Developmental mechanisms underlying evolutionary diversities in the eggshell shape of Drosophila species. Y. Nakamura , K. Matsuno. Dept Biol Sci & Tech, Sci Univ Tokyo, Chiba, Japan.

   The Drosophila egg has the dorsal appendages at the dorsal-anterior region. Their numbers and shapes evolutionarily diversify among the Drosophila species. For example, D. melanogaster and D. virilis have two and four dorsal appendages, respectively. However, the developmental mechanisms underlying their diversities have not been elucidated. In D. melanogaster, the formation of two dorsal appendages depends on the activation of mitogen-activated protein kinase (MAPK) through the Drosophila EGF receptor (DER) signaling pathway. The activation of DER has been shown to be regulated by Rhomboid-1 and Argos in positive and negative manners, respectively. Thus region-specific expression of Rhomboid-1 and Argos play essential roles in the morphogenesis of the dorsal appendages. Here, we cloned rhomboid-1 and argos cDNA from D. virilis and investigated expression of rhomboid-1 and argos during D. melanogaster and D. virilis oogenesis. These two genes were expressed in spatially distinct patterns between the two species. Interestingly, rhomboid-1 was expressed in two domains in D. melanogaster, but four in D. virilis. Furthermore, in D. virilis, our analysis revealed MAPK is activated in similar regions as rhomboid-1. Broad-Complex, a marker protein for the precursor cells of the dorsal appendages was expressed in larger regions than those of rhomboid-1 and not in four clear domains. However, the cells expressing Broad-Complex eventually migrated and gathered in the four domains expressing rhomboid-1. Our results suggest that evolutionary diversity in the number of the dorsal appendages arises from species-specific activation of DER signaling during oogenesis.