Artificial induction of third-stage dispersal juveniles of Bursaphelenchus xylophilus using newly established inbred lines

The pine wood nematode, Bursaphelenchus xylophilus, is the causal agent of pine wilt disease. This nematode has two developmental forms in its life cycle; i.e., the propagative and dispersal forms. The former is the form that builds up its population inside the host pine. The latter is specialized for transport by the vector. This form is separated into two dispersal stages (third and fourth); the third-stage dispersal juvenile (J(III)) is specialized for survival under unfavorable conditions, whereas the fourth-stage juvenile (J(IV)), which is induced by a chemical signal from the carrier Monochamus beetle, is transported to new host pines and invades them. Because of its importance in the disease cycle, molecular and chemical aspects of the J(IV) have been investigated, while the mechanism of J(III) induction has not been sufficiently investigated. In an effort to clarify the J(III) induction process, we established inbred lines of B. xylophilus and compared their biological features. We found that the total number of nematodes (propagation proportion) was negatively correlated with the J(III) emergence proportion, likely because nematode development was arrested at J(III); i.e., they could not develop to adults via the reproductive stage. In addition, J(III) induction seemed to be regulated by a small number of genes because the J(III) induction proportion varied among inbred lines despite the high homozygosity of the parental line. We also demonstrated that J(III) can be artificially induced by the nematode’s secreted substances. This is the first report of artificial induction of J(III) in B. xylophilus. The dauer (dispersal) juvenile of the model organism Caenorhabditis elegans corresponds functionally to J(III) of B. xylophilus, and this stage is known to be induced by a chemical signal referred to as daumone, derived from the nematodes’ secretion. The artificial induction of J(III) suggests the presence of daumone-like material in B. xylophilus.