Species displacement facilitated by ascarosides between two sympatric sibling species: a native and invasive nematode

Competitive displacement is not only the most extreme outcome of interspecific competition, but also an important strategy for invasive species to be successful invaders. Pinewood nematode, Bursaphelenchus xylophilus, the causal agent for pine wilt disease and global quarantine pest, usually displaces Bursaphelenchus mucronatus, a native sympatric sibling species, during its invasion process. Despite this prevalent outcome, the driving forces behind this displacement remain elusive. Ascarosides, an evolutionarily conserved family of nematode pheromones, are versatile in structure and function. We hypothesize these nematode pheromones play a role in species displacement. To investigate this hypothesis, we compared the ascarosides composition of B. xylophilus and B. mucronatus by LC–MS/MS followed by bioassays to test the responses of two nematodes to both crude and synthetic ascarosides. We found that asc-C5 (ascr#9) was the most abundant component and that there were no differences in pheromone composition between the two nematode species. B. xylophilus had faster growth rates under competition conditions. Furthermore, low concentrations of both crude and synthetic ascarosides [asc-C5, asc-C6 (ascr#12) and their mixture] enhanced female fecundity and body length growth in B. xylophilus but not in B. mucronatus. In contrast, body length of B. mucronatus was suppressed by a crude extract of its own ascarosides as well as by synthetic ascarosides (asc-C5, asc-C6 and their mixture). Our results strongly suggest that ascarosides play a role in the competitive displacement between two nematode species, which could explain the phenomena observed in B. xylophilus-invaded forests where B. mucronatus widely existed prior to B. xylophilus invasion.