Oogenesis and Embryology of Two Plant-parasitic Nematodes, Pratylenchus penetrans and P. zeae

The process of oogenesis was studied in the bisexual species, Pratylenchus penetrans, and the monosexual species, P. zeae. The nucleus of the oocyte of P. penetrans underwent two divisions after sperm penetration. The chromosome number of P. penetrans observed at metaphase of the first maturation division was 2n = 10 and the reduced chromosome number observed at anaphase of the first maturation division was n = 5. Two polar bodies were found within the egg, indicating that this species reproduces by amphimixis. The nucleus o f the oocyte of P. zeae underwent one mitotic division and the chromosome number was 2n = 26. The presence of only 1 polar body indicates that this species reproduces by mitotic parthenogenesis. The development of the embryo was similar in P. penetrans and P. zeae. Unsegmented eggs were usually deposited by females. Following the 9-celled stage, the number of cells increased rapidly until a blastula was formed. Cell differentiation immediately followed, as evidenced by the formation of darker and larger inner endodermal cells and smaller ectodermal cells. Six to 7 days after egg deposition, the first stage larva was coiled three to f o u r times within the egg shell. During the first molt, the styler apex was formed first, then the larva moved frequently and vigorously and the styler was repeatedly thrust into the egg shell. Finally, the shell was broken and the second stage larva emerged. It took 10 days from the unsegmented egg to hatching at 23 C.