Functional analysis of two homologous cell adhesion molecules, Capricious and Tartan, during Drosophila tracheal morphogenesis. C. Krause ¹, C. Wolf ¹, C. Samakovlis ², R. Schuh ¹. 1) MPI fuer biophysikalische Chemie, Goettingen, Germany; 2) Department of Developmental Biology, Wenner-Gren Institute, Stockholm University.

   One of the ideal systems to study directed cell outgrowth and migration is Drosophila tracheal system, a highly complex three-dimensional tubular structure such as kidney, lung or blood vessel in the vertebrates, since the formation of stereotypical trachea is completed solely through cell shape change, directed cell extension or migration, and cell-to-cell contact. For the formation of proper tracheal system, an interplay between ectodermal tracheal cells and the surrounding mesodermal cells is necessary. For example, it was recently revealed that a single mesodermal cell called bridge-cell serves as a guidance post to direct the cell extensions of outgrowing tracheal dorsal trunk cells towards each other.
    Here we show that two highly homologous genes that both encode cell adhesion molecules play essential roles in mediating the signal between the mesodermal cells and the ectodermal tracheal cells. Both capricious and tartan encode protein with a single transmembrane domain and 14 Leucine-Rich-Repeat motifs in the extracellular domain. However, while capricious is expressed specifically in the bridge-cells, tartan is expressed more broadly in the mesoderm. In the mutant embryos, which lack capricious in the bridge-cells, the pathfinding of dorsal trunk cells is impaired and therefore, the dorsal trunk becomes discontinuous. In tartan mutant embryos, the proper formation of not only the dorsal trunk but also the lateral trunks is disrupted. When both capricious and tartan are lacking, then the dorsal trunk is more frequently interrupted than in either single mutant embryos. These observations indicate that both Capricious and Tartan are necessary for interconnecting the outgrowing tracheal cells with the surrounding mesodermal cells in order to form the continuous tubular tracheal system.