Early embryogenesis of the pinewood nematode Bursaphelenchus xylophilus

The early embryogenesis and cell lineage of the pinewood nematode Bursaphelenchus xylophilus was followed from a single‐cell zygote to a 46‐cell embryo under Nomarski optics, and elongation of the microtubules was studied by immunostaining. As a B. xylophilus oocyte matures, it passes through a passage connecting the oviduct with the quadricolumella, the distal part of the uterus, and reaches the quadricolumella where it stays for a few minutes and is fertilized. After fertilization, the germinal vesicle disappears, an eggshell is formed, and the male and female pronuclei appear. The pronuclei move toward each other and fuse at the center of the egg. Around this time, the microtubule‐organizing center appears. The presumptive region of sperm entry into the oocyte becomes the future anterior portion of the embryo. This anterior–posterior axis determination is opposite to that of Caenorhabditis elegans, where the sperm entry site becomes the posterior portion of the embryo. The optimal growth temperatures of these two nematodes also differ in that temperatures of about 30°C afford the fastest growth rate and highest hatching frequency in B. xylophilus. Otherwise, the lineage resembles that of C. elegans with respect to timing, positioning and the axis orientation of each cell division.