Nuclear division and migration during early embryogenesis of Bradysia tritici coquillet (syn. Sciara ocellaris) (diptera : Sciaridae)

Nuclear division and migration of cleavage nuclei in the embryos of Bradysia tritici (Diptera : Sciaridae) have been studied by light microscopy and nuclear staining. There are 8 cleavage cycles up to the syncytial blastoderm stage (4.5 hr), and during the 11th cycle cellularization begins (6.5 hr). The first 3 divisions take about 30 min each. During the 5th and 6th cycles, the maximum rate of division is reached (12 min/cycle at 22°C). After pole cell formation, the duration of the following mitotic cycles increases progressively. During nuclear migration, the presumptive germ line nuclei reach the egg cortex first, followed by anterior somatic nuclei and finally, posterior somatic nuclei reach the egg cortex. Possibly as a result of this region-specific nuclear migration, nuclear divisions become parasynchronous after 3 hr of embryogenesis (4th cycle). Several mitotic cycles later, between the 8th and 10th cycle in different embryos, X-chromosome elimination in somatic nuclei begins at the anterior egg pole and progresses in anteroposterior direction. Our observations suggest that the observed region-specific differences may be due to the activity of localized factors in the egg that control migration and nuclear cycle of the somatic nuclei.