Most animals have two centrioles in spermatids (the distal and proximal 
centrioles), but insect spermatids seem to contain only one centriole (Fuller 
1993), which functionally resembles the distal centriole. Using fluorescent 
centriolar markers, we identified a structure near the fly distal centriole that 
is reminiscent of a proximal centriole (i.e., proximal centriole-like, or PCL). 
We show that the PCL exhibits several features of daughter centrioles. First, a 
single PCL forms near the proximal segment of the older centriole. Second, the 
centriolar proteins SAS-6, Ana1, and Bld10p/Cep135 are in the PCL. Third, PCL 
formation depends on SAK/PLK4 and SAS-6. Using a genetic screen for PCL defect, 
we identified a mutation in the gene encoding the conserved centriolar protein 
POC1, which is part of the daughter centriole initiation site (Kilburn et al. 
2007) in Tetrahymena. We conclude that the PCL resembles an early intermediate 
structure of a forming centriole, which may explain why no typical centriolar 
structure is observed under electron microscopy. We propose that, during the 
evolution of insects, the proximal centriole was simplified by eliminating the 
later steps in centriole assembly. The PCL may provide a unique model to study 
early steps of centriole formation.