It has been hypothesized that horizontal gene/chromosome transfer and parasexual recombination following hyphal fusion between different strains may contribute to the emergence of wide genetic variability in plant pathogenic and other fungi. However, the significance of vegetative (heterokaryon) incompatibility responses, which commonly result in cell death, in preventing these processes is not known. In this study, we have assessed this issue following different types of hyphal fusion during colony initiation and in the mature colony. We used vegetatively compatible and incompatible strains of the common bean pathogen Colletotrichum lindemuthianum in which nuclei were labelled with either a green or red fluorescent protein in order to microscopically monitor the fates of nuclei and heterokaryotic cells following hyphal fusion. As opposed to fusion of hyphae in mature colonies that resulted in cell death within 3 h, fusions by conidial anastomosis tubes (CAT) between two incompatible strains during colony initiation did not induce the vegetative incompatibility response. Instead, fused conidia and germlings survived and formed heterokaryotic colonies that in turn produced uninucleate conidia that germinated to form colonies with phenotypic features different to those of either parental strain. Our results demonstrate that the vegetative incompatibility response is suppressed during colony initiation in C. lindemuthianum. Thus, CAT fusion may allow asexual fungi to increase their genetic diversity, and to acquire new pathogenic traits.
Colletotrichum lindemuthianum, causal agent of anthracnose in the common bean, has wide genetic variability. Differential bean cultivars and morphological and physiological characteristics were used to analyze 74 isolates of C. lindemuthianum collected in two counties in the state of Minas Gerais, Brazil. Six different races were found, with a predominance of race 65 at both locations. Isolates were classified according to their sensitivities to the fungicide thiophanate-methyl, normally used in the control of common bean anthracnose. In all, ≈10% of isolates were resistant to the fungicide in vitro. Characteristics such as indexes of mycelia growth rate, colony diameter, sporulation capacity, and percentage of germination demonstrated the high genetic variability of C. lindemuthianum. We also observed variation in conidial cytology. The conidia of most isolates showed septa formation after germination, in contrast to septa absence, previously reported in the literature. Sexual and asexual reproduction were evaluated for mechanisms that may contribute in the generation of variability in C. lindemuthianum. Conidial anastomosis tubes were commonly found, indicating that asexual reproduction can help increase variability in this species. Information from this study confirmed high variability in C. lindemuthianum and will guide future studies in basic knowledge and applied technologies.
Colletotrichum lindemuthianum, the causal agent of anthracnose in the common bean (Phaseolus vulgaris), presents a wide genetic and pathogenic variability that gives rise to complications in the development of resistant bean cultivars. The aim of this study was to identify the variability within race 65 of C. lindemuthianum, the race most commonly encountered in Brazil, through randomly amplified polymorphic DNA (RAPD) and anastomosis analyses. Thirteen isolates of race 65, collected in different years and from various host cultivars located in diverse areas of the state of Minas Gerais, Brazil, were investigated. Twenty-four RAPD primers were employed and 83 polymorphic bands amplified. Genetic similarities were estimated from the Sorensen-Dice coefficient and ranged from 0.54 to 0.82. The dendrogram obtained by cluster analysis classified the isolates into 11 separate groups. For the purposes of the analysis of anastomosis, isolates were considered to be compatible when the fusion of hyphae from different isolates could be observed. The proportion of compatible reactions for each isolate was estimated and similarity estimates, based on the Russel & Rao coefficient, ranged from 0.28 to 0.85. Isolates were classified into 11 anastomosis groups, 10 of which were formed by only one isolate. Although isolates LV61, LV73 and LV58 were classified in the same anastomosis group, they were genetically distinct according to RAPD analysis. Results from both RAPD and anastomosis analyses revealed great variability within C. lindemuthianum race 65.
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