Early embryonic development of the dipteran insect Heteropeza pygmaea in the presence of cytoskeleton-affecting drugs

The Spindle Assembly Checkpoint Functions during Early Development in Non-Chordate Embryos

In eukaryotic cells, a spindle assembly checkpoint (SAC) ensures accurate chromosome segregation, by monitoring proper attachment of chromosomes to spindle microtubules and delaying mitotic progression if connections are erroneous or absent. The SAC is thought to be relaxed during early embryonic development. Here, we evaluate the checkpoint response to lack of kinetochore-spindle microtubule interactions in early embryos of diverse animal species. Our analysis shows that there are two classes of embryos, either proficient or deficient for SAC activation during cleavage. Sea urchins, mussels, and jellyfish embryos show a prolonged delay in mitotic progression in the absence of spindle microtubules from the first cleavage division, while ascidian and amphioxus embryos, like those of Xenopus and zebrafish, continue mitotic cycling without delay. SAC competence during early development shows no correlation with cell size, chromosome number, or kinetochore to cell volume ratio. We show that SAC proteins Mad1, Mad2, and Mps1 lack the ability to recognize unattached kinetochores in ascidian embryos, indicating that SAC signaling is not diluted but rather actively silenced during early chordate development.

From egg to pole cells: ultrastructural aspects of early cleavage and germ cell determination in insects

Insect eggs are giant and very complex cells covered by an extremely resistant shell. Both the egg cell and surrounding eggshell express anteroposterior and ventrodorsal polarity. The molecular and cytoplasmic organization of both axes originates during oogenesis and leads to the production of an ooplasmic system which consists of euplasm and deutoplasm (yolk) and contains a nucleus as well as extranuclear determinants of maternal origin. Both are part of the store of information for early embryogenesis. In addition, the deutoplasm serves as raw material and early nutrient supply for building the embryo. The insect egg cell, which is arrested in the first maturation division when released from the ovary during oviposition, will be activated by different stimuli among different species to complete meiosis and start embryogenesis. The zygote nucleus undergoes a number of synchronous mitotic divisions leading to cleavage energids which initially form a syncytial blastoderm and subsequently the cellular blastoderm. In many insects, prior to blastoderm formation, polar granules (or oosome material) are incorporated in a single cell or a small number of cells which bud off at the posterior pole. These so called pole cells give rise to the primordial germ cells. Therefore, polar granules or the oosome material mark the germ line, and while structural counterparts of determinants of body pattern formation have so far not been found, the polar granules or oosome serve as an autonomous ooplasmic determinant for the pole or germ cells. Anteroposterior body polarity can arise independent of the germ plasm.