Gcl protein specifically associates with those nuclei that later become the nuclei of the germ cell precursors (Jongens, 1992).
Pole cells and posterior segmentation in Drosophila are specified by maternally encoded genes whose products accumulate at the posterior pole of the oocyte. Among these genes is tudor (tud). Progeny of hypomorphic tud mothers lack pole cells and have variable posterior patterning defects. A null allele was isolated to further investigate tud function. While no pole cells are ever observed in embryos from tud-null mothers, 15% of these embryos have normal posterior patterning. Oskar and Vasa proteins, and nanos RNA, all initially localize to the pole plasm of tud-null oocytes and embryos from tud-null mothers, while localization of germ cell-less (gcl) and polar granule component (pgc), is undetectable or severely reduced. In embryos from tud-null mothers, polar granules are greatly reduced in number, size, and electron density. Thus, tud is dispensable for somatic patterning, but essential for pole cell specification and polar granule formation (Thomson, 2004).
In Drosophila, the germline precursor cells, i.e. pole cells, are formed at the posterior of the embryo. As observed for newly formed germ cells in many other eukaryotes, the pole cells are distinguished from the soma by their transcriptional quiescence. To learn more about the mechanisms involved in establishing quiescence, a potent transcriptional activator, Bicoid (Bcd), was ectopically expressed in pole cells. Bcd overrides the machinery that downregulates transcription, and activates not only its target gene hunchback but also the normally female specific Sex-lethal promoter, Sxl-Pe, in the pole cells of both sexes. Unexpectedly, the terminal pathway gene torso-like is required for Bcd-dependent transcription. However, terminal signaling is known to be attenuated in pole cells, and this raises the question of how this is accomplished. Evidence is presented indicating that polar granule component (pgc) is required to downregulate terminal signaling in early pole cells. Consistently, pole cells compromised for pgc function exhibit elevated levels of activated MAP kinase and premature transcription of the target gene tailless (tll). Furthermore, pgc is required to establish a repressive chromatin architecture in pole cells (Deshpande, 2004).
The germline of Drosophila is derived from a special group of cells called pole cells that are formed during early embryonic development. The Drosophila embryo initially develops as a syncytium of rapidly dividing nuclei that undergo multiple rounds of synchronized mitotic cycles. Prior to the tenth division cycle, several nuclei migrate into the specialized cytoplasm or pole plasm at the posterior of the embryo. These nuclei cellularize precociously and these newly formed cells divide two or three times to produce ~30-35 germline precursor cells. The remaining nuclei migrate to the surface of the embryo at nuclear division cycle 10-11. They then undergo several more synchronous divisions and cellularize at the end of nuclear cycle 14 to form the cellular blastoderm (Deshpande, 2004 and references therein).
In addition to their earlier cellularization and slower rate of mitosis, pole cells differ in their transcriptional activity. Somatic nuclei substantially upregulate RNA polymerase II transcription after they migrate to the surface of the embryo. The activation of zygotic gene expression is essential for these nuclei to respond appropriately to the maternal pathways that assign positional information along the axes of the embryo. By contrast, pole cell nuclei shut down RNA polymerase II transcription when they enter the pole plasm and they then remain transcriptionally quiescent until much later stages of embryogenesis. Transcriptional quiescence is a hallmark of germline precursor cells in many organisms. For example, in C. elegans, RNA polymerase II transcription is repressed in the germ cell lineage by the product of the pie-1 gene. Transcriptional inactivity appears to be crucial in establishing germ cell identity as mutations in pie-1 switch the fate of these cells to that of a somatic lineage (Deshpande, 2004 and references therein).
A number of maternally derived gene products are likely to contribute to transcriptional quiescence in the pole cells of Drosophila. One of these is Germ cell less (Gcl), a component of the germ plasm that is necessary for the formation of pole cells. gcl appears to be involved in the establishment of transcriptional quiescence and in embryos lacking gcl activity, newly formed pole buds are unable to silence the transcription of genes such as sisterless-a and scute. Conversely, when Gcl protein is ectopically expressed in the anterior of the embryo it can downregulate the transcription of terminal group genes such as tailless (tll) and huckebein (Leatherman, 2002). A second maternally derived gene product involved in transcriptional quiescence is Nanos. In the soma, Nanos, together with Pumilio, plays a key role in posterior determination by blocking the translation of maternally derived hunchback (hb) mRNA. Nanos (Nos) also plays a role in down-regulating transcription in pole cells, and in embryos produced by nos mutant mothers: genes that are normally active only in somatic nuclei are inappropriately transcribed in pole cells. These include the pair-rule genes fushi tarazu and even skipped, and the somatic sex determination gene Sex-lethal (Deshpande, 2004 and references therein).
The global effects of nos and gcl mutations on RNA polymerase II activity in pole cells are analogous to those seen in pie-1 mutants in C. elegans. In pie-1 mutants, genes that are normally expressed only in somatic lineages are turned on in the germ cell lineage. In wild-type C. elegans embryos, the inhibition of transcription in the germ cell lineage is correlated with a marked reduction in phosphorylation of the CTD repeats of the large subunit of RNA polymerase II (Seydoux, 1997). The CTD repeats are phosphorylated when polymerase is transcriptionally engaged. PIE-1 protein may prevent transcription by inhibiting this modification. As in C. elegans, the RNA polymerase II CTD repeats are underphosphorylated in the pole cells of wild-type Drosophila embryos. In the pole cells of gcl and nos mutant embryos, however, the level of CTD phosphorylation is elevated (Leatherman, 2002; Deshpande, 2004 and references therein).
Previous studies have shown that when a heterologous transcriptional activator, GAL4-VP16, is expressed in pole cells, it is unable to activate transcription of target gene(s) (Van Doren, 1998). This finding suggests that even if a potent activator were to be produced in pole cells, it would not be able to overcome the inhibition of the basal transcriptional machinery by gcl, nos and other factors. However, since GAL4-VP16 is a chimera of a yeast DNA-binding domain and a mammalian activation domain, an alternative possibility is that co-factors essential for its activity may be absent or inactive in Drosophila pole cells. For these reasons, tests were performed to see whether a transcription factor that is normally present and active in the somatic cells of early Drosophila embryos can promote the transcription of target genes when inappropriately expressed in pole cells. The homeodomain protein Bicoid (Bcd), which activates the zygotic transcription of hb and other genes specifying anterior development, was tested. A Bcd protein gradient is generated in precellular blastoderm embryos from the translation of maternal mRNA localized at the anterior pole. Although Bcd is not present in the posterior of wild-type embryos, increasing the bcd gene dose results in expansion of the gradient toward the posterior and a concomitant change in the pattern of zygotic gene expression. This result suggests that co-factors crucial for Bcd function are likely to be ubiquitous (Deshpande, 2004 and references therein).
Ectopic expression of Bcd in pole cells can induce the transcription of the bcd target gene hb. In addition to activating hb transcription, Bcd protein perturbs the migration of the pole cells to the primitive somatic gonad and causes abnormalities in cell cycle control. These effects on germ cell development resemble those observed in embryos from nos mutant females. Moreover, as in the case of nos- pole cells, the Sxl promoter Sxl-Pe is also turned on in pole cells by Bcd in a sex-nonspecific manner. Surprisingly, transcriptional activation in pole cells by Bcd requires the activity of the terminal signaling system. This observation is unexpected, since previous studies have established that the transcription of a downstream target gene of the terminal pathway, tailless (tll) is shut down completely in pole cells. Moreover, the doubly phosphorylated active isoform of MAP kinase ERK, which serves as a sensitive readout of the terminal pathway, is nearly absent in pole cells. Taken together, these findings argue that the activity of terminal signaling pathway in pole cells of wild-type embryos must be substantially attenuated, but not shut off completely. What mechanisms are responsible for downregulating terminal signaling in the presumptive germline? Evidence indicates that polar granule component (pgc) functions to attenuate the terminal pathway in newly formed pole cells. pgc encodes a non-translated RNA that is localized in specialized germ cell-specific structures called polar granules (Nakamura, 1996). Loss of pgc function in newly formed pole cells results in the ectopic phosphorylation of ERK and the activation of the ERK dependent target gene tll. pgc is required to block the establishment of an active chromatin architecture in pole cells (Deshpande, 2004).
Thus Bcd protein expressed from a bcd-nos3'UTR transgene (the 3' UTR of nos serves to localize the bcd message to pole cells) can activate the transcription of its target gene hb in pole cells, overcoming whatever mechanisms are responsible for transcriptional quiescence. In addition to activating transcription of hb, Bcd has other phenotypic effects. It prevents the pole cells from properly arresting their cell cycle and disrupts their migration to the somatic gonad. Because similar defects in pole cell development can be induced by the inappropriate expression of Sxl protein in these cells, one plausible hypothesis is that Bcd not only activates the hb promoter, but also turns on the Sxl establishment promoter, Sxl-Pe. Consistent with this idea, the Sxl-Pe:lacZ reporter is turned on in the pole cells of male and female bcd-nos 3' UTR embryos and Sxl protein accumulates in these cells. Although previous studies indicate that Sxl-Pe is responsive to Bcd, it is somewhat surprising that Sxl-Pe is not only inappropriately turned on in pole cells by Bcd, but that it is activated in both sexes. This suggests that Bcd activation of Sxl-Pe in pole cells must proceed by a mechanism that bypasses the X/A chromosome counting system which controls Sxl-Pe activity in the soma. It is interesting to note that the activation of Sxl-Pe in pole cells in the absence of nos function also seems to depend upon a mechanism(s) that circumvents the X/A chromosome counting system (Deshpande, 2004).
That Bcd protein depends upon other ancillary factors to turn on transcription in pole cells is demonstrated by the requirement for tsl function in the activation of both the hb and Sxl-Pe promoters. tsl is a component of the maternal terminal signaling pathway that activates the zygotic genes, tll and huckebein (hkb), at the poles of the embryo. In addition, the terminal pathway has opposing effects on the expression of bcd-dependent gap genes. At the anterior pole, where terminal signaling activity is highest, Bcd targets such as hb and orthodenticle (otd) are repressed. At a distance from the anterior pole, where both the concentration of Bcd protein and the strength of the terminal signaling cascade is much lower, the terminal pathway has an opposite, positive effect on hb and otd expression. Two mechanisms are thought to account for the positive effects of the terminal pathway on bcd target genes: (1) Bcd is a direct target for phosphorylation by the terminal signaling cascade; (2) regulatory regions of bcd target genes have sites for other transcription factors whose activity can be directly modulated by the terminal system (Deshpande, 2004).
The concentration of Bcd protein produced by the bcd-nos 3' UTR transgene in pole cells is much less than it is at the anterior pole. Similarly, the activity of the terminal signaling cascade in pole cells is much reduced compared with that in the somatic nuclei at the anterior and posterior poles. Thus, in both of these respects, the conditions in the bcd-nos 3' UTR pole cells would appear to most closely approximate those in the region of the embryo where the terminal signaling cascade potentiates rather than inhibits Bcd activity. This would explain why activation of transcription in pole cells by Bcd depends on the terminal signaling pathway and why in this particular instance this pathway does not antagonize the activity of the ectopically expressed Bcd protein (Deshpande, 2004).
The fact that the terminal pathway can function in pole cells, yet does not turn on its target gene tll indicates that the activity of this pathway is attenuated in the germline. It seems likely that several different mechanisms may be responsible for preventing pole cells from responding to the terminal pathway and turning on tll transcription. One mechanism appears to be an inhibition of the signaling cascade itself. In the posterior and anterior soma of pre-cellular blastoderm embryos, the terminal signaling cascade directs the phosphorylation of the MAP kinase ERK. While phosphorylated ERK can also be detected in wild-type pole cells, the amount of activated kinase is much less than in the surrounding soma. Consistent with this observation, potentiating the terminal system using either a gain-of-function torso receptor mutant or by expressing elevated levels of the receptor in pole cells using a torso transgene (which has the nos 3' UTR) had only a small effect on the activity of a tll-lacZ reporter in the germline. By contrast, gain-of-function torso mutation substantially upregulates the tll reporter in the soma (Deshpande, 2004).
To identify factors that could be involved in repressing the terminal pathway in pole cells, three genes, nos, gcl and pgc, were examined that are known to play an important role in the early development of the germline and have been implicated in transcriptional quiescence. Of these three, only pgc appears to have significant effects on the terminal signaling pathway in pole cells. The expression of a tll reporter is turned on in pole cells of embryos deficient in pgc activity. That this is due at least in part to a failure to properly attenuate the terminal signaling pathway in the germline is suggested by the fact that the level of activated ERK is greatly elevated in pgc pole cells compared with wild type. Although these findings implicate pgc in downregulating the terminal pathway, how this is accomplished and whether pgc has a direct rather than an indirect role in this process remains to be determined. In addition, these studies indicate that pgc has functions in addition to attenuating this signaling cascade: (1) it was found that there are abnormalities in the formation of pole cells in pgc embryos and Vasa-positive 'cells' are observed in cycle 9-10 embryos at abnormal locations; (2) the loss of pgc activity may lead to the inappropriate activation of genes in addition to tll. Two markers for global transcriptional activity, CTD phosphorylation and histone H3 K4 methylation, are present in pole cells of pgc embryos (Deshpande, 2004).
The results also suggest that multiple and interrelated levels of regulation are responsible for ensuring transcriptional quiescence in the pole cells. For example, Sxl-Pe can be upregulated by the terminal pathway in the soma and requires this pathway to be activated by Bcd in pole cells. However, this promoter is not activated in pole cells in the absence of pgc function. Thus, the activation of the terminal signaling cascade in pole cells is not sufficient in itself to induce Sxl-Pe. This suggests that mechanisms are in place in pgc pole cells that would override any effects of activated ERK on Sxl-Pe activity. Similarly, although loss of nos activity leads to the activation of Sxl-Pe in pole cells, and the upregulation of tll in the posterior soma, the tll promoter is not turned on in nos pole cells. It is presumed that tll is not activated in pole cells because it requires the terminal system that still remains attenuated in nos pole cells. Redundancy is also suggested by the finding that although the loss of gcl leads to the expression of the X chromosome counting genes sis-a and scute in pole cells (Leatherman, 2002), Sxl-Pe is not activated, suggesting that nos function is sufficient to keep Sxl-Pe off in gcl mutant pole cells even though several X chromosome counting genes are activated. Similarly, no obvious effect was observed of nos mutations on scute expression in pole cells. This implies that gcl and nos may be responsible for repressing the transcription of different sets of genes (Deshpande, 2004).
Finally, although transcription is upregulated in pgc pole cells between nuclear cycles 9/10-13, a high level of transcriptional activity is not maintained in the pole cells that are present by the time the cellular blastoderm is formed. The tll reporter is turned off, and both CTD phosphorylation and histone H3 K4 methylation disappear. One possible interpretation of this finding is that pgc has an early function in establishing transcriptional quiescence, but is not required after nuclear cycle 13 because of the activity of other factors such nos or gcl. However, since the number of pole cells at cellularization is reduced compared with the number present earlier, it also possible that the only pole cells that remain are the ones in which the amount of pgc activity is sufficient to establish some degree of transcriptional repression. Further studies with bona fide null alleles will be required to resolve this question, and to understand how pgc functions during pole cell formation and germ cell determination (Deshpande, 2004).
The first cell fate specification process in the Drosophila embryo, formation of the germline precursors, requires posteriorly localized germ plasm. germ cell-less is required for germline formation. Posterior localization of the gcl messenger RNA (mRNA) requires the function of those genes essential for the localization of both nanos RNA, which specifies the abdomen, and the germ cell determinants. Mothers with reduced gcl function give rise to sterile adult progeny that lack germ cells. In embryos with reduced maternal gcl product, the germ cell precursors fail to form properly. These observations suggest that gcl functions in the germ cell specification pathway (Jongens, 1992).
The maternally supplied plasm at the posterior pole of a Drosophila embryo contains determinants that specify both the germ-cell precursors (pole cells) and the posterior axis. One pole plasma component, the product of the germ cell-less gene, has been found to be required for specification of pole cells, but not posterior somatic cells. Mothers with reduced levels of gcl give rise to progeny that lack pole cells, but are otherwise normal. Mothers overexpressing gcl, on the other hand, produce progeny exhibiting a transient increase of pole cells. Ectopic localization of gcl to the anterior pole of the embryo causes nuclei at that location to adopt characteristics of pole cell nuclei, with concurrent loss of somatic cells. Evidence indicating that the gcl protein associates specifically with the nuclear pores of the pole cell nuclei. This localization suggests a novel mechanism in the specification of cell fate for the germ line (Jongens, 1994).
The germ cell precursors of Drosophila (pole cells) are specified by maternally supplied germ plasm localized to the posterior pole of the egg. One component of the germ plasm, germ cell-less (gcl) mRNA, encodes a novel protein that specifically localizes to the nuclear envelope of the pole cell nuclei. In addition to its maternal expression, gcl is zygotically expressed through embryonic development. A null allele of gcl has been characterized to determine its absolute requirement during development. gcl activity is required only for the establishment of the germ cell lineage. Most embryos lacking maternal gcl activity fail to establish a germline. No other developmental defects were detected. Examination of germline development in these mutant embryos has revealed that gcl activity is required for proper pole bud formation, pole cell formation, and pole cell survival. Using this null mutant the activity of forms of Gcl protein with altered subcellular distribution were assayed; localization to the nuclear envelope is crucial for promoting pole cell formation, but not necessary to initiate and form proper pole buds. These results indicate that gcl acts in at least two different ways during the establishment of the germ cell lineage (Robertson, 1999).
Although gcl encodes a germ plasm component and is expressed in several tissues throughout development, a requirement was detected for only its maternal expression. Embryos that lack maternal gcl activity form either no or fewer pole cells than control embryos. Most of the resulting adults are sterile, with no other developmental defects being observed. This effect on germline formation is not due to a decrease in germ plasm integrity or due to a failure to establish transcriptional repression in the early pole cells. Analysis of the gcl null mutant phenotype, in combination with assaying the ability of mislocalized forms of Gcl protein to rescue this mutant, has revealed that gcl is required up to three times during the establishment of the germ cell lineage. Furthermore, the results confirm the suggestion, inferred from antisense studies, that maternal gcl is required only for the establishment of the germ cell lineage (Robertson, 1999).
gcl activity is initially required at or prior to the time of pole bud formation, since pole buds fail to appear or have poor morphology in the gclD embryos. The overexpression and ectopic localization of gcl leads to extra and ectopic pole bud formation, respectively (Jongens, 1994). Taken together these results indicate that gcl is both necessary and sufficient to induce pole bud formation although, since some level of pole bud formation is observed in the gclD embryos, there is clearly another activity in the germ plasm capable of initiating this process. The failure to form proper pole buds is probably the major cause of sterility in the gclD progeny, since pole cell formation is presumably dependent on proper pole bud formation. It is clear, however that this is not the only reason that a germline fails to form in the gclD progeny (Robertson, 1999).
Attempts to rescue the gcl null phenotype with mislocalized forms of Gcl protein have revealed that gcl is required in two distinct ways for efficient pole cell formation to occur. Although pole bud formation can occur when Gcl protein is restricted to the cytoplasm or nucleoplasm, efficient pole cell formation requires that Gcl protein localizes to the nuclear envelope. This indicates that the dramatic reduction in the number of pole cells formed in the gclD embryos is due to the loss of gcl activity at two distinct times, once for pole bud formation and a second time for efficient pole cell formation. At this point, it is not known if pole bud formation and pole cell formation require different activities encoded by gcl or if the two processes have unique subcellular localization requirements for the same activity (Robertson, 1999).
Analysis of germ cell precursor development in the gclD embryos reveal that gcl activity may also be required after pole cell formation. During gastrulation the pole cells undergo a patterned migration to the embryonic gonad. Previous studies have noted that during this migratory phase some of the pole cells die or migrate aberrantly; only 70% of the pole cells successfully reach the embryonic gonad (Technau, 1986; Hay, 1988). Although the host line used in these experiments had a slightly higher success rate (91%), only 39% of the pole cells formed in the gclD embryos successfully reached the embryonic gonad (Robertson, 1999).
Although this reduction in pole cell survival may be due to pole cells being poorly formed or being poorly determined at the time of formation, it is also possible that it is due to a requirement for gcl activity after pole cell formation. It is interesting to note with respect to this last possibility that Gcl protein is detected in the pole cells up until stage 10 (Jongens, 1992). The presence of Gcl protein up until this time may be important for keeping pole cells directed toward germ cell fate (Robertson, 1999).
Most, if not all, of the pole cells that successfully reach the embryonic gonad in the gclD embryos appear to develop into functional germ cells. The percentage of embryos that have one pole cell or more at stage 14 matches the percentage of fertile gclD progeny. An interesting observation with respect to this point is that upon examination of the gclD females, the minimum number of ovarioles observed in a single ovary was 14. Since the germarium, at the tip of each ovariole, contains a minimum of two germline stem cells, these results show that a single pole cell entering the embryonic gonad can give rise to a minimum of 28 germline stem cells. Technau (1986) has shown that a mechanism that limits the maximum number of pole cells that reach the embryonic gonad exists. The current result, in combination with those of Technau (1986), indicates that mechanisms exist to regulate both the minimum and the maximum number of germ cell precursors in the gonad (Robertson, 1999).
The analysis of germline formation in the gclD embryos has revealed that gcl is required at the time of pole bud formation. Mutant forms of Gcl that are restricted to the nucleoplasm or cytoplasm can rescue pole bud formation in gclD embryos. Thus, the most likely scenario is that Gcl acts in the cytoplasm to promote pole bud formation, prior to its entry into the pole bud nuclei. At this point it is not known how gcl activity affects the cytoskeletal reorganization required for this process (Robertson, 1999).
The results show that Gcl protein must localize to the nuclear envelope for efficient pole cell formation to occur. Previous characterization of the subcellular distribution of Gcl protein revealed that it is mostly localized to the nucleoplasmic surface of the nuclear envelope (Jongens, 1994). This distribution precludes it from having a direct role in the cytokinesis event required to form the pole cells and indicates that gcl activity may act through some intracellular signaling pathway. An interesting point with respect to this possibility is the apparent myristoylation modification required to localize Gcl protein to the nuclear envelope. This N-terminal protein modification is commonly found on components of intracellular signaling pathways which are membrane bound (Robertson, 1999).
The dependence of Gcl protein localization to the nuclear envelope on a myristoylation modification draws into question a previously proposed model whereby Gcl protein localizes to the nuclear envelope through an interaction with the nucleoplasmic surface of the nuclear pore complex (NPC) (Jongens, 1994). Clearly, the combination of an NLS and a myristoylation site present in the germ cell-less sequence should be sufficient to localize Gcl protein to the nucleoplasmic surface of the nuclear envelope. Therefore the localization of Gcl protein probably occurs independent of an association with the NPC (Robertson, 1999).
Given results obtained through antisense, overexpression, and ectopic-expression studies of gcl, it was expected that the germ cell lineage would be affected in embryos lacking maternal gcl activity. However, it was surprising to find that gcl is not absolutely required for this process. This expectation was held for two reasons: (1) Gcl protein is found on all of the pole cell nuclei (Jongens, 1992); (2) using antisense methodology to reduce maternal gcl mRNA levels, sterility rates were obtained that were as high as 90%, even when some maternal gcl mRNA is still detected in the embryo (Jongens, 1992). Thus the expectation was that if all of the maternal contribution of gcl mRNA was removed, all of the progeny would be sterile. This is clearly not the case; roughly 30% of the gclD progeny can form a functional germline. Therefore, a stronger effect was observed with the antisense approach compared to the null mutant. One possibility for this difference is the existence of another gene with similar activity and a high degree of sequence similarity to gcl that is also affected by antisense gcl RNA expression. To investigate this possibility, low-stringency Southern analysis was performed on Drosophila genomic DNA, but no gcl homolog was found. Also transgene was introduced that provided high levels of antisense gcl RNA expression into the gcl null background: no enhancement of the phenotype was detected. Thus at this time the higher sterility rate obtained in the antisense experiments cannot be explained (Robertson, 1999).
Nonetheless a fairly accurate requirement of gcl activity was uncovered by the antisense approach. The failure to identify a gcl homolog leaves unanswered the reason for the incomplete penetrance of the gcl null mutant. It is conceivable that some gcl-like activity is provided by a homolog whose sequence divergence prevents detection with low-stringency hybridization approaches or by a gene with no similarity to gcl. This redundant germ cell-less-like activity observed during pole cell formation could also be present later in development and mask the requirement for gcl zygotic activity (Robertson, 1999).
In Drosophila, mitochondrially encoded ribosomal RNAs (mtrRNAs) form mitochondrial-type ribosomes on the polar granules, distinctive organelles of the germ plasm. Since a reduction in the amount of mtrRNA results in the failure of embryos to produce germline progenitors, or pole cells, it has been proposed that translation by mitochondrial-type ribosomes is required for germline formation. This study reports that injection of kasugamycin (KA) and chloramphenicol (CH), inhibitors for prokaryotic-type translation, disrupted pole cell formation in early embryos. The number of mitochondrial-type ribosomes on polar granules was significantly decreased by KA treatment, as shown by electron microscopy. In contrast, ribosomes in the mitochondria and mitochondrial activity were unaffected by KA and CH. It was further found that injection of KA and CH impairs production of Germ cell-less (Gcl) protein, which is required for pole cell formation. The above observations suggest that mitochondrial-type translation is required for pole cell formation, and Gcl is a probable candidate for the protein produced by this translation system (Amikura, 2005).
Increased reproduction is frequently associated with a reduction in longevity in a variety of organisms. Traditional explanations of this 'cost of reproduction' suggest that trade-offs between reproduction and longevity should be obligate. However, it is possible to uncouple the two traits in model organisms. Recently, it has been suggested that reproduction and longevity are linked by molecular signals produced by specific reproductive tissues. For example, in C. elegans, lifespan is extended in worms that lack a proliferating germ line, but which possess somatic gonad tissue, suggesting that these tissues are the sources of signals that mediate lifespan. In this study, evidence of such gonadal signals was tested in Drosophila melanogaster. The germ line was ablated using two maternal effect mutations: germ cell-less and tudor. Both mutations result in flies that lack a proliferating germ line but that possess a somatic gonad. In contrast to the findings from C. elegans, it was found that germ line ablated females had reduced longevity relative to controls and that the removal of the germ line led to an over-proliferation of the somatic stem cells in the germarium. The results contrast with the widely held view that it is downstream reproductive processes such as the production and/or laying of eggs that are costly to females. In males, germ line ablation caused either no difference, or a slight extension, in longevity relative to controls. The results indicate that early acting, upstream reproductive enabling processes are likely to be important in determining reproductive costs. In addition, it is suggested that the specific roles and putative patterns of molecular signalling in the germ line and somatic tissues are not conserved between flies and worms (Barnes, 2006).
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date revised: 2 January 2007
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