Program of the 2004 East Coast Worm Meeting - Caenorhabditis ...

3. Robust germline amplification and the precise timing of initial meiosis are dependent
upon interactions with specific cells of the developing gonadal sheath
Darrell J. Killian, E. Jane Albert Hubbard
New York University, Department of Biology
An early step in animal development is the establishment of the germ line, an "immortal" cell
lineage that produces gametes as a means to contribute genetic information to the next
generation. In animals as diverse as flies, nematodes, and mammals, the adult germ line is a
spatial gradient of differentiation from germline stem cells progressing through meiosis to gamete
formation. However, germline development begins from a pool of presumably equivalent
primordial germ cells (PGCs). The development of the germ line from PGCs to a patterned adult
tissue is dependent upon interactions with the developing somatic gonad. We are interested in
somatic gonad/germline interactions that influence proliferation and differentiation of the
developing germ line.
In C. elegans, soma/germline interactions occur throughout germline development and can be
studied through genetic and anatomical perturbations. The PGCs, Z2 and Z3, associate with the
somatic gonad precursor cells Z1 and Z4. This interaction is essential for initial germ cell divisions
and their competence to differentiate. Later in development, germline proliferation is dependent
on the somatic distal tip cell (DTC). Ablation of the DTC or disruption of the underlying
LAG-2/GLP-1 signaling pathway results in a loss of germline stem cells to meiosis (Kimble and
White 1981; Austin and Kimble 1987). Conversely, constitutive activation of GLP-1 leads to
germline hyper-proliferation at the expense of differentiation (Berry et al., 1997). Yet another
germline pattern defect is proximal proliferation (Pro phenotype) characterized by ectopic
germline proliferation in the proximal region of the germ line. Pro mutant germ lines contain, from
distal to proximal, germline stem cells, meiotic cells, gametes, and a proximal germline tumor. In
several Pro mutants the germline tumor is derived of a subset of the pre-meiotic germ line that
failed to differentiate (Seydoux et al., 1990; Pepper et al., 2003; Killian and Hubbard 2004).
We have shown that a reduction-of-function mutation in pro-1 results in weak distal germline
proliferation, delayed meiosis, and a highly penetrant Pro phenotype. These phenotypes are due
to loss of pro-1 activity in the sheath/spermatheca (SS) lineage of the somatic gonad, not the
germ line. Consistent with this finding, the earliest germline defects in pro-1 mutants are detected
just following the division of the SS cells. Furthermore, a stronger reduction of pro-1 function by
RNAi deletes the SS lineage, further impairs germline proliferation, but does not cause a Pro
phenotype. pro-1 encodes a member of a highly conserved but poorly characterized sub-family of
the WD-repeat containing proteins (Killian and Hubbard 2004).
Our studies with pro-1 prompted an investigation of early SS lineage/germline interactions with
respect to germline proliferation, the timing of initial meiosis, and the molecular function of
PRO-1. Our time-course analysis of coordinate somatic gonad/germline development and our
cell-killing experiments complement and extend earlier findings by McCarter et al. (1997). We find
that the distal SS daughter, sheath 1, is in direct contact with mitotic germ cells in the L3 and L4
and this contact is crucial for robust proliferation. Sheath 1 does not contact the mitotic germ line
in adults (Hall et al., 1999) when the germline is in a steady state. In addition, we find that
ablation of the proximal daughter of the SS cell, the precursor to sheath 2-5 and spermathecal
cells, delays the onset of meiosis in the germ line. Together, our results support important and
antagonistic roles of the SS daughter cells in early germline pattern formation. We are also
investigating the role of PRO-1 with respect to the function of the SS lineage cells. The S.
cerevisiae ortholog of pro-1, Ipi3, has been implicated in the rRNA processing step of ribosome.
Our genetic analysis supports an analogous role for PRO-1. Up-regulation of ribosome
biogenesis via mutation in either ncl-1 or lin-35/Rb significantly rescues pro-1(na48) germline
pattern defects. Defects in ribosome biogenesis have been linked to both inhibition of cell growth
and cell cycle arrest. We are currently investigating a potential developmental control of ribosome
biogenesis related to the functions of the SS lineage cells and germline patterning.