Lack of Functional Conservation in Early Axial Patterning of the Drosophila Embryo. Jackie F. Gavin-Smyth¹, Daniel R. Matute², Martin Krietman¹, John Reinitz¹. 1) Ecology and Evolution, University of Chicago, Chicago, IL; 2) Dept. of Human Genetics, University of Chicago, Chicago, IL.

   A long held tenet of developmental biology is that the early, most critical patterning genes and networks are also the most functional constrained and conserved throughout evolution. Recent work (Matute et al. 2010), however, has shown that within a relatively short evolutionary time frame, a large number of genes are sufficiently diverged as to disallow viability when hemizygous in a hybrid embryo between two species. This hemizygous inviability in the hybrid context implies a lack of functional compensation between the gene networks of the different species' genomes. Specifically, many of the early embryonic patterning genes are not able to functionally compensate in hybrid embryos derived from Drosophila melanogaster and Drosophila santomea, a species whose last common ancestor with D. melanogaster was less than 13 million years ago. Here we present evidence that the early, critical axial patterning networks of the Drosophila embryo, including the gap gene giant (gt), have diverged rapidly on both a sequence and functional level. This divergence leads to novel, antimorphic patterning defects when juxtaposed in the hybrid context. Quantitative imaging analysis and genetic experiments confirm the loss of gt expression and subsequent phenotypic defects which result in the failure to develop a viable hybrid.