The vasa gene product of Drosophila melanogaster is required only in the female germ line. Progeny of females homozygous for vasa mutations lack posterior structures and pole cells. Isolation and characterization of vasa genomic and complementary DNA clones show that the transcript is abundant in the female germ line and early embryos only. The predicted amino acid sequence is very similar to those of the translation initiation factor eIF-4A and the human nuclear antigen p68.

Sam68 is a member of a growing family of proteins that contain a single KH domain embedded in a larger conserved domain of ϳ170 amino acids. Loops 1 and 4 of this KH domain family are longer than the corresponding loops in other KH domains and contain conserved residues. KH domains are protein motifs that are involved in RNA binding and are often present in multiple copies. Here we demonstrate by coimmunoprecipitation studies that Sam68 self-associated and that cellular RNA was required for the association. Deletion studies demonstrated that the Sam68 KH domain loops 1 and 4 were required for self-association. The Sam68 interaction was also observed in Saccharomyces cerevisiae by the two-hybrid system. In situ chemical cross-linking studies in mammalian cells demonstrated that Sam68 oligomerized in vivo. These Sam68 complexes bound homopolymeric RNA and the SH3 domains of p59 fyn and phospholipase C␥1 in vitro, demonstrating that Sam68 associates with RNA and signaling molecules as a multimer. The formation of the Sam68 complex was inhibited by p59 fyn , suggesting that tyrosine phosphorylation regulates Sam68 oligomerization. Other Sam68 family members including Artemia salina GRP33, Caenorhabditis elegans GLD-1, and mouse Qk1 also oligomerized. In addition, Sam68, GRP33, GLD-1, and Qk1 associated with other KH domain proteins such as Bicaudal C. These observations indicate that the single KH domain found in the Sam68 family, in addition to mediating protein-RNA interactions, mediates protein-protein interactions.

The Drosophila gene vasa (vas) encodes an RNA-binding protein required for embryonic patterning and germ cell specification. In vas mutants, translation of several germline mRNAs is reduced. Here we show that VAS interacts directly with the Drosophila homolog of yeast translation initiation factor 2, encoded by a novel gene, dIF2. Embryos produced by vas/+; dIF2/+ females have pattern defects and fewer germline progenitor cells, indicating a functional interaction between endogenous vas and dIF2 activities. Mutations in other translation initiation factors do not enhance the vas phenotype, suggesting that dIF2 has a particular role in germ plasm function. We conclude that VAS regulates translation of germline mRNAs by specific interaction with dIF2, an essential factor conserved from bacteria to humans.

SummaryGermline stem cells (GSCs) in Drosophila are a valuable model to explore of how adult stem cells are regulated in vivo. Genetic dissection of this system has shown that stem cell fate is determined and maintained by the stem cell's somatic microenvironment or niche. In Drosophila gonads, the stem cell niche-the cap cell cluster in females and the hub in males-acts as a signaling center to recruit GSCs from among a small population of undifferentiated primordial germ cells (PGCs). Shortrange signals from the niche specify and regulate stem cell fate by maintaining the undifferentiated state of the PGCs next to the niche. Germline cells that do not receive the niche signals because of their location assume the default fate and differentiate. Once GSCs are specified, adherens junctions maintain close association between the stem cells and their niche and help to orient stem cell division so that one daughter is displaced from the niche and differentiates. In females, stem cell fate depends on bone morphogenetic protein (BMP) signals from the cap cells; in males, hub cells express the cytokine-like ligand Unpaired, which activates the Janus kinase-signal transducers and activators of transcription (Jak-Stat) pathway in stem cells. Although the signaling pathways operating between the niche and stem cells are different, there are common general features in both males and females, including the arrangement of cell types, many of the genes used, and the logic of the system that maintains stem cell fate. KeywordsDrosophila; germline stem cell; GSC; PGC; primordial germ cell; stem cell fate; stem cell niche IntroductionStem cells are defined by their capacity to self-renew and to generate daughters that differentiate into one or more terminal cell types. Proper regulation of this property is critical for animal development, growth control, and reproduction. Understanding stem cell function is potentially very important for future developments in gene therapies and regenerative medicine. Research in Drosophila on germline stem cells (GSCs) has been instrumental in defining the important function of the stem cell's somatic microenvironment or niche in the control of its division and self-renewal (1,2). The Drosophila germline is an excellent model of stem cell biology because the system is genetically tractable, the sterility and rudimentary gonads resulting from GSC loss are easily recognized, and the stem cells can be easily identified based on molecular markers and position in the gonad. Germline morphology and development, from embryogenesis to the differentiation of gametes in adult flies, has been well characterized and offers a solid basis for the study of GSC fate determination, maintenance, and differentiation.Drosophila GSCs are derived from embryonic pole cells, the first cell type defined in the embryo. They migrate from the posterior to meet the somatic gonadal precursors (SGPs) and form the embryonic gonad, a simple structure made up of about ten primordial germ cells (PGCs) intermingled with and surr...

The genetic interval 35C to 36A on chromosome arm 2L of Drosophila melanogaster has been saturated for mutations with visible or lethal phenotypes. 38 loci have been characterized, including several maternal-effect lethals (vasa, Bic-C, chiffon, cactus and cornichon) and several early embryonic lethals, including snail and fizzy. About 130 deletions have been used to order these loci. Complex interactions between mutant alleles have been uncovered in the immediate genetic environs of the snail gene, as has further evidence for an interaction between this region and that including the nearby genes no-ocelli and elbow.