Population structure of Bursaphelenchus xylophilus within single Pinus thunbergii trees inoculated with two nematode isolates

A study was performed to clarify the population structure of the pinewood nematode, Bursaphelenchus xylophilus, within single Pinus thunbergii trees after double infection of nematode populations using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Two nematode isolates, which had different levels or the same level of virulence, were inoculated into 6-year-old trees at the same or different times and then the propagated nematodes were collected from the trees after 1, 6 and 9 months. When a virulent and an avirulent isolate were inoculated into a single tree, an overwhelming propagation of the virulent isolate was observed there irrespective of the inoculation order of isolates or collection time of nematodes. However, when two virulent isolates were inoculated, propagation through the interbreeding between the two isolates was observed. In the case of the staggered inoculations with two virulent isolates, the frequency of nematodes with a PCR-RFLP pattern of the primarily inoculated isolate increased with the time after nematode inoculations. This suggested that the population structure of B. xylophilus within a single tree varied by the virulence level of nematode populations transmitted and their transmission order.