What are the essential functions of centrioles and centrosomes?
The fly shown at the top is a normal wild type (WT) fly, while the fly on the bottom is a DSas-4 mutant that compeletly lacks centrioles, centrosomes, cilia and flagella. As a result it is uncoordinated (that is why it is holding its wings and legs in an odd way) and will die shortly after it ecloses (as it cannot feed or drink). Otherwise, however, it looks completely morphologically normal, and has no hair, eye or bristle defecects.
We unexpectedly found a way to produce flies that completely lack centrioles, centrosomes, cilia and flagella (Basto et al., 2006). We had been studying the Drosophila homologue of the centrosomal protein CenpJ, which causes primary autosomal microcephaly when the gene encoding it is mutated in humans. The human and fly proteins are weakly related to SAS-4, a centriolar protein in C. elegans that is essential for centriole duplication. We showed that the Drosophila protein (DSas-4) is also a centriolar protein required for centriole duplication. We isolated a mutation in DSas-4 and found that homozygous mutant embryos derived from heterozygous mothers proceeded through early development normally, using the maternal supply of DSas-4. As embryogenesis proceeded, however, the mutant cells started to lose centrioles, and, by the 3rd instar stage of larval development, no centrioles, centrosomes, cilia or flagella were detectable. Thus, DSas-4 mutants provide us with a unique opportunity to assess the essential functions of centrioles, centrosomes, cilia and flagella within the context of a complex multicellular organism like the fly.
Remarkably, DSas-4 mutant animals developed at near normal rates and eclosed as morphologically normal adults at near normal Mendelian ratios. The adult flies, however, were uncoordinated and died soon after eclosion because they lacked cilia, which are essential for the function of certain sensory neurons. These findings were extremely surprising, as they suggested that centrosomes are not essential for any of the complex cell behaviours that are required for the normal development of an adult fly.
We have gone onto show that normal cell division is largely unperturbed by the absence of centrosomes, but that ~30% of the asymmetric divisions of the larval neuroblasts are abnormal. Oogenesis, however, is not detectably perturbed in the absence of centrosomes (Stevens et al., 2007). Thus, it will be important to examine in detail the various cell-processes that have previously been thought to involve centrosomes, to test whether they are not detectably required for these processes (as appears to be the case during oogenesis) or whether centrosomes have a role, but flies can effectively compensate for their absence (as appears to be the case in larval neuroblasts). We are currently testing whether centrioles and centrosomes have important functions in a variety of cell processes.
These images are of larval eye discs from WT (left) and DSas-4 mutants (right) that have been stained to reveal the distribution of nuclei (blue), neurons (green) and centrioles (red). The DSas-4 mutant tissue lacks detectable centrioles (the red dots in the WT tissue), but the organisation of the neurons seems to be unperturbed.