Rev 19-09-2023
As of 2021, over 130 valid species of the genus were recognized (Kanzaki, et al., 2020; Ryss et al., 2021).
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Females:
Ref: Nickle, 1970.
Male tail is curved and pointed with short, oval caudal alae at tail tip - hence the genus name.
Caudal papillae occur throughout the Aphelenchina.
Bursaphelenchus xylophilus is native to North America and also occurs primarily in East Asia and Europe (Dwinell & Nickle, 1989). A closely related species, B. mucronatus, is widely distributed in the Eurasian continent (Braasch et al., 2011).
Bursaphelenchus xylophilus is the primary causal agent of pine wilt disease which has extensively damaged pine forests in East Asia and Europe where it is virulent on Pinus species in those areas.
In North America, where Bursaphelenchus xylophilus is native (Dwinell & Nickle, 1989), Pinus trees generally appear to be resistant to pine wilt disease. A closely related species, B. mucronatus, is widely distributed in the Eurasian continent (Braasch et al., 2011) and has little virulence to Pinus trees there (Kanzaki et al., 2012).
Of the >130 valid species of the genus Bursaphelenchus, two are devastating plant pathogens, B. xylophilus and B. cocophilus and several are moderate to weak plant pathogens, e.g., B. sexdentati and B. mucronatus (e.g., Ryss and Subbotin, 2017; Kanzaki and Giblin-Davis, 2018).
The genus contains several biologically interesting species, e.g., B. okinawaensis, a hermaphroditic species; B. sycophilus, a highly derived obligate plant parasite; B. sinensis, which sxhibits phenotypic plasticity in stylet dimorphism (phenotypic plasticity, B. doui, which has a dauer juvenile and phoretic adult form as different dispersal stages, and B. ulmophilus which has two dauer (dispersal) juvenile stages (Ekino et al., 2017; Kanzaki et al., 2014; Kanzaki et al., 2019; Kanzaki et al., 2013; Kanzaki et al., 2021; Ryss et al., 2015).
Most species of the genus appear to be fungal-feeding, however, there are several plant-feeding endoparasites of aboveground plant parts, including tree trunks. Many have phoretic relationships with insects. Of the fungal-feeding species found in dead and decaying wood, the tree species of the wood seems to be determined bythe carrier insects; the nematode is essentially a saprophagous (mycophagous) speciesoccupying the substrate (Kanzaki et al., 2021).
A phoretic relationship with cerambycid bark beetles is frequently observed. For example, the recent discovery in Japan tahat adults of Acalolepta sejuncta and Monochamus saltuarius emerged from pine logs (Pinus densiflora) carrying Bursaphelenchus doui. Other known phoretic vectors of B. doui are M. subfasciatus and A. fraudatris (Aikawa et al., 2020). Many different insect species have been reported to carry Bursaphelenchus spp, including soil-dwelling bees, nitidulid beetles, ambrosia beetles, stag beetles, and fig wasps (Kanzaki et al., 2015)..
Several obligate and facultative plant parasitic nematodes occur in the genus Bursaphelenchus e.g., B. xylophilus, the pathogen of pine wilt disease, B. cocophilus, the pathogen of red ring disease, B. sexdentati, which has moderate to strong pathogenicity to pine trees, and B. sycophilus, a parasite of fig syconia. The basic morphology of the ingestive/digestive organs in the plant-parasitic species is similar to that of mycophagous Bursaphelenchus species. However, the plant-parasitic species generally differ in having a more strongly-developed stylet and larger esophageal glands (Kanzaki, et al., 2014).
Several types of parasitic associations between Bursaphelenchus spp. and insects were summarized by Kanzai et al (2015): entomopathofenic examples such as insect-parasitic adult stages of B. luxuriosae and B. doui and parasitic dauer juveniles of B. populi, B. trypophloei and B. masseyi have been reported from the body cavities of host insects; parasitic dauer juveniles of B. fagi and B. tiliae are reported from Malpighian tubules of their hosts (reviewed by Kanzaki et al.,, 2015)..
There are over 100 described species of Bursaphelenchus. Most species have a phoretic relationship with insects, especially bark beetles and wood borers and are associated with dead or dying conifers.
All species feed on fungi.
Bursaphelenchus species in phoretic association with Monochamus beetles may infect pine trees in two ways, both of which are impoprtant but may have different fitness implications:
1. They enter through feeding wounds made by the beetles on healthy pine twigs (Mamiya and Enda, 1972; Jikumaru and Togashi, 2001). Ozawa et al (2021) suggest that this infection pathway may render less virulent nemaodes unable to overcome the resistance mechanisms of healthy pine tree and fail to become established.
2. They enter via oviposition wounds created by adult female beetles on declining or recently killed pine trees (Wingfield, 1983; Ishiguro and Aikawa, 2016). Evidence supporting this mechanisms of transmission of the nematodes is provided by the frequent occvurrence of Bursaphelenchus in the reproductive organs of both sexes of the beetle. Ozawa et al (2021) consider this infection pathway to be of critical importance for the long-term survival and success of the nematode population because the weakened or dying trees are less likely to have effective resistance mechanisms.
Phenotypic plasticity in which, regardless of genotype, the developmental phenotype of individuals is determined by environmental conditions, has been reported in B. okinawaensis. That species reproduces primarily as a self-fertilizing hermaphrodite when cultured on a filamentous fungus. The nematodes produced abundant eggs on the fungus plate but produced very few sperm (for self-fertilization) when cultured on yeast, Some individuals on the yeast plate did not produce any sperm or eggs, indicating that they had developed as females rather than heremaphrodites (Fujimora et al., 2022).