Blastoderm Formation and Cellularisation inDrosophila melanogaster

Abstract

Immediately following fertilisation in Drosophila and many other arthropods, the embryo undergoes a series of rapid syncytial nuclear divisions. These divisions are driven by maternally supplied components and occur in the absence of zygotic transcription. Unlike typical cell divisions, these divisions alternate between S and M phases, resulting in cell cycles that last only from 10 to 25 min. After four rounds of division, the nuclei undergo axial expansion, a process that relies on microfilaments. Subsequently migration of the nuclei to the cortex relies on microtubules. Once at the cortex, the nuclear divisions occur on a single plane and rely on partial cleavage furrows to maintain an even distribution. The cortical nuclear divisions continue until the mid‐blastula transition (MBT), at which time cellularisation results in the formation of a multicellular embryo. Key Concepts: Fertilisation triggers a series of events that induces the first mitotic cycle, a gonomeric division between the male and female pronucleus. After fertilisation, the embryo undergoes 13 synchronous divisions within a syncytium. Divisions 10–13 occur at the cortex of the embryo and require reorganisation of actin and membrane into metaphase furrows. At cycle 14, the cell cycle pauses and cellularisation occurs forming individual somatic cells. Cellularisation, a key feature of the mid‐blastula transition, marks the time at which zygotic transcription occurs and maternal products are degraded.

Tum/RacGAP functions as a switch activating the Pav/kinesin-6 motor

“…That a step in centralspindlin localization involves the centrosome readily explains the unusual localization pattern of this complex observed in some cell types. For example, during the cortical syncytial divisions of the early Drosophila embryo, Rappaport-like cytokinesis furrows, known as metaphase furrows, form between astral microtubule arrays emanating from centrosomes of neighbouring nuclei 53 54 . Studies reveal that centralspindlin concentrates at regions of astral microtubule overlap and this localization is essential for metaphase furrow formation 32 55 .…”

Centrosomal and Non-Centrosomal Microtubule-Organizing Centers (MTOCs) in Drosophila melanogaster

“…Both the GC and PCL then replicate during the zygotic interphase to yield two functional centrosomes that assist in the first mitotic division [ 169 , 171 ]. The newly-created centrosomes assemble astral MTs that facilitate the migration of the female pronucleus farthest from the cortex towards the male pronucleus in a Ncd and Klp3A motors-dependent manner [ 164 , 171 , 172 , 173 , 174 , 175 , 176 ]. When male and female pronuclei fuse, the centrosomes separate to form the spindle poles of the first mitosis.…”

Chk1 and Wee1 kinases coordinate DNA replication, chromosome condensation, and anaphase entry

“…In contrast to the 24-h cell cycle of the typical mammalian cell, the syncytial divisions of the Drosophila embryo are 15–20 min in length (Kotadia et al ., 2010). In spite of their short duration, these cycles possess robust S-phase and spindle checkpoints mediated by conserved checkpoint pathways (Song, 2005).…”