Molecular characterization and functional analysis of daf-8 in the pinewood nematode, Bursaphelenchus xylophilus

Bursaphelenchus xylophilus, causal agent of pine wilt disease, causes extensive damage worldwide. Strategies are needed to inhibit population growth or block the spread of the invasive nematode to control pine wilt disease. The gene daf-8 plays crucial roles in larval development and dauer formation in Caenorhabditis elegans, but little is known about its orthologue in B. xylophilus. In the present molecular characterization and functional analysis of daf-8 in B. xylophilus (Bx-daf-8), RT-qPCR showed that the expression of Bx-daf-8 gradually increased during the embryonic stage, peaked in the second-stage juvenile (J2), then dramatically dropped in the J3, and remained at that low level for the rest of its life. Bx-daf-8-transgenic C. elegans was employed to mimic the spatiotemporal expression of Bx-daf-8, which was expressed close to the pharynx during early embryogenesis and weakly throughout the whole body during late embryogenesis. It was observed in head neurons and tail ganglions throughout all postembryonic stages. B. xylophilus embryos were severely abnormal, and hatching rate decreased sharply after Bx-daf-8 knockdown. daf-16-1 and da-f16-2, downstream genes in the IIS pathway, also dropped sharply after Bx-daf-8 knockdown. We propose that TGFβ may crosstalk with the IIS pathway upstream of Bx-daf-16 and that daf-8 may act as a master regulator of daf-16 in B. xylophilus. However, knockdown of Bx-daf-8 did not lead to constitutive developmental arrest at the dauer larval stage, indicating that dauer entry in B. xylophilus might be controlled by several genes and is more complicated than in C. elegans. Bx-daf-8 alone did not control the dauer entry in B. xylophilus, but it was indispensable for embryogenesis, providing a potential target for suppressing population growth of B. xylophilus.