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.