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Hyphal anastomosis criteria have been used extensively to place isolates of Rhizoctonia into taxonomically distinct groups called anastomosis groups. In practice, hyphal anastomosis is determined in several ways. The most commonly employed practice involves pairing two isolates of Rhizoctonia on a glass slide and allowing them to grow together. The area of merged hyphae is stained and examined microscopically for the resulting hyphal interaction(s).Pairing of isolates belonging to the same AG-results in hyphal fusion (anastomosis), leading to either acceptance (self-pairings) or rejection (somatic incompatibility). Pairings between AGs do not result in hyphal fusion, suggesting greater genetic differences between isolates (i.e., different species, etc.) Interpretation of anastomosis reaction is not always straightforward because the four hyphal interaction phenotypes (C0 to C3) represent a continuum. Within an AG, two types of hyphal interactions (C2 and C3) are most relevant for the study of population biology. The C2 reaction (also referred as killing reaction), represents a somatic incompatibility response between genetically distinct individuals. The C3 reaction (perfect fusion) between two isolates is indicative of genetic identity or near identity.Very little is known about the genetic mechanisms controlling this recognition process in Rhizoctonia. In other filamentous fungi, somatic incompatibility is controlled by several genes with multiple alleles. For two fungal isolates to be compatible, all somatic compatibility loci must be the same.Isolates of R. solani have been assigned to 12 AGs. Recent protein and DNA-based studies support the separation of R. solani into genetically distinct groupings, but has also revealed considerable genetic diversity within an anastomosis group. Hyphal anastomosis and molecular methods are currently being used to further examine the taxonomy, ecology and pathology of R. solani.
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