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is unknown. However, overexpression of centrosomal proteins can also form cent-
riolar structures that are clearly abnormal (Gopalakrishnan et al. 2010 ; Rodrigues-
Martins et al. 2007a ; Stevens et al. 2010a ). The capacity to induce centriole formation
by overexpression seems to be tissue-specific and is more prevalent in the oocyte/
embryo than in the testes (Peel et al. 2007 ; Stevens et al. 2009 ).
Whether centrioles can form de novo in Drosophila melanogaster without
overexpression of centriolar components is not clear. It was shown that in other
Drosophila species, oocytes assemble a number of cytoplasmic asters after acti-
vation with centrioles and centrosomal proteins (Ferree et al. 2006 ; Riparbelli and
Callaini 2003 ). In addition, in sas-4 null mutants that cannot form centrioles, foci
containing centriolar markers appear transiently, suggesting that nascent procen-
trioles form de novo but fail to develop further in the absence of Sas-4 (Gopala-
krishnan et al. 2011 ).
In the ''centriolar pathway'', a preexisting centriole acts as a template to ensure
that only a single daughter centriole is formed per cell cycle. However, it does not
appear to impart structural information to the daughter (Phillips 1967 ; Rodrigues-
Martins et al. 2007b ). Before cell division, centriole duplication under the ''centriolar
pathway'' results in four centrioles; each mother/daughter centriole pair forms a
centrosome that migrates to one of the cell's two poles where it helps orient the
mitotic spindle, a structure essential for cell division. Having precisely one centriole
pair in interphase and two centriole pairs during mitosis is believed to be critical for
proper cell division and having too many centrioles can initiate tumorigenesis (Basto
et al. 2008 ; Cunha-Ferreira et al. 2009 ; Fukasawa 2007 ; Ganem et al. 2009 ).
How the mother centriole ensures that only a single daughter centriole forms is
not clear. However, it is readily observed that there is already only one daughter
centriole at the vicinity of the mother centriole by the time a procentriole is
present. Therefore, an approach to address this question is to study proteins that are
involved early in centriole formation when the formation of the procentriole is
initiated. In recent years, extensive proteomic, genetic, and bioinformatic studies
have identified many of the key proteins critical for centriole formation (Andersen
et al. 2003 ; Avidor-Reiss et al. 2004 ; Fritz-Laylin and Cande 2010 ; Gonczy et al.
2000 ; Keller et al. 2005 ; Kilburn et al. 2007 ; Li et al. 2004 ; Mahoney et al. 2006 ).
Some of these proteins were analyzed in flies and were shown to function early in
procentriole initiation (Table 1.1 ). Some of the important discoveries regarding
these proteins that were made using flies are summarized below.
1.6 Identification of Centrosomal Proteins and Mutants
in Drosophila melanogaster
Identification of centrosomal components that play a specific role in centrosome
have been hampered by initial difficulties in obtaining sufficient quantities of
material for biochemical isolation and a lack of clear phenotypes expected from
mutation of essential centrosomal proteins. Over time, several approaches have
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