Distribution and relationship of chromosome-specific dispensable DNA sequences in diverse isolates of Colletotrichum gloeosporioides

DNA hybridization probes specific to a dispensable 1 2 Mb chromosome identified in some isolates of biotype B of Colletotrichum gloeosporioides pathogenic on the legume Stylosanthes guianensis were used to assess the distribution and chromosomal location of homologous sequences in a diverse collection of other Colletotrichum isolates. Homologous sequences were not detected in isolates representing C. lindemuthianum, C. musae, C. crassipes, C. trifolii, C. dematium and C. truncatum but were in some C. gloeosporioides isolates. Thus these sequences were not conserved in the genus and deleted in some C. gloeosporioides strains. Partly homologous DNA sequences were detected in genetically distinct biotypes of C. gloeosporioides pathogenic on hosts other than Stylosanthes, which suggests that these DNA sequences may have evolved within the species C. gloeasporioides prior to host specialization. Highly homologous chromosomes were identified in some isolates of the fungus from Stylosanthes from outside Australia including one isolate from the Philippines which appeared to have a 1.2 Mb chromosome almost identical to that of Australian biotype B strains. The DNA probes revealed a range of sizes of homologous chromosomes in other strains of C. gloeosporioides but in general the chromosome-specific DNA probes were linked, suggesting a common origin with subsequent gross chromosomal rearrangement or exchange. The possibility that the chromosome-specific markers may be useful diagnostic probes for biotype B isolates virulent on S. guianensis cv. Graham was tested using 10 new isolates with this virulence phenotype and three of these isolates lacked both the chromosome and the DNA sequence homology. This suggests that the 1.2 Mb mini-chromosome is not associated with race-specificity The significance of these results for the origin and function of dispensable chromosomes in C. gloeosporioides is discussed