GENETIC DIVERSITY IN SCLEROTIUM (ATHELIA) ROLFSII AND RELATED SPECIES

Pairings among 119 isolates of Sclerotium rolfsii, 11 of S. delphinii, and two of S. coffeicola on potato-dextrose agar to establish mycelial compatibility groups (MCGs) revealed 49, 3, and 2 MCGs, respectively, in a worldwide collection. Within an MCG, isolates were often from the same host and geographical area; however, widely diverse isolates also were grouped within the same MCG. The host of origin of the isolate was not correlated with the MCG except in S. delphinii. Many MCGs were comprised of only one isolate. Variation in nuclear rDNA [internal transcribed spacer (ITS) regions] was examined following restriction enzyme digests. Restriction fragment length polymorphisms (RFLPs) were obtained with AluI, HpaII, RsaI, and MbaI and could distinguish amongst the three Sclerotium spp. and three Athelia spp. (an outgroup). Combined banding patterns for the four enzymes were used to characterize intraspecific variation in the three Sclerotium spp. There were 12 subspecific groupings in S. rolfsii, one in S. delphinii, and two in S. coffeicola; some of these groupings correlated with their MCG. However, isolates within an MCG could show different ITS-RFLP patterns and certain patterns were also dispersed among different MCGs. The total sum of digested fragment sizes exceeded the undigested polymerase chain reaction product in several S. rolfsii isolates. This length discrepancy was not due to artifacts or incomplete digestion and, therefore, must have resulted from variation among rDNA copies in the presence of restriction sites. Furthermore, segregation of two MboI restriction patterns in the ITS region among 29 single-basidiospore strains, derived from four parental field isolates of S. rolfsii with one pattern, suggested the presence of two distinct rDNA types in the field isolates. The rDNA types may reflect a heterokaryotic nuclear condition in field isolates of S. rolfsii. Restriction maps and phylogenetic analyses supported a close affinity of the three Sclerotium spp., which may be more appropriately designated as subspecific varieties of S. rolfsii.