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time when
de novo
DNA methylation takes place (
Aravin
et al
., 2007b;
Kuramochi-Miyagawa
et al
., 2008
). In the same direction, a
piwi
mutant
defective in slicing activity is capable of rescuing the transposable element
silencing thus suggesting a slicer-activity independent PIWI function in this
context (
Saito
et al
., 2010
). This activity could be related to the fact that
PIWI proteins are capable of recruiting Heterochromatin protein 1A
(HP1A) to specific genomic loci, potentially leading to gene silencing
(
Brower-Toland
et al
., 2007; Pal-Bhadra
et al
., 2004
).
4.1.2. piRNA function in controlling protein-encoding
genes in germline
In addition to their function in transposon silencing, piRNAs were found to
target repetitive sequences of the protein-coding
Stellate
gene (
Aravin
et al
.,
2001
) and suggested to control
Fasciclin 3
(
Fas3
) levels in the follicle cells of
D. melanogaster
ovaries (
Saito
et al
., 2009
). In the fly testes, piRNAs might target
the
vasa
gene, which is a helicase necessary for pole plasm formation (
Nishida
et al
., 2007
). Interestingly, it has been recently shown that piRNAs may induce
degradation of maternally deposited mRNAs, such as
nanos
, during maternal-
to-zygotic transition (
Rouget
et al
., 2010
). In this case, piRNA function is
required for recruiting the CCR4 deadenylation complex to the specific RNA.
4.2. piRNAs function in germ cell specification
The role of Piwi proteins in germ cell specification in
Drosophila
is well
studied. Here, Piwi is localized to the polar granules and is required for pole
cell formation in a dose-dependent manner (
Megosh
et al
., 2006
) such that an
increase in maternal
piwi
dosage leads to a proportional increase in PGC
formation. Noteworthy, as Piwi protein was shown to associate with a small
number of miRNAs in addition to its association with piRNAs (
Megosh
et al
., 2006
), it is not clear if Piwi affects germ cell specification through
associated piRNAs or miRNAs. An additional piece of evidence linking
piRNAs to the process of germ cell specification relates to the phenotype
of embryos laid by female flies mutated for the
aubergine
gene that encodes for
a Piwi homolog that is localized to polar granules as well (
Harris and
Macdonald, 2001; Thomson
et al
., 2008
). Such embryos fail to form germ
cells reflecting defects in controlling the translation of
oskar
RNA, but thus
far the involvement of piRNAs in the process has not been demonstrated.
4.3. piRNA function in germline stem cell maintenance
and gametogenesis
Piwi proteins in
D. melanogaster
are expressed in somatic and germline cells of
both male and female (
Cox
et al
., 1998
), and flies mutated for
piwi
fail to
maintain GSCs in both sexes, while overexpressing Piwi in the somatic cells
