Biology Reference
In-Depth Information
1.3.4 Phenotypic Characterization of Centrosomal Mutants
The spermatid flagellum is formed in a unique way from that of sensory cilia
(Avidor-Reiss et al.
2004
; Han et al.
2003
). Sensory cilia are formed within a
distinct compartment separated from the rest of the cell by a transition zone that is
thought to function as the ciliary gate (Betleja and Cole
2010
; Craige et al.
2010
;
Omran
2010
; Williams et al.
2011
). As a result, sensory cilium formation requires
a complex machinery known as the intra-flagellar transport (IFT) (Rosenbaum
2002
; Scholey and Anderson
2006
). On the other hand, the spermatid flagellum is
formed in the cytoplasm and does not depend on IFT, a process called cytoplasmic
or non-compartmentalized ciliogenesis (Avidor-Reiss et al.
2004
; Han et al.
2003
).
In flies, this distinction allows phenotypic analysis to rapidly determine if a protein
is an essential centriolar protein, if it is important for compartmentalized cilio-
genesis, or if it is important for the conversion of a centriole into a basal body.
Mutants in essential centriolar proteins will have the following phenotype that
reflects the role of centrioles in their development. Meiosis will be abnormal in
males due to the importance of centrosomes in meiosis, sperm will be nonmotile as
a result of defects in non-compartmentalized ciliogenesis, and adult flies will be
uncoordinated due to defects in compartmentalized ciliogenesis. Such mutants
include Sas-6, Sas-4, Asl, Ana-1, Spd-2, and Plk4 (Basto et al.
2006
; Bettencourt-
Dias et al.
2005
; Blachon et al.
2009
; Blachon et al.
2008
; Giansanti et al.
2008
;
Martinez-Campos et al.
2004
; Rodrigues-Martins et al.
2007a
).
Mutations in proteins that are essential for compartmentalized ciliogenesis will not
affect meiosis or sperm motility, but will still result in adult uncoordination due to
defects in compartmentalized ciliogenesis. Examples include mutation in IFT proteins
such as Oseg 1 and 2 as well as IFT88 (Avidor-Reiss et al.
2004
; Han et al.
2003
).
A third group of centriolar proteins seems to affect compartmentalized and non-
compartmentalized ciliogenesis, but do not have an impact on meiosis. This group
seems to function in the conversion of a centriole into a basal body. Such mutants
include unc (Baker et al.
2004
).
1.4 The Role of the Centrosome
Immediately after the discovery of the centrosome by Flemming (1875) and van
Beneden (1876), two major hypotheses regarding its function were postulated.
Boveri hypothesized that the centrosome is a cellular organelle found close to the
nucleus and with paramount importance in cell division. On the other hand, the
Henneguy-Lenhossek theory (1898) claimed that the centrosome and basal body
are the same organelle located in two distinct sites, with the centrosome located at
the cell center near the nucleus, and the basal body existing at the base of the cilia
at the plasma membrane. This theory was the first to emphasize the importance of
the centrosome in cilia formation.
