Molecular evidence for chromosome transfer between biotypes of Colletotrichum gloeosporioides

Two genetically distinct biotypes (A and B) of Colletotrichum gloeosporioides that cause different anthracnose diseases on the legumes of Stylosanthes spp. have been identified in Australia. All virulent isolates of these biotypes are anamorphic. Laboratory pairings of nitrate reductase (nit) mutants representing the two biotypes indicated that biotypes A and B form separate vegetative compatibility groups (VCGs). We have investigated the mechanisms leading to a major karyotype polymorphism in this fungus. All biotype A isolates studied to date carry a 2-Mb chromosome while some biotype B isolates carry an apparently dispensable but homologous 1.2-Mb chromosome. A biotype B-like field isolate termed Bx was recently obtained from S. guianensis in Northern Australia. The Bx isolate was shown to carry two homologous chromosomes, 1.2 Mb and 2 Mb in size. These two chromosomes lacked two repeat sequences that were present on other chromosomes in biotype B and these repeats appeared to be monomorphic in biotype B and Bx. This suggests the 1.2-Mb and 2-Mb chromosomes may be recent additions to the Bx genome. Analysis of the DNA sequence organization of the 2-Mb and 1.2-Mb chromosomes in Bx by means of chromosome-specific DNA markers indicated that these chromosomes were each identical to the 2-Mb and 1.2-Mb sized homologous chromosomes in biotype A and some biotype B isolates, respectively. Extensive analysis of the rest of the genome of Bx by restriction fragment length polymorphisms, random amplified polymorphic DNAs, and minichromosome profiling indicated that the background genome of Bx was like that of biotype B. No other biotype A-like markers outside of the 2-Mb chromosome were identified in Bx. Pairing of nit mutants indicated that Bx was part of the biotype B VCG. The results therefore indicate that a dispensable 2-Mb chromosome in the biotype B isolate Bx most probably originated by a relatively recent "horizontal" transfer from biotype A. The results suggest that the occasional transfer of specific chromosomes may occur between apparently genetically isolated clonal lines of asexual pathogens.