Oogenesis in the parthenogenetic stick insect Sipyloidea sipylus Westwood (Orthoptera, Phasmidae).

Abstract

Oogenesis in the parthenogenetic stick insectSipyloidea sipylus Westwood is described. The female has 80±2 chromosomes and is presumably tetraploid. Meiosis consists of two equational divisions and parthenogenesis is thus apomictic. In the end chamber meiosis proceeds as far as pachytene. After pachytene the chromosomes despiralize while the germinal vesicle of the growing oocyte develops. Endomeiosis occurs since, very likely, only univalents enter in the diffuse stage of the germinal vesicle. The vitellarium contains 10–12 growing oocytes, only the oldest of which shows yolk accumulation. Meiosis goes on in the cortical cytoplasm ventrally in the egg about 11 days before oviposition when the first prometaphase is observed. First metaphase lasts from about 6 days before oviposition until about 4 hours after oviposition. Meiosis ends about 9 to 20 hours after oviposition. The first metaphase chromosomes are tetrapartite through an extra chromosome doubling which requires that meiosis consists of two divisions instead of one. These divisions are normal. The first polar body also forms chromosomes and may divide abnormally before degenerating. The second polar body degenerates immediately. The pronucleus divides mitotically in the environment of the polar bodies during its migration to the micropylar area.