Abstract
The Bursaphelenchus mucronatus, which was highly similar with Bursaphelenchus xylophilus in terms of morphological characteristics and biological properties—but had weaker pathogenicity to forests—was a native species often displaced by B. xylophilus when occupying the same niche. Since the draft genome of the invasive B. xylophilus has been published, the absence of a reference genome of B. mucronatus still prevents us from understanding the molecular evidences behind competitive displacement. In this study, we employed Single Molecule, Real-Time (SMRT) sequencing and a Hi-C scaffolding approach to yield a near chromosome-level assembly of B. mucronatus, including six pseudo-chromosomes. The assembly size is 73 Mb, with scaffold N50 of 11.50 Mb and contig N50 of 1.48 Mb. Comparative genomics results showed high similarity between B. xylophilus and B. mucronatus. However, the losing of orphan genes and species-specific orthologous genes in B. mucronatus may indicate weaker adaptability to the environment. The gene family contractions of GPCRs (G Protein-Coupled Receptors) and cellulases in B. mucronatus may jointly contribute to its displacement by B. xylophilus. Overall, we introduced a valuable genomic resource for molecular and evolutionary studies of B. mucronatus, especially for studying the competitive displacement by the pinewood nematode, which could help us control the pathogenicity of pine wilt diseases.
Highlights
The pinewood nematode, Bursaphelenchus xylophilus, was introduced from North America toAsia and Europe; it is a global quarantine pest and the main cause of pine wilt disease
Combined with gene family results, we found that the number of single-copy genes, srn-1(srh family), whose function in C. elegans is amphid sensory neurons [58] is much higher in B. xylophilus (22 genes) than in B. mucronatus (13 genes)
We find that the daf-38 that was mediating ascaroside perception in C. elegans has five copies in B. xylophilus, and two copies in B. mucronatus, which may result in a weaker ability to conduct chemical communications within B. mucronatus populations
Summary
The pinewood nematode, Bursaphelenchus xylophilus, was introduced from North America toAsia and Europe; it is a global quarantine pest and the main cause of pine wilt disease. B. xylophilus in terms of morphological characteristics and biological properties [3], but has weaker pathogenicity to forests [4,5,6,7]. The tail of female B. mucronatus is conical and has a distinct tail tip [6,8], whereas B. xylophilus has a rounded terminus [8]. They occupy the same niche, reproduce in the resin canals, feed on epithelial cells in living hosts, consume the host’s symbiotic fungi in dead hosts, and are even transported to host pine trees by the same insect vectors, Monochamus beetles [4,9]. Previous work, which mostly focused on comparisons of biological characteristics (morphology, pathogenicity, reproduction, etc.) between B. xylophilus and B. mucronatus [7,11,12,13] could not sufficiently explain why and how the competitive replacement took place
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