The Hybrid male rescue gene encodes a rapidly evolving Myb-related protein. D.A. Barbash ^1,2, D.F. Siino ¹, A.M. Tarone ¹, P. Awadalla ¹, J. Roote ². 1) Section of Evolution & Ecology, Univ. California, Davis, Davis, CA, USA; 2) Department of Genetics, Univ. of Cambridge, Cambridge, UK.

   Matings between different species of animals or plants often result in sterile or lethal hybrids. The cross of D. melanogaster females to males of its sibling species D. simulans, D. mauritiana and D. sechellia produces lethal sons and semi-viable but sterile daughters. We have cloned the first hybrid incompatibility gene in D. melanogaster, Hybrid male rescue (Hmr). Mutations in Hmr suppress the lethality of both hybrid sexes and partially rescue hybrid female sterility, demonstrating that Hmr is a major component of post-zygotic isolation. We have cloned Hmr by chromosome walking and by constructing P-element transformant lines that mimic wild type Hmr[+] activity. Hmr encodes a protein homologous to a family of DNA binding proteins that includes the Drosophila transcription factor ADH transcription factor 1 (Adf1) and the Myb oncoprotein. Comparative sequencing has shown that Hmr is one of the most rapidly evolving genes in D. melanogaster, with an extraordinary number of amino acid changes and insertions and deletions between D. melanogaster and its sibling species. Current experiments to test the functional significance of this sequence divergence as well as the mechanism of hybrid lethality will be presented. In conjunction with reports from other organisms our results suggest that rapid evolution may be a general characteristic of genes involved in speciation and hybrid incompatibilities.