PANFILIO, K.A.; LIU, P.Z.; KAUFMAN, T.C.; University of Cambridge, UK; Indiana University, Bloomington; Indiana University, Bloomington: Knock-down of Hox3/zerkn�llt in a representative insect results in complete eversion of the embryo: Evolution of embryogenesis and of gene function

The Hox3 locus in insects has diverged significantly from the canonical role of Hox genes in segmental specification along the anterior-posterior axis. One of the changes in function gave rise to the zerkn�llt (zen) gene, which is no longer expressed in the embryo, but rather in the extraembryonic tissues that surround the embryo during early development. The evolutionary history of this change in function remains unclear. The only published functional data for this gene come from Drosophila melanogaster, in which zen is necessary for maintenance of the single, vestigial extraembryonic tissue, the amnioserosa. We have examined zen function in the milkweed bug, Oncopeltus fasciatus, an insect that has well-developed and distinct amniotic and serosal tissues. Depletion of Of-zen transcript by RNA interference results in the complete eversion of the embryo: it develops inside out. This appears to be the consequence firstly of a failure of the embryonic movements known as katatrepsis, leaving the embryo upside down and backward. Secondly the amnion fails to rupture, thereby confining the embryonic flanks at the time of dorsal closure. This phenotype differs dramatically from D. melanogaster zen null mutants, which result in a so-called �crumpled� germband phenotype that arrests at a much earlier developmental stage. Though the loss-of-zen phenotypes differ morphologically, these orthologues are both involved in the function of extraembryonic tissue. Nevertheless, the mechanism of action may not be conserved. I will discuss scenarios for the evolution of extraembryonic membranes and of zen function in insects.