Ascarosides Promote the Prevalence of Ophiostomatoid Fungi and an Invasive Pathogenic Nematode, Bursaphelenchus xylophilus

Abstract

Understanding the coevolution of pathogens and their associated mycoflora depend upon a proper elucidation of the basis of their chemical communication. In the case of pine wilt disease, the mutual interactions between cerambycid beetles, invasive pathogenic nematodes, (Bursaphelenchus xylophilus) and their symbiotic ophiostomatoid fungi provide a unique opportunity to understand the role of small molecules in mediating their chemical communication. Nematodes produce ascarosides, a highly conserved family of small molecules that serve essential functions in nematode biology and ecology. Here we demonstrated that the associated fungi, one of the key natural food resources of pine wood nematodes, can detect and respond to these ascarosides. We found that ascarosides significantly increase the growth of L. pini-densiflorae and Sporothrix sp. 1, which are native fungal species in China that form a symbiotic relationship with pinewood nematodes. Hyphal mass of L. pini-densiflorae increased when treated with asc-C5 compared to other ophiostomatoid species. Field results demonstrated that in forests where higher numbers of PWN were isolated from beetle galleries, L. pini-densiflorae had been prevalent; the same results were confirmed in laboratory studies. Furthermore, when treated with asc-C5, L. pini-densiflorae responded by increasing its production of spores, which leads to a higher likelihood of dispersal by insect vectors, hence explaining the dominance of L. pini-densiflorae over S. sp. 1 in the Tianwang and Nanlu Mountains within the Northern Forestry Centre of China. These findings provide an emphatic representation of coevolution of pine wood nematode and its associated fungi. Our results lay a broader foundation for a better understanding of inter-kingdom mutualisms and the chemical signals that mediate their establishment.

Infective juveniles of entomopathogenic nematodes (Steinernema and Heterorhabditis) secrete ascarosides and respond to interspecific dispersal signals

“…Kaplan et al (2011) did not detect ascarosides in "worm water" conditioned by C. elegans dauers, though small quantities of two ascarosides were found within the dauer bodies, while Choe et al (2012) detected several ascarosides in worm water from dauers of Caenorhabditis species 7. Ascarosides have also been identified in worm water of IJs of diverse taxonomic and ecological groups including the animal parasite Nippostrongylus brasiliensis (Choe et al, 2012), various plant parasites (Manosalva et al, 2015;Zhao et al, 2018) and the insect-associated Ocheius carolinensis and Steinernema species (S. carpocapsae, S. scapterisci, S. riobrave and S. glaseri) (Choe et al, 2012)). Our results are the first to show that IJs or dauers continue to release ascarosides to the external medium for longer than 24 h. Ascr#9 was the dominant signal detected in the exo-metabolome of the three Steinernema species tested here, with smaller signals for several other ascarosides.…”

“…Our results are the first to show that IJs or dauers continue to release ascarosides to the external medium for longer than 24 h. Ascr#9 was the dominant signal detected in the exo-metabolome of the three Steinernema species tested here, with smaller signals for several other ascarosides. This is in line with the findings that the ascaroside blends of various insect-associated nematodes (the entomopathogens Steinernema spp., H. bacteriophora and Oscheius tipuli, the necromenic Pristionchus pacificus, and the beetle-vectored pine wilt nematode Bursaphelenchus xylophilus) (Choe et al 2012;Zhao et al, 2018) all had high amounts of this ascaroside. Choe et al (2012) noted that ascaroside biosynthesis patterns appear in part to correlate with lifestyle or ecological niche as well as with phylogeny.…”

“…There is evidence that EPN IJs exhibit group movement (Fushing et al, 2008;Shapiro-Ilan et al, 2014;Ruan et al, 2018); ascarosides may play a role in intraspecific communication to promote aggregation. There is increasing evidence that ascarosides can be detected by other trophic groups (Hsueh et al, 2013;Manosalva et al, 2015;Zhao et al, 2016Zhao et al, , 2018. This may be to the advantage of the nematodes; for example, ascarosides produced by B. xylophilus juveniles promoted metamorphosis of their beetle vector and the prevalence of its fungal symbiont (Zhao et al, 2016(Zhao et al, , 2018.…”

“…There is increasing evidence that ascarosides can be detected by other trophic groups (Hsueh et al, 2013;Manosalva et al, 2015;Zhao et al, 2016Zhao et al, , 2018. This may be to the advantage of the nematodes; for example, ascarosides produced by B. xylophilus juveniles promoted metamorphosis of their beetle vector and the prevalence of its fungal symbiont (Zhao et al, 2016(Zhao et al, , 2018. Other soil organisms may "eavesdrop" on nematode communication, to their disadvantage, for example triggering plant defences (Manosalva et al, 2015), and trap formation by nematophagous fungi (Hsueh et al, 2013).…”

Ophiostomatoid Fungi Associated with Cryphalus Piceae in Shandong province in eastern China

“…Ophiostoma ips was one of the most frequently isolated ophiostomatoid fungi in China and this study (Lu et alternatus, but its speci c function in this symbiotic relationship remained unresolved (Zhao et al, 2018). Therefore, it is not unreasonable to hypothesize that this symbiotic fungus also in uences the life history and population of its vector and associated nematode.…”