To address how cell proliferation, differentiation, and morphogenesis are coordinated during organogenesis, we are studying gonadogenesis in C. elegans . In wild-type hermaphrodites, 12 somatic gonadal cells are generated by early L2. By L3, ten of these cells coalesce into the gonad center to form the somatic gonadal primordium (SGP), while the two distal tip cells (DTC) remain at the gonad ends. The DTCs and anchor cell (AC) differentiate at this stage, while the remaining cells divide further during L3 and L4 to make adult tissues. The
spf-1 gene is defined by three recessive, loss-of-function mutations that map to the right arm of LGI.
spf-1 mutants have two dramatic effects on gonadogenesis. First, specification of the DTCs does not occur, as demonstrated by a lack of gonadal arm extension and absence of markers of DTC fate. By lineage analysis of
spf-1 gonads, the cells that normally would differentiate as DTCs, Z1.aa and Z4.pp, divide in mid-L2. In wild-type, these cells never divide. Furthermore, the timing of other somatic gonadal cell divisions is aberrant in
spf-1 mutants. Therefore,
spf-1 is required for the normal specification of DTCs and for the normal pattern of somatic gonadal cell divisions. Second, in
spf-1 mutant hermaphrodites, the somatic gonadal cells fail to form the SGP. Instead, they position themselves at the periphery of the gonadal mass and encapsulate the germline cells. Therefore,
spf-1 is required for the morphogenetic rearrangement to make the hermaphrodite SGP. (By contrast, the male SGP forms essentially normally, and males can produce cross-progeny.) Despite these early defects, the somatic gonadal cells make differentiated gonadal tissues later, although these tissues are not organized into typical structures. We cloned
spf-1 and found it to encode a novel cytoplasmic protein. The
spf-1 cDNA detects a single transcript by Northern analysis. A SPF-1::GFP construct is expressed in all somatic gonadal cells just prior to SGP formation and remains on in all somatic gonadal cells that continue to divide until late L4 when they terminally differentiate. The role of
spf-1 is being tested further by misexpression, laser ablation and antibody staining.