Nematodes carried by beetles and remaining in malt extract agar (MEA)

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