Biology Reference
In-Depth Information
are missing in organisms that lack cilia. Since centrioles are only observed in
ciliated organisms, this screen can also be used to identify centriolar proteins
(Carvalho-Santos et al.; Hodges et al.). Examples of Drosophila centrosomal genes
identified using such approaches are poc1 and poc18 (Keller et al.
2005
).
1.6.4 Male Sterile Mutants
The centrosomal mutant asterless (asl) was identified in a study of male sterile
mutants (Bonaccorsi et al.
1998
). Unfortunately, for nearly 10 years, it was
incorrectly believed that Asl was solely important for centrosomal function and
aster formation (hence its name: Asterless) while its essential role in centriole
formation remained unknown (Bonaccorsi et al.
1998
; Giansanti et al.
2001
;
Oliferenko and Balasubramanian,
2002
; Varmark et al.
2007
; Wakefield et al.
2001
). Nonetheless, it was later demonstrated that Asl is instead essential for
centriole duplication and that the absence of aster formation in asl mutants is
mainly due to a lack of centrioles (Blachon et al.
2008
). This finding came from
analysis of asl
mecD
, a new mutant that was discovered in a mechanosensory mutant
screen (see below). Asl is a conserved centrosomal protein known as Cep152 in
vertebrates (Blachon et al.
2008
; Varmark et al.
2007
).
Male sterility is a common phenotype in centrosomal mutants and is a valuable
method to identify centriolar mutants. Male sterility in centriolar mutants can arise
by several distinct mechanisms:
First, centrosomes are essential for male meiosis and flies with abnormal
meiosis will fail to form functional sperm. This type of defect is most commonly
diagnosed by examining spermatids immediately after meiosis by light microscopy
and
observing
abnormal
numbers
and
shapes
of
nuclei
and
mitochondria
(Fig.
1.11
) (Bonaccorsi et al.
1998
; Li et al.
1998
).
Second, the centriole acts in templating the sperm flagellum. As a result,
abnormalities or absences of the centriole translate to abnormalities or absences of
the sperm flagellum. This type of defect is diagnosed by observing that the fly
sperm is not motile, or by electron microscopy analysis where fly sperm cross-
section shows a missing or abnormal axoneme (Baker et al.
2004
; Blachon et al.
2008
; Mottier-Pavie and Megraw,
2009
; Rodrigues-Martins et al.
2007a
).
Third, centrosomes are essential for mechanosensory cilia formation (Blachon
et al.
2009
; Blachon et al.
2008
; Martinez-Campos et al.
2004
). In the absence of
normal centrioles, flies have abnormal proprioception and cannot court the female
and mate with her.
Fourth, it is possible that an additional mechanism of male sterility is the failure
to form a normal PCL. If the PCL becomes one of the zygotic centrosomes, it is
expected that a nonfunctional PCL will cause male sterility even if the sperm is
delivered to the oocyte and fertilization takes place. In this case, genes whose
mutations cause PCL failure are expected to fall into a class of interesting mutants
referred to as paternal effect genes (Fitch and Wakimoto
1998
).
