Biology Reference
In-Depth Information
microtubules, centrioles in the syncytial blastoderm are made of nine doublets of
microtubules. In addition, syncytial blastoderm centrioles are shorter than classic
centrioles: *200 nm long instead of 400-500 nm long observed in vertebrates.
Finally, the centrioles of the syncytial blastoderm have a structure known as the
''cartwheel'' within their core, which in vertebrate cells is characteristic of a
young, developing centriole (procentriole) and is absent from mature centrioles.
This raises the hypothesis that syncytial blastoderm centrioles are centriolar
structures arrested in the procentriole stage. It is possible that since the syncytial
blastoderm nuclei divide rapidly and there is no need for cilia formation (see
below), the centrioles do not have the time, nor the need, to develop into their
mature states (Fig.
1.6
).
1.2.3 The Drosophila Zygote and Syncytial Blastoderm Develop
Using Proteins Generated in the Mother
Protein deposition in the oocyte that supports early embryonic development is
called maternal contribution. An important implication of the presence of maternal
contribution in Drosophila development is that, despite the fact that an embryo
may be genetically homozygous for a mutation in an essential centrosomal gene, it
will still contain the wild-type protein, allowing it to produce normal centrosomes
as long as the maternal contribution persists. As a result, studying those mutants
cannot reveal the role of centrosomes in early embryogenesis. Indeed, studies
using homozygous mutants for an essential centrosomal gene, have demonstrated
that the fly embryo can develop normally (Basto et al.
2006
; Blachon et al.
2008
;
Rodrigues-Martins et al.
2007a
).
Investigating the role of centrosomes in early embryogenesis requires the study
of an embryo that is produced from an oocyte generated in an environment that is
also mutated. Since flies with mutations in essential centrosomal proteins are
unable to walk, mate, or lay eggs (see below), it is not possible to use embryos
produced by homozygous females. However, this obstacle can be overcome using
other approaches.
One way is to study centrosomal proteins that are essential for aspects of
centrosome function but are not necessary to produce a fertile female. For
example, mutations in centrosomin (Cnn) result in flies that are viable but female
sterile (Megraw et al.
1999
; Vaizel-Ohayon and Schejter
1999
). Cnn is a PCM
protein that plays an important role in PCM formation and is required for the
centrosome's activity as a microtubule organization center (Li and Kaufman
1996
). Studies of cnn mutants in early embryogenesis reveal an impairment of
several aspects of embryo development that depend on the function of the cyto-
skeleton (Megraw et al.
1999
; Vaizel-Ohayon and Schejter
1999
). In particular, it
appears that Cnn is essential for the organization of actin into cleavage furrows.
New information suggests that the centrosome functions as a site where Cnn
