Tribolium embryogenesis: a SEM study of cell shapes and movements from blastoderm to serosal closure

Embryogenesis in the beetle Tribolium is of increasing interest to both mol ecular and evolutionary biology because it differs from the Drosophila para digm by its type of segment specification (short- vs. long germ) and by the extensive epithelial envelopes - amnion and serosa - that are typical of m ost insects but not of higher dipterans. Using scanning electron microscopy of DAPI staged embryos we document development in Tribolium castaneum from blastoderm to completion of the envelopes, recording many details not othe rwise accessible; we also provide a time table of the respective stages at 30 degrees C. The nascent blastoderm cells remain basally confluent with th e yolksac until after the 13th (=last synchronous) mitotic cycle. The cells in the prospective serosa - the first domain to segregate visibly from the uniform blastoderm - carry surface protrusions likely to contact the overl ying vitelline envelope. The embryonic rudiment, the other (and larger) bla stodermal domain, gives rise to amnion and germ anlage. In the latter, visi ble differentiation begins with a "primitive pit" reminiscent of the poster ior midgut rudiment of Drosophila. The subsequent invagination of the mesod erm resembles Drosophila gastrulation, except in the head region where the median groove extends through the entire preoral region. The prospective am nion starts differing visibly from the germ anlage during early gastrulatio n. It then folds underneath the spreading serosa and, advancing with the la tter, closes the amniotic cavity at the ventral face of the germband. The l argest (=posterior) amniotic fold covers a crestlike protrusion of the yolk sac. Together with marked changes in the shape and arrangement of the amnio n cells, this protrusion may contribute to the fold's elevation and early p rogress.