Complete reversal of antero-posterior polarity in a centrifuged insect embryo.

Spatial pattern formation during embryogenesis is ascribed to differential gene expression, which in turn is thought to result in part from interactions of nuclei with cytoplasmic determinants. In the chironomid midge Smittia, and probably in other dipterans as well, blastoderm cells seem to make an early decision as to whether they contribute to cephalic and thoracic or to abdominal (and possibly thoracic) structures. Inactivation or translocation of cytoplasmic components involved in this antero-posterior decision could conceivably lead to duplications of head and thorax, or abdomen, or to complete but inverted embryos forming the head posteriorly and the abdomen anteriorly in the egg. Whereas the former two malformations have been described, completely inverted embryos are reported for the first time, to our knowledge, in this letter. Reversal of partial germ bands has previously been observed following combined ligation and cytoplasmic translocation in eggs of the leaf hopper, Euscelis plebejus.