Early Embryogenesis and Anterior-Posterior Axis Formation in the White-Tip Nematode Aphelenchoides besseyi (Nematoda: Aphelenchoididae).

We followed the early embryogenesis of Aphelenchoides besseyi from fertilization to the 4-cell stage under Nomarski optics and examined the chromosome number and structure by DAPI staining. After an oocyte is fertilized by a sperm, the eggshell forms and the male and female pronuclei are reconstructed. The male pronucleus moves toward the female pronucleus, which is located at the center of the egg. They meet, rotate 90°, and fuse. The embryo then divides unequally into a larger anterior AB cell and a smaller posterior P(1) cell. The site of sperm entry into the oocyte seems to become the future anterior pole of the embryo, and thus the formation of an anterior-posterior axis formation is the same as that for Bursaphelenchus xylophilus, but opposite to that for Caenorhabditis elegans. From immunostaining, the fertilizing sperm appears to bring the centrosome into the oocyte. The chromosome structure during the pronuclear meeting as observed by DAPI staining suggests that a haploid sperm (N = 3) fertilizes a haploid oocyte (N = 3) to form a diploid embryo (2N = 6) and that all chromosomes appear to be of a similar size. Unlike C. elegans does, the P(1) cell first divides anterior-posteriorly followed by the AB anterior-posteriorly. These divisions produced the 4-cell stage embryo with 4 cells arranged in a linear fashion, again in contrast to that for C. elegans or B. xylophilus configured in a rhomboid shape.