Telomere fingerprinting for assessing chromosome number, isolate typing and recombination in the entomopathogen Beauveria bassiana

Beauveria bassiana is a popular biocontrol agent used as 'green' pesticide in crop insect pest management. Chromosome number has been variously reported as five, six, seven and eight in this species. The range of chromosome number and the minimum chromosome number in this economically important fungus were assessed through telomere fingerprint analysis of a sample of 17 isolates from different and similar hosts and distant and same geographic origin. Genomic DNA digested with EcoRI, which has no cutting site in the telomere repeat sequence arrays was probed with a radioisotope-labelled (5'-TTAGGG-3')(8) oligonucleotide. The probe-hybridised regions appeared as discrete bands - each representing a telomere. The number of bands in each lane was counted and halved to arrive at the chromosome number of that isolate. The chromosome number varied from 5 to 10 in the different isolates. The telomere probe hybridised bands were also scored for presence or absence in a 0-1 matrix and a dendrogram based on similarities between the isolates was constructed using the NTSYS-pc ver. 2.02i software. The isolates showed very little similarity; the overall similarity was 14%. Only two isolates which were of diverse host and geographic origin showed 100% similarity. Isolates from the same epizootic that showed 43% similarity in their telomere fingerprints had 96% similarity in their RAPD (Random amplified polymorphic DNA) fingerprints with 10 primers. The genetic distances computed from any one DNA fingerprinting method thus do not reflect the true genetic similarities of the isolates. The frequency distribution pattern of the pair-wise similarities computed from telomere fingerprints hinted at the occurrence of recombination in this fungus. Telomere fingerprinting proved very useful in typing isolates since each of them was found to have a unique fingerprint. Isolates with the same chromosome number neither showed a distinct morphology or virulence character nor a close similarity in telomere or RAPD fingerprints to merit their subgrouping into a taxonomically relevant or practically useful unit.