Biology Reference
In-Depth Information
These two hypotheses were based only on observations of the centrosome and
have remained untested for many years. Recent studies in Drosophila have
allowed these theories to be tested directly. A genetic approach that has been
employed is elimination of the centrosome using mutants and the study of its effect
on cilia formation and cell division. Ideally, one would employ a null mutation in a
centrosome-specific component that is absolutely essential for centrosome for-
mation. Studying the phenotype of these mutants can test if the centrosome is
essential for cell division and/or cilia formation.
Study of fly mutants that block centrosome formation (sas-4 and asl
mecD
) has
suggested that flies lacking centrosomes die due to mechanosensory defects caused
by a failure to form cilia (Basto et al.
2006
; Blachon et al.
2008
). These flies also
exhibit a failure in male meiosis and their sperm form without an axoneme. However,
it is important to note that during larval and pupal development, the centrosome does
not play an essential role in cell division; division, though delayed, still takes place.
This argues that, at least in larvae, pupae, and adult files, centrosomes and centrioles
behave in a way consistent with the Henneguy-Lenhossek theory.
In flies, the oocyte carries a large amount of maternally derived proteins that support
the early development of the embryo. Therefore, analyzing the role of the centrosome
in the early embryo requires the embryo's mother or oocyte to be mutant (see above).
Interestingly, such studies suggest that centrosomes are vital in the zygote and early
embryo. However, whether this is because centrosomes are essential to mediate
nuclear division is not clear. Regardless, in this developmental stage, fly cells do not
have cilia and it is thus possible that the early embryonic syncytium requires the
centrosome for ''cell'' division in a way that is consistent with the Boveri hypothesize.
However, an important caveat in this idea is that centrosomes may have an important
function in the early embryo that is neither related to cilia formation nor cell division.
For example, it appears that centrosome function is essential for the migration and
fusion of the gamete pronuclei and in acting as a signaling hub (see above).
1.5 Two Pathways of Centriole Formation
A centriole forms by one of two pathways (Anderson and Brenner
1971
; Delattre and
Gonczy
2004
; Loncarek and Khodjakov
2009
; Loncarek et al.
2007
). In the
'' acentriolar pathway'', a daughter centriole forms de novo without a preexisting
centriole. This occurs when multiple centrioles are required in a cell or under the
unusual situation when preexisting centrioles are absent. The acentriolar pathway
produces an imprecise number of centrioles. In flies, centrioles can form de novo
when certain centrosomal proteins are overexpressed in oocytes or fertilized
embryos (e.g., PLK4, Asl, and Ana-2) (Peel et al.
2007
; Rodrigues-Martins et al.
2007b
; Stevens et al.
2010a
). To what extent these centrioles are similar to centrioles
that normally form is not clear, but in some cases, they appear to have normal
centriolar structures, have the capacity to recruit PCM proteins, and can form astral
microtubules. Whether, these induced centrosomes can form cilia or mediate meiosis
