Robust methods for in situ hybridization and gene knockdown via RNA interference (RNAi) have now been
developed for both platyhelminths and acoels, and these
are providing insight into the molecular basis of the
various functions of neoblasts [1,4,8] (Figure 1c). One
group of genes, the PIWI genes, has received particular
attention as conserved regulators of stem-cell function.
PIWI genes are a subfamily of the PIWI/Argonaute gene
family and, in most animals investigated, are expressed
specifically in the germ line, where they are thought to
function in silencing transposons and translational gene
regulation. In Schmidtea and Macrostomum, transcripts
of piwi homologs are present not only in the germ line
but also in the neoblasts, and gene knockdown by RNAi
results in the eventual loss of both the germ-line and the
neoblast pool [6,9]. De Mulder et al. [4] now find that a
piwi homolog is also expressed in both germ line and
neoblasts in Isodiametra. RNAi knockdown of its
expression causes loss of the germ line but, unexpectedly,
does not affect the neoblast pool or its proliferation.
Whether there are other piwi homologs expressed
redundantly in the neoblasts of this species remains to be
determined