence: [email protected] 2Center for Molecular Biology of Heidelberg University (ZMBH), Heidelberg, Germany Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Vangalis et al. BMC Biology (2021) 19:169 Page 2 of 19 Background parasexual interactions in the laboratory [9–11], and Generation of genetic and phenotypic diversity is of ut- the possibly heterokaryosis-mediated horizontal gene most importance for the survival and adaptability of spe- transfer has gained increasing attention in the post- cies over evolutionary timescales [1, 2]. Genetic genomic era as a driver of fungal evolution [24, 25]. reshuffling in meiosis provides a constant supply of gen- The soil-borne asexual ascomycete Verticillium dah- etic substrates for natural selection and enables efficient liae is the causal agent of Verticillium wilt, a disease that purging of deleterious mutations from sexual popula- affects a wide range of plants and accounts for signifi- tions [3, 4]. A complete lack of recombination in organ- cant annual economic losses [26]. This haploid species is isms such as obligate asexual species can result in the considered strictly asexual and propagates predomin- accumulation of problematic alleles, a phenomenon antly by clonal expansion [27]. Despite the apparent lack known as “Muller’s Ratchet” [5]. Some source of recom- of sexual activity, V. dahliae populations exhibit signs of bination is, therefore, usually deemed essential for the recombination, which, although not frequent, has the long-term survival of species. potential to create novel clonal lineages [28, 29]. The In earlier days of fungal genetics, an alternative mech- parasexual cycle has been described in Verticillium spe- anism for genetic recombination, termed the parasexual cies and presents interesting features [30–32]. These in- cycle, was described in fungi [6]. This refers to a com- clude high instability of transient diploid nuclei, which bination of non-sexual processes that lead to genetic di- are formed within heterokaryons following cell fusion, versification through the somatic cell and nuclear fusion, and frequent mitotic recombination during haploidiza- followed by mitotic recombination during ploidy reduc- tion, often leading to recombined progeny [30, 33, 34]. tion (i.e., haploidization) [6, 7]. Meiotic sexual cycles Furthermore, overcoming cell wall-related barriers by may have evolved from ancestral parasexual processes protoplast fusion or nuclear microinjection resulted in during the early eukaryotic evolution [8]. Variations of interspecific heterokaryons between V. dahliae and V. the parasexual cycle are present in several and diverse albo-atrum at frequencies similar to intraspecific combi- extant eukaryotes, i.e., many filamentous fungi and nations [35]. Our previous finding that heterokaryosis in yeasts [9–12], protists [13, 14], and mammalian cells, V. dahliae is occasionally possible even between strains where it can contribute to the phenotypic plasticity of of different VCGs (thus being considered incompatible) tumor cell populations [15]. [36] raises the question whether parasexuality in asexual The parasexual cycle is normally initiated in hetero- fungi may be more important than previously recog- karyons, i.e., cells that host genetically distinct nuclei in nized. We recently established fusion between spores via a common cytoplasm. Heterokaryosis in fungi often conidial anastomosis tubes (CATs) as a convenient sys- arises from somatic non-self fusion (or anastomosis) tem for addressing this question [37]. between hyphae or spores of different strains [16, 17]. In this work, our analyses of the extent and mecha- However, fusion between different individuals and es- nisms of heterokaryosis in the strictly asexual fungus V. tablishment of viable heterokaryons are believed to be dahliae challenge the general assumption of absolute largely prevented in nature by non-self recognition sys- vegetative incompatibility barriers in asexual fungi. Fre- tems, a phenomenon described as vegetative (or hetero- quent heterokaryon formation and transfer of genetic karyon) incompatibility [18–20]. Incompatibility material even between supposedly “incompatible” part- barriers can block cell communication to prevent fu- ners, under non-selective conditions, suggest that para- sion, or they can trigger post-fusion rejection mecha- sexuality could play an important role in genomic and nisms that result in the induction of programmed cell phenotypic diversification. Overall, our study paves the death in the incompatible heterokaryotic cell [19–21]. way for the elucidation of non-sexual genetic interac- The versatile allorecognition mechanisms have been tions in asexual fungi and the evolutionary significance analyzed mostly in the sexual fungi Neurospora crassa, of these processes in adaptation and survival. Podospora anserina,andCryphonectria parasitica, where they are believed to function as defense mecha- Results nisms against the rapid spread of infectious elements Conidial fusion occurs frequently between “incompatible” and somatic parasitism [18, 19]. Fungal strains unable V. dahliae strains under starvation to form viable vegetative heterokaryons because of Starvation induces significantly CAT-mediated fusion of these barriers are referred to as vegetatively incompat- V. dahliae conidia/germlings [37], and this motivated us ible, whereas compatible strains that can fuse and es- to investigate the possibility of conidial fusion between tablish stable heterokaryons are classified into the same strains of different VCGs under such conditions. Seven Vegetative Compatibility Group (VCG) [22]. Despite representative strains (Additional file 1: Table S1 [36– the large perceived extent of incompatibility in natural 38]; Fig. 1a) were labeled with sGFP- or mCherry-tagged populations [22, 23], many fungi seem competent in histone H1 (nuclear localization) for their microscopic Vangalis et al. BMC Biology (2021) 19:169 Page 3 of 19 Fig. 1. Spontaneous non-self fusion via CATs occurs frequently between “incompatible” strains of V. dahliae under starvation. a Wild-type strains used in
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