Potential vector switching in the evolution of Bursaphelenchus xylophilus group nematodes (Nematoda: Aphelenchoididae)

Abstract

To show the importance of vector switching of nematodes in the evolution of the Bursaphelenchus xylophilus group, we tested a hypothesis that “ Bursaphelenchus doui (or its ancestor) was transferred by Acalolepta fraudatrix , Acalolepta sejuncta , and/or Monochamus subfasciatus (or their ancestral species) from broad‐leaved trees to conifers, switched vectors from these cerambycid beetles to Monochamus beetles in conifers, and then evolved into the common ancestor of Bursaphelenchus mucronatus and B. xylophilus .” We used a simple nematode‐loading method to beetles and produced 20 binary combinations of five B. xylophilus group species and four cerambycid beetle species in the tribe Lamiini. The affinity of the nematodes for the beetles was examined based on phoretic stage formation of the nematodes. Phoretic stages of B. doui appeared in all beetle species examined, namely Acalolepta luxuriosa , Psacothea hilaris , A. fraudatrix , and Monochamus alternatus , although the affinity of the nematode for M. alternatus was weak. This finding indicates that B. doui could switch vectors to conifer‐using Monochamus beetles after transfer by A. fraudatrix from broad‐leaved trees to conifers. We conclude that vector switching of nematodes could have potentially happened during the evolutionary history of the B. xylophilus group.

Cytoplasmic incompatibility in the semivoltine longicorn beetle Acalolepta fraudatrix (Coleoptera: Cerambycidae) double infected with Wolbachia

“…Two artificial diets established for rearing M. alternatus [28,32] were used for rearing A. fraudatrix larvae and adults. In diet I, a mixture of 4.0 g of milled inner bark of P. densiflora, 6.6 g of artificial silkworm diet (Silkmate 2M, Norsan Co., Kanagawa, Japan), and 0.8 g of dried yeast (EBIOS, Asahi Group Foods, Ltd., Tokyo, Japan) was kneaded with 8.6 ml of distilled water, an approach modified from that described in [28].…”

“…In diet I, a mixture of 4.0 g of milled inner bark of P. densiflora, 6.6 g of artificial silkworm diet (Silkmate 2M, Norsan Co., Kanagawa, Japan), and 0.8 g of dried yeast (EBIOS, Asahi Group Foods, Ltd., Tokyo, Japan) was kneaded with 8.6 ml of distilled water, an approach modified from that described in [28]. Diet II was composed of 1.6 g of current and one-year-old needles of P. densiflora dried at 70˚C for one day and milled into powder, 5.4 g of Silkmate 2M, 0.6 g of EBIOS, and 12.4 ml of distilled water [32]. Approximately 20 g of each diet was placed into a 50 ml Erlenmeyer flask.…”

How did Bursaphelenchus nematodes acquire a specific relationship with their beetle vectors, Monochamus?

“…Although further detailed analysis is required, it is possible that the dauer stage of B. okinawaensis may be induced by the same or similar compounds on Monochamus beetles. Notably, dauers of B. okinawaensis induced by M. alternatus in this study could rarely ride on beetles (1.3%), although dauer formation was promoted by M. alternatus to the same level as B. xylophilus (Maehara et al, 2020). Indeed, in a previous study, a small percentage of dauers of B. okinawaensis was vectored by M. alternatus (0.14%) (Maehara and Kanzaki, 2016).…”

“…In the xylophilus group of Bursaphelenhus (Braasch, 2001;Kanzaki and Futai, 2002;Kanzaki and Giblin-Davis, 2018), nematodes develop into third-stage dispersal (DIII) larvae from the propagative form when the population density increases (Tanaka et al, 2017). The DIII larvae molt to fourthstage dispersal (DIV) larva, which is analogous to the dauer stage in Caenorhabditis elegans (Maupas, 1899) Dougherty, 1953 (Nematoda, Rhabditidae) and the insect-phoretic stage, when late pupae and callow adults of cerambycid beetles (Monochamus spp., Acalolepta spp., and Psacothea hilaris Pascoe, 1857) are present in a pupal chamber (Morimoto and Iwasaki, 1973;Maehara and Futai, 1996;2001;Necibi and Linit, 1998;Maehara et al, 2020). The formation of the DIII larva of B. xylophilus is promoted by crude extracts of cultured nematodes (Tanaka et al, 2017), suggesting that these larvae are induced by the water-soluble pheromone ascaroside, as is the freeliving nematode C. elegans (Golden and Riddle, 1982;Jeong et al, 2005;Butcher et al, 2007;Butcher et al, 2008).…”

Gene sdaB Is Involved in the Nematocidal Activity of Enterobacter ludwigii AA4 Against the Pine Wood Nematode Bursaphelenchus xylophilus

“…Bursaphelenchus xylophilus was inoculated on a PDA culture of B. cinerea and incubated at 25 • C in the dark until the fungal mycelia were completely consumed by B. xylophilus. Nematodes were collected using the modified Baermann funnel technique (Kitazume et al, 2018;Cesarz et al, 2019;Maehara et al, 2020) and washed with a mixture of 0.1% streptomycin sulfate and 0.002% actinone three times to remove surface microbial contaminants (Liu et al, 2016). Then, these nematodes were used for PWN-killing activity test and in planta biocontrol assays.…”