MITOCHONDRIAL-DNA LENGTH VARIATION IN MELOIDOGYNE-INCOGNITA ISOLATES OF ESTABLISHED GENETIC-RELATIONSHIPS - UTILITY FOR NEMATODE POPULATIONSTUDIES

Six Meloidogyne incognita isolates with previously characterized genet ic relationships were used to test the utility of a 63 base-pair mitoc hondrial Variable Number Tandem Repeat (VNTR) as a marker for populati on studies. The polymerase chain reaction (PCR) was used to amplify th is locus and to measure copy number and allele frequencies of the 63 b p VNTR. Individual nematodes were typically heteroplasmic and maintain ed mitochondrial DNA (mtDNA) molecules containing up to thirteen disti nguishable VNTR size classes. Each allele was composed of one to 21 re peat copies. Hierarchical statistics revealed that diversity was low ( 7%) among the isolates whereas 60% of the total genetic diversity meas ured for these six isolates resides within individuals. Likelihood rat io tests revealed that diversity indices were independent of genetic r elatedness and race designation, limiting the utility of this locus fo r studies of population differentiation. As M. incognita is an obligat e parthenogen, paternal contribution to heteroplasmy is excluded and d iversity within individual nematodes at this mtDNA locus is primarily a consequence of mutation to different repeat copy numbers. (C) Orstom /Elsevier, Paris.