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function is then important for the maintenance of germline stem cells (GSCs) in
their niche and for coordinating GSC differentiation into the gametes. The
second family of small RNA molecules, the short endo-siRNAs, regulate gene
expression posttranscriptionally as well but are also important for transposable
elements regulation and thus for genome stability. Last, the Piwi-interacting
small RNAs (piRNAs) are specifically expressed in the germline, where they
primarily control transposon activity, guarding the genome from potential dam-
age associated with excessive transposition. Together, the function of these
three groups of small RNAs ensures proper development and continuation of
the germline through the generations.
1. Introduction: The Germ Cell Lineage
and Small RNA Molecules
The germline passes the genetic information from one generation to
the next. The first cells that exclusively give rise to the germline by mitotic
divisions are the primordial germ cells (PGCs). The PGCs then migrate
toward the region where the gonad will form, where they settle in their
niche and are maintained as germline stem cells (GSCs) that give rise to
differentiated progeny. A characteristic of PGCs and GSCs is the possession
of a unique structure, the germ plasm, also termed “nuage” (Extavour and
Akam, 2003) , which is an electron-dense structure associated with mito-
chondria and contains germline specific mRNAs and proteins. Relevant for
this review, small noncoding RNAs have been shown to reside in the
germ plasm ( Findley et al ., 2003 ). These small regulatory molecules are
crucial for the regulation of various processes in the life of germ cells
including the specification of germ cell progenitors, maintenance of
GSC fate, protection of the genome, and allowance of proper differentia-
tion into the gametes. Three major classes of small RNAs have been
characterized, and while they are produced and function in different
pathways, related proteins are required for their biogenesis and activity.
The first to be discovered and perhaps the best-characterized small RNA
molecules are the microRNAs (miRNAs), followed by the later identifica-
tion of endogenous small interfering RNAs (endo-siRNAs) and Piwi-
interacting RNAs (piRNAs) ( Aravin et al ., 2001; Hamilton et al ., 2002;
Lee et al ., 1993 ).
In this review, we highlight and discuss the roles of small RNAs
belonging to these three groups in various aspects of germline development
in animals—from specification of the PGC progenitors to the final differ-
entiation steps producing the gametes. The function of small RNAs in the
plant germline has been reviewed elsewhere ( Borges et al ., 2011; Grant-
Downton, 2010; Jones-Rhoades et al ., 2006; Voinnet, 2009 ).
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