Development 113, 419-430 (1991) Printed in Great Britain © The Company of Biologists Limited 1991 419 Segmentation gene expression in the housefly Musca domestica RALF SOMMER* and DIETHARD TAUTZ* Institut fiir Genctik and Mikrobiologie, Umvcrsittit MUnchen, Mana-Wardstrasse la, 8000 MUnchen 19, FRG •Present address: Institut fllr Zoologie, UniversitSt MUnchen, Luisenstr. 14, 8000 MUnchen 2, FRG Summary Drosophila and Musca both belong to the group of higher dipteran flies and show morphologically a very similar early development. However, these two species are evolutionary separated by at least 100 million years. This presents the opportunity for a comparative analysis of segmentation gene expression across a large evolution- ary distance in a very similar embryonic background. We have analysed in detail the early expression of the maternal gene bicoid, the gap genes hunchback, Kriippel, knirps and tailless, the pair-rule gene hairy, the segment- polarity gene engrailed and the homoeotic gene Ultra- bithorax. We show that the primary expression domains of these genes are conserved, while some secondary expression aspects have diverged. Most notable is the finding of hunchback expression in 11-13 stripes shortly before gastrulation, as well as a delayed expression of terminal domains of various genes. We conclude that the early developmental gene hierarchy, as it has been defined in Drosophila, is evolutionary conserved in Musca domestica. Key words: Musca domestica, development, segmentation gene evolution. Introduction Extensive genetic and molecular studies have led to a detailed insight into the processes underlying pattern formation in Drosophila (reviewed by Ingham, 1988). Segmentation of the Drosophila embryo depends on the successive activities of several classes of genes. On top of the hierarchy are the maternal genes, which provide the positional information for the expression of the zygotic gap genes. The gap genes in turn regulate the expression of the pair-rule and segment-polarity genes, which eventually subdivide the embryo into its segmental units. The segmental units are then specified by the homoeotic genes. The genes involved in the segmentation process in Drosophila have been identified on a functional basis in appropriate mutagenesis screens. Cloning and ex- pression analysis of these genes has yielded a very detailed understanding of the molecular interactions involved in the early pattern formation process in Drosophila. It is, therefore, now of great interest to ask, whether these molecular interactions have been conserved in evolution. However, it seems unreason- able to perform a similar extensive genetic screen, as in Drosophila, for any other insect system. Instead, it should be possible to make these inferences on the basis of the expression analysis of the homologous genes in other species. This requires, first, to find the homolo- gous genes and, second, to assume that they have homologous functions. Such an assumption can be tested, if the first step in the analysis is the comparison of the expression of homologous genes in a species that shows a homologous embryonic development. We have therefore compared the expression domains of several key genes of the segmentation gene hierarchy of the two higher dipteran fly species Drosophila and Musca. Though the two species are evolutionary separated by probably at least 100 million years (Hennig, 1981), the morphology and the early embry- ology of the higher dipterans are very similar (Weis- mann, 1866). The only difference so far detected is the mitosis behavior of the blastoderm nuclei (Lundquist, 1981; Sommer and Tautz, 1991), which is most likely not related to the segmentation process (van der Meer et al. 1982). One could expect that the molecular mechanisms that lead to the segmentation of the embryo should be conserved. On the other hand, a divergence of the primary segmentation gene expression pattern seems also a theoretical possibility. We know that redundant segmentation pathways exist in Drosophila (Hiilskamp and Tautz, 1991) and one could, therefore, imagine that these are subject to modification during evolution. One particular function of a gene could for example be lost in favor of the redundant function of another gene and one would then expect a diverged expression pattern of the respective genes. We believe that it is important to test the assumption of homologous segmentation gene expression in a homologous embryonic background. A recurrent feature of most of the segmentation genes in Drosophila is that they are not only involved in