Cell-Cell Interactions Determine Cell Fate in the Drosophila Retina 
protein is expressed on many cells within an om- 
matidium in addition to the R7 precursor cell. 
Nevertheless, only one cell assumes an R7 cell 
fate. Four of these additional cells contact the 
boss-expressing R8 cell, and the others are one or 
more cell diameters away. We have shown that 
multiple mechanisms limit the induction to only 
one cell, through studies in which we examined 
R7 development in various mutant backgrounds 
and in response to ectopically expressed boss 
protein. These studies revealed that the cells that 
do not contact the R8 cells are competent to re- 
spond to the inductive cue and do so if presented 
with a surface-bound form of boss. Conversely, 
the sev-expressing cells that normally contact R8 
are restricted from responding to the cue as a re- 
sult of a very early developmental event that 
commits them to alternative fates. 
We believe that similar molecular and develop- 
mental strategies involving the carefully con- 
trolled expression of surface-bound ligands and 
competing alternative developmental pathways 
will be important in establishing the remarkable 
cellular diversity and organization of more- 
complex nervous systems, such as the vertebrate 
brain. 
472 
