Quorum sensing-mediated inter-specific conidial anastomosis tube fusion between Colletotrichum gloeosporioides and C. siamense

Conidia Fusion: A Mechanism for Fungal Adaptation to Nutrient-Poor Habitats

Conidia fusion (CF) is a commonly observed structure in fungi. However, it has not been systematically studied. This study examined 2457 strains of nematode-trapping fungi (NTF) to explore the species specificity, physiological period, and physiological significance of CF. The results demonstrated that only six species of Arthrobotrys can form CF among the sixty-five tested NTF species. The studies on the model species Arthrobotrys oligospora (DL228) showed that CF occurred in both shed and unshed plus mature and immature conidia. Additionally, the conidia fusion rate (CFR) increased significantly with the decrease of nutrient concentration in habitats. The studies on the conidia fusion body (CFB) produced by A. oligospora (DL228) revealed that the more conidia contained in the CFB, the faster and denser the mycelia of the CFB germinated in weak nutrient medium and soil plates. On the one hand, rapid mycelial extension is beneficial for the CFB to quickly find new nutrient sources in habitats with uneven nutrient distribution. On the other hand, dense mycelium increases the contact area with the environment, improving the nutrient absorption efficiency, which is conducive to improving the survival rate of conidia in the weak nutrient environment. In addition, all species that form CF produce smaller conidia. Based on this observation, CF may be a strategy to balance the defects (nutrient deficiency) caused by conidia miniaturization.

Whole genome resequencing reveal patterns of genetic variation within Colletotrichum acutatum species complex from rubber trees in China

The Colletotrichum acutatum species complex possesses a diverse number of important traits, such as a wide host range and host preference, different modes of reproduction, and different strategies of host infection. Research using comparative genomics has attempted to find correlations between these traits. Here, we used multi-locus techniques and gene genealogical concordance analysis to investigate the phylogenetic relationships and taxonomic status of the Colletotrichum acutatum species complex using field isolates obtained from rubber trees. The results revealed that the dominant species was C. australisinense, followed by C. bannaense, while strain YNJH17109 was identified as C. laticiphilum. The taxonomic status of strains YNLC510 and YNLC511 was undetermined. Using whole-genome single nucleotide polymorphism data to analyze population structure, 18 strains of C. australisinense were subsequently divided into four populations, one of which was derived from an admixture of two populations. In addition, the strains LD1687, GD1628, and YNLC516, did not belong to any populations, and were considered to be admixtures of two or more populations. A split decomposition network analysis also provided evidence for genetic recombination within Colletotrichum acutatum species complex from rubber trees in China. Overall, a weak phylogeographic sub-structure was observed. Analysis also revealed significant differences in morphological characters and levels of virulence between populations.

Genetic Analysis of Colletotrichum siamense Populations from Different Hosts and Counties in Hainan, China, Using Microsatellite Markers

Colletotrichum siamense was demonstrated as the dominant species among Colletotrichum spp. that infected rubber tree, areca palm, and coffee in Hainan, China. However, the extent of genetic differentiation within the species C. siamense in relation to geographical regions and host species is not known. In this study, 112 C. siamense isolates were genotyped with 12 microsatellite markers. In total, there were 99 multilocus genotypes. Results from permutational multivariate analysis of variance and analysis of molecular variance indicated that there was no significant genetic differentiation between fungal populations with respect to host, location (county), and year. Discriminant analysis of principal components and STRUCTURE analysis showed that C. siamense isolates grouped into three clusters; further analysis confirmed that there were significant (P < 0.001) genetic differences among the three clusters. However, each cluster had isolates from different hosts, counties, or years, supporting the lack of genetic differentiation with respect to host, county, and year. Statistical analyses of allelic associations indicated some evidence for recombination within the populations defined on the basis of host or county. The present findings provide insights into the genetic structure of C. siamense on the three perennial host species in Hainan and suggest that the disease on these three crops can be effectively considered as one disease and, hence, needs to be controlled simultaneously in mixed plantations.

Characterization and necrosis-inducing activity of necrosis- and ethylene-inducing peptide 1-like proteins from Colletotrichum australisinense, the causative agent of rubber tree anthracnose

Colletotrichum australisinense, a member of the Colletotrichum acutatum species complex, is an important pathogen causing rubber tree anthracnose. Genome-wide comparative analysis showed this species complex contains more genes encoding necrosis- and ethylene-inducing peptide 1-like proteins (NLPs) than other Colletotrichum species complexes, but little is known about their necrosis-inducing roles in host. The aim of this study was to analyze NLPs number and type in C. australisinense, and characterize their necrosis-inducing activity in host or non-host. According to phylogenetic relationship, conserved the cysteine residues and the heptapeptide motif (GHRHDWE), 11 NLPs were identified and classified into three types. Five of the eleven NLPs were evaluated for necrosis-inducing activity. CaNLP4 (type 1) could not induce necrosis in host or non-host plants. By contrast, both CaNLP5 and CaNLP9 (type 1) induced necrosis in host and non-host plants, and necrosis-inducing activity was strongest for CaNLP9. CaNLP10 (type 2) and CaNLP11 (type 3) induced necrosis in host but not non-host plants. Substitution of key amino acid residues essential for necrosis induction activity led to loss of CaNLP4 activity. Structural characterization of CaNLP5 and CaNLP9 may explain differences in necrosis-inducing activity. We evaluated the expression of genes coding CaNLP by reverse transcription polymerase chain reaction (RT-PCR) and quantitative real-time PCR (qRT-PCR) at different time-points after pathogen infection. It was found that genes encoding CaNLPs with different activities exhibited significantly different expression patterns. The results demonstrate that CaNLPs are functionally and spatially distinct, and may play different but important roles in C. australisinense pathogenesis.

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

Filamentous fungi are characterised by specific features, such as multinuclearity, coexistence of genetically different nuclei and nuclear movement across the mycelial network. These attributes make them an interesting, yet rather underappreciated, system for studying (eco)evolutionary dynamics. This is especially noticeable among theoretical studies, where rather few consider nuclei and their role in (eco)evolutionary dynamics. To encourage such theoretical approaches, we here provide an overview of existing research on nuclear genotype heterogeneity (NGH) and its sources, such as mutations and vegetative non-self-fusion. We then discuss the resulting intra-mycelial nuclear dynamics and the potential consequences for fitness and adaptation. Finally, we formulate a nuclei-based conceptual framework, which considers three levels of selection: a single nucleus, a subpopulation of nuclei and the mycelium. We compare this framework to other concepts, for example those that consider only the mycelium as the level of selection, and outline the benefits of our approach for studying (eco)evolutionary dynamics. Our concept should serve as a baseline for modelling approaches, such as individual-based simulations, which will contribute greatly to our understanding of multilevel selection and (eco)evolutionary dynamics in filamentous fungi.

Hyphal Fusions Enable Efficient Nutrient Distribution in Colletotrichum graminicola Conidiation and Symptom Development on Maize

Hyphal and germling fusion is a common phenomenon in ascomycetous fungi. Due to the formed hyphal network, this process enables a coordinated development as well as an interaction with plant hosts and efficient nutrient distribution. Recently, our laboratory work demonstrated a positive correlation between germling fusion and the formation of penetrating hyphopodia on maize leaves outgoing from Colletotrichum graminicola oval conidia. To investigate the probable interconnectivity of these processes, we generated a deletion mutant in Cgso, in which homologs are essential for cellular fusion in other fungal species. However, hyphopodia development was not affected, indicating that both processes are not directly connected. Instead, we were able to link the cellular fusion defect in ∆Cgso to a decreased formation of asexual fruiting bodies of C. graminicola on the leaves. The monitoring of a fluorescent-labelled autophagy marker, eGFP-CgAtg8, revealed a high autophagy activity in the hyphae surrounding the acervuli. These results support the hypothesis that the efficient nutrient transport of degraded cellular material by hyphal fusions enables proper acervuli maturation and, therefore, symptom development on the leaves.

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

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.”

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01101-5.

Differential Physiological Prerequisites and Gene Expression Profiles of Conidial Anastomosis Tube and Germ Tube Formation in Colletotrichum gloeosporioides

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.