Summary
A bioRxiv preprint reports nine high-confidence hybrids between invasive Bursaphelenchus xylophilus and native B. mucronatus in Japanese field samples. Genome-wide analyses identified both F1-like and advanced backcross-like individuals, including some missed by a single-locus test.
A bioRxiv preprint reports genome-wide evidence of repeated gene flow between the invasive pinewood nematode Bursaphelenchus xylophilus and the native B. mucronatus in Japan. Among 342 field-collected nematodes genotyped across 2023 and 2024, the researchers identified nine high-confidence hybrids—2.6% of the sample—including individuals with genetic profiles resembling both first-generation hybrids and later backcrosses.
The findings point to introgression: genetic material passing between species through hybrid offspring and subsequent generations. The work is a preprint, and its field-sample result is not a population-wide estimate of how often the species hybridise.
Genome-wide screening found hybrids missed by a single marker
The team used multiplexed inter-simple sequence repeat genotyping by sequencing, then applied pedigree-calibrated analysis with HIest v2.0 to 48 highly informative diagnostic single-nucleotide polymorphisms (SNPs). They also examined a broader dataset of 2,756 SNPs using ADMIXTURE, NeighborNet and principal component analyses.
The nine hybrids included relatively balanced, F1-like profiles and strongly parentally biased profiles classified as advanced backcross-like. The latter are consistent with hybrid descendants crossing back towards one parental species, leaving ancestry that may be difficult to spot from a small number of genetic markers. Four of the nine hybrids—44.4%—would have gone undetected with the single-locus diagnostic marker used for comparison. That result highlights how a marker-based screen can miss individuals whose inherited ancestry varies across the genome.
The evidence therefore indicates recurrent, multigenerational introgression in the sampled Japanese contact zone, rather than only a single set of first-generation crosses. The study links this gene flow to a forest pathogen: B. xylophilus causes pine wilt disease.
Crossing assays point to incomplete reproductive barriers
Chemotaxis assays detected no volatile-mediated pre-mating discrimination between the nematodes. Crosses also showed that reproductive barriers after mating were incomplete and asymmetric. When B. xylophilus was the maternal parent, hatchability was reduced, particularly in crosses with B. mucronatus mucronatus. Reciprocal crosses generally had higher hatchability, although some showed reduced fecundity.
Together, the field genotypes and crossing assays offer evidence for both natural gene flow and a possible reproductive route between the species. The reported results concern genetic ancestry and reproductive outcomes; they do not establish that hybrids cause more severe pine wilt disease.
The authors also identify a possible implication for Europe, where they say B. mucronatus kolymensis is widespread and can produce viable hybrid offspring with B. xylophilus in both crossing directions. They suggest that establishment of the pathogen there could create opportunities for introgression. This is a conditional risk, not a report of gene flow already detected in European populations.