Abdominal patterning in Drosophila requires the function of nanos (nos) to prevent translation of hunchback (hb) mRNA in the posterior of the embryo. nos function is restricted to the posterior by the translational repression of mRNA that is not incorporated into the posteriorly localized germ plasm during oogenesis. The wasp Nasonia vitripennis (Nv) undergoes a long germ mode of development very similar to Drosophila, although the molecular patterning mechanisms employed in these two organisms have diverged significantly, reflecting the independent evolution of this mode of development. Here, we report that although Nv nanos (Nv-nos) has a conserved function in embryonic patterning through translational repression of hb, the timing and mechanisms of this repression are significantly delayed in the wasp compared with the fly. This delay in Nv-nos function appears to be related to the dynamic behavior of the germ plasm in Nasonia, as well as to the maternal provision of Nv-Hb protein during oogenesis. Unlike in flies, there appears to be two functional populations of Nv-nos mRNA: one that is concentrated in the oosome and is taken up into the pole cells before evidence of Nv-hb repression is observed; another that forms a gradient at the posterior and plays a role in Nv-hb translational repression. Altogether, our results show that, although the embryonic patterning function of nos orthologs is broadly conserved, the mechanisms employed to achieve this function are distinct.
Two maternally localized mRNAs, bicoid (bcd) and nanos (nos) are crucial for patterning the anterior and posterior of the Drosophila embryo, respectively (Struhl, 1989). The patterning mechanisms employed by these two factors are quite distinct. bcd mRNA is localized to the anterior pole of the oocyte during oogenesis (Berleth et al., 1988) and is translated from this localized source after the activation of the egg. Its protein product diffuses through the embryo, forming an anterior-to-posterior concentration gradient of a transcription factor that activates target genes in a concentration-dependent manner (Driever and Nusslein-Volhard, 1988a; Driever and Nusslein-Volhard, 1988b; Driever et al., 1989; Struhl et al., 1989). Similar to bcd, nos mRNA is localized during oogenesis, but at the posterior. Nos protein is produced from this localized source and diffuses through the embryo, resulting in a posterior to anterior concentration gradient (Wang and Lehmann, 1991). Although Bicoid acts primarily as an instructive transcription factor that activates anterior genes, including zygotic hunchback (hb), Nos has a permissive function that allows posterior patterning by translationally repressing ubiquitous maternal hb mRNA (Tautz, 1988; Hulskamp et al., 1989; Irish et al., 1989; Struhl, 1989), which contains a Nos response element (NRE) in its 3′ UTR (Wharton and Struhl, 1991).
Mutations in nos result in ectopic production of Hb protein at the posterior of the embryo, leading to the complete loss of abdominal segmentation (Lehmann and Nusslein-Volhard, 1991). This phenotype can be mimicked by a hb transcript that is unresponsive to Nos repression (Struhl, 1989). Even more dramatically, when loss of nos is combined with the absence of maternal hb expression, normal segmentation of the embryo is restored (Hulskamp et al., 1989; Irish et al., 1989; Struhl, 1989), showing that repressing hb translation is the only patterning function of nos.
Localized maternal nos mRNA is also a component of the germ plasm, a structure that contains the determinants for germline formation that will be included in the germ cells as soon as they form at the posterior of the syncytial embryo. Formation and localization of the germ plasm is crucial for proper function of Nos in patterning the embryonic posterior, as mutants in genes responsible for assembling the germ plasm, such as oskar, vasa or tudor, all result in loss of nos mRNA localization (and thus loss of translation), and produce segmentation defects similar to those observed in nos mutations (Ephrussi et al., 1991; Lehmann and Nusslein-Volhard, 1991).
Although nos mRNA is highly concentrated in the posterior germ plasm, this localized population represents only 4% of the total nos mRNA, while the remainder is found diffusely throughout the embryo (Bergsten and Gavis, 1999). However, unlocalized nos is specifically repressed by Smaug and its co-factors (Smibert et al., 1996; Dahanukar et al., 1999; Smibert et al., 1999), resulting in a Nos protein gradient whose only source is the germ plasm localized mRNA. The translational repression of unlocalized nos is crucial in Drosophila because ectopic Nos protein prevents anterior translation of the NRE, which contains hb and bcd mRNAs, resulting in severe anterior patterning defects (Simpson-Brose et al., 1994; Wharton and Struhl, 1991; Gavis and Lehmann, 1992).
nos is present throughout the metazoa, including the most basal lineages (Mochizuki et al., 2000), with broadly conserved functions in the germline (Tsuda et al., 2003). Within the insects, some degree of maternal localization of nos has been observed in all species so far examined (Curtis et al., 1995; Lall et al., 2003; Goltsev et al., 2004; Chang et al., 2006; Dearden, 2006), indicating that the posterior patterning function of nos may be conserved.
Furthermore, the 3′ UTRs of both Tribolium hb and orthodenticle-1 (otd1) mRNAs contain NREs. Both genes show evidence of translational repression from the posterior. The resulting protein gradients appear to be crucial for patterning the anterior segments of the embryo (Wolff et al., 1995; Schroder, 2003), although the true roles of these gradients in patterning the early Tribolium embryo have recently been questioned (Marques-Souza et al., 2008; Kotkamp et al., 2010). NREs have also been found in the 3′UTRs of hb and otd1 in the wasp Nasonia vitripennis (Nv), and both of these genes show evidence of translational repression from the posterior (Pultz et al., 2005; Lynch et al., 2006b).
Like Drosophila, Nasonia undergoes a mode of long germ embryogenesis where the embryonic anlage occupies the entire anterior-posterior extent of the egg fate map and all segments form at roughly the same time. The role of nos in patterning the Nasonia embryo is of particular interest because, although Nasonia embryogenesis appears morphologically very similar to that of Drosophila, it uses molecular mechanisms for embryonic patterning that are quite distinct (da Fonseca et al., 2009). For example, mRNA for Nasonia orthodenticle 1 (Nv-otd1) is localized to both the anterior and posterior poles, and is required to pattern segments in both of these regions (Lynch et al., 2006b). Maternally localized giant (Nv-gt) mRNA at the anterior plays a permissive role in allowing anterior patterning (Brent et al., 2007), while posteriorly localized caudal (Nv-cad) mRNA obviates the need for translational control to restrict its function from the anterior (Olesnicky et al., 2006).
With the availability of parental RNAi (pRNAi) (Lynch and Desplan, 2006) and a sequenced genome (Werren et al., 2010), Nasonia is an ideal model for detailed comparative analyses of gene function and regulation within insects. Taking advantage of these tools, we investigated the mode of posterior localization, embryonic patterning function and translational regulation of Nv-nos. We found that although the translational repressive function of Nos function is conserved, the localization of its mRNA and its genetic regulation appear to be unique to Nasonia.
MATERIALS AND METHODS
Identification and cloning of Nasonia genes
A fragment of Nv-nos was isolated from genomic DNA using degenerate primers designed using the CODEHOP (Rose et al., 1998). This fragment was extended by RACE PCR using the SMART RACE kit from Clontech.
dsRNA production and injection
Wasp rearing, embryo collection and fixation, and RNAi experiments were performed as described previously (Lynch and Desplan, 2006). Previous experiments using mock injections with water or gfp dsRNA showed no effects on embryonic development (Lynch et al., 2006b). Nasonia orthologs of other genes were identified by BLAST and have the following Accession Numbers: Nv-smaug, XM_001602088; Nv-vasa, XM_001603906; Nv-pumilio, XM_001607039.
Fluorescent in situ hybridization and immunohistochemistry
Antisense probes and dsRNAs were generated with the T7 Maxiscript or Megascript Kit (Ambion), respectively. Concentration of dsRNA injected for each gene was as follows: Nv-nos, 3 mg/ml; Nv-vas, 1 mg/ml; Nv-smaug, 1 mg/ml; Nv-pumilio, 0.1 mg/ml.
In situ hybridization experiments were performed using a protocol adapted from vertebrates (Brent et al., 2003). For two-color in situs, digoxigen-labelled otd1 and fluorescein-labeled nos probes were added simultaneously and were detected using anti-digoxigen::POD antibody (1:200, Roche) coupled with AlexaFluor 488 tyramide detection (Invitrogen), and anti-fluorescein::AP antibody (1:2500, Roche) coupled with HNPP Fluorescent detection (Roche), respectively. Hb protein expression was detected using an anti-Nv-Hb antibody (Pultz et al., 2005) at a dilution of 1:1000 and a goat-anti-rabbit Alexafluor-488 (Molecular probes)-conjugated secondary antibody at a concentration of 1:750. Accessory nuclei were detected using anti-nuclear pore antibody (Sigma) at a concentration of 1:2000 combined with AlexaFluor 488 TSA kit (Invitrogen). DNA staining was carried out by incubating embryos or ovaries in a 1 μg/ml solution of 4′-6-Diamidino-2-phenylindole (DAPI).
Patterns of Nv-nos and otd1 ovarian localization
Nv-nos is expressed maternally in the ovary of Nasonia and its mRNA is localized to the posterior cortex of the oocyte (Olesnicky and Desplan, 2007). Another maternal mRNA, that of Nv-otd1, appears to be localized early to the posterior pole in a similar fashion, but later also shows localization to the anterior, consistent with its function in patterning both poles of the embryo (Lynch et al., 2006b). In order to analyze the localization of Nv-nos and compare it with that of posterior Nv-otd1 in more detail, we have established a two-color fluorescent in situ hybridization protocol for Nasonia. Nv-nos mRNA is detected in the nurse cells and oocyte in early stages, where it overlaps with Nv-otd1 expression (Fig. 1A-A′). mRNA for both Nv-nos and Nv-otd1 appears initially to accumulate strongly around the oocyte nucleus (Fig. 1A-A′, topmost two follicles); then, in later follicles, Nv-nos and otd1 mRNAs move to the posterior pole of the oocyte (Fig. 1A-A′, bottom-most follicle).
Unlike Nv-nos, Nv-otd1 has a second phase of localization at the anterior pole of the oocyte. This anterior accumulation is correlated with the abrupt transfer of the remaining Nv-otd1 mRNA from the nurse cells (Fig. 1B′, lower follicle). These events are in turn correlated with the onset of nurse cell dumping in the wasp, where the nurse cells begin to shrink as their contents are transferred to the oocyte (J.A.L., personal observation). At this stage, localization of Nv-nos at the posterior pole becomes tighter and takes on a discoid shape (Fig. 1B).
Genetic regulation of Nv-nos localization in the ovary
We next asked whether genes required in Drosophila for posterior localization of nos are also required in Nasonia. We have previously shown that RNAi against a Nasonia ortholog of tudor, although causing oocyte polarity defects, does not affect localization of Nv-nos mRNA (Olesnicky and Desplan, 2007). We therefore wondered whether vasa, another crucial germ plasm component that is absolutely required for posterior nos localization in Drosophila (Lehmann and Nusslein-Volhard, 1991; Wang et al., 1994) is involved in Nv-nos localization. When Nv-vasa is knocked down by RNAi, Nv-nos localization to the posterior is not abolished (Fig. 1C), although in later follicles, the localization becomes less tight at the posterior cortex (compare Fig. 1D with 1B). Strikingly, however, Nv-otd1 mRNA, which is initially localized normally at the posterior (Fig. 1C′, upper follicle), begins to delocalize, spreads throughout the oocyte (Fig. 1C′, middle follicle), and is eventually strongly depleted at the posterior pole (Fig. 1C′, bottom follicle, and Fig. 1D′) while increasing anteriorly.
Embryonic Nv-nos expression
In the embryo, Nv-nos mRNA is associated with the oosome, a structure found in many Hymenoptera (Bilinski and Jaglarz, 1999; Grbic, 2003) that is analogous to the germ plasm in Drosophila. The Nasonia oosome exhibits a dynamic behavior in pre-blastoderm embryonic stages. In freshly laid embryos, Nv-nos and the oosome are associated with the ventral posterior cortex (Fig. 2A). During the first few nuclear cleavages (cycles 1-4), the oosome detaches from the cortex, becomes spheroid and moves anteriorly in the interior cytoplasm, and can sometimes be observed near the middle of the embryo (Fig. 2B). The observed position of the oosome along the AP axis is quite variable in this stage (the extreme case shown in Fig. 2B represents about 8% of all embryos at this stage), but it is not clear whether this is due to the inherent variability of the migration, or to different amounts of time spent by the oosome at particular positions along the axis. The oosome later returns to the posterior pole during the next few cleavages (cycles 4-6) (Fig. 2C).
The formation of pole cells in Nasonia is initiated by a large protrusion from the posterior pole (Bull, 1982) (Fig. 2D,E) that eventually buds from the embryo and rapidly begins dividing. This initial bud often contains multiple nuclei (Bull, 1982) (data not shown). The oosome, with its associated Nv-nos mRNA, is extruded into this initial bud (Fig. 2D,E). Nv-nos mRNA not associated with the oosome remains in the embryo proper, and appears to accumulate preferentially around the somatic nuclei in a posterior to anterior gradient at the early stages of blastoderm formation (Fig. 2D,E). As syncytial divisions continue, the posterior embryonic domain of Nv-nos expression begins to fade and eventually disappears, whereas expression remains strong in the pole cells (Fig. 2F). For the remainder of embryogenesis, expression is seen exclusively in pole cells and prospective germline (Fig. 2F, data not shown).
Nv-vasa is required for the assembly of the oosome and formation of pole cells
vasa orthologs are crucial components of the germ plasm across the metazoa (Raz, 2000), which made surprising the weak effects of Nv-vasa pRNAi on the localization of Nv-nos to the posterior of the oocyte, as localization of nos mRNA is dependent on the assembly of germ plasm in Drosophila (Ephrussi et al., 1991; Lehmann and Nusslein-Volhard, 1991). We therefore asked whether Nv-vasa has a role in establishing the germline in the Nasonia embryo. When Nv-vasa function is knocked down by RNAi, the oosome is not formed, and Nv-nos is diffusely localized at the posterior pole (Fig. 2G). Furthermore, pole cells do not form, and Nv-nos remains in a cap-like expression domain at the posterior after the formation of the blastoderm (Fig. 2H).
Nv-nos is required for translational repression of Nv-hb
We next asked whether Nv-nos has a similar role in axial patterning to its fly ortholog. In Nasonia, maternal hb (Fig. 3A) is expressed ubiquitously as in Drosophila, but Nv-Hb protein is also present early and is found ubiquitously until blastoderm formation (Pultz et al., 2005) (Fig. 3A′). After pole cell and blastoderm formation, Nv-hb mRNA is still present throughout the embryo (Fig. 3B), while Nv-Hb protein has been cleared from nuclei in the posterior half (Fig. 3B′), indicating that Nv-hb is being translationally repressed in the posterior at this stage (Pultz et al., 2005). In embryos where Nv-nos has been knocked down by parental RNAi, the production of Nv-Hb prior to pole cell formation is not affected (data not shown), but it is derepressed at the posterior of early syncytial blastoderm stage embryos (63%, n=71). In many cases (31%, n=71), ectopic Nv-Hb can be seen all the way to the posterior pole (Fig. 3C).
Regulation of Nv-nos function requires vasa and smaug
In Drosophila, only the small amount of nos mRNA that is localized to the germ plasm is translated and functions in embryonic patterning, whereas the majority is translationally repressed. We have shown that Nv-vasa, although not required for targeting Nv-nos to the posterior, is required to assemble it into the oosome (germ plasm) (Fig. 2G,H). Nv-Hb protein is derepressed in the posterior of embryos where Nv-vasa has been knocked down (Fig. 3D), despite the higher levels of Nv-nos mRNA observed at the posterior in these embryos (Fig. 2G,H). This indicates that Nv-vasa function is required for Nv-nos-mediated translational repression of Nv-hb.
In Drosophila, translational repression of unlocalized nos is mediated by Smaug protein. Although it has not yet been possible to directly detect Nv-Nos protein expression, the delay in the appearance of translational repression of Nv-Hb until after blastoderm formation suggests that Nv-nos function is blocked during the early embryonic stages when we can observe ubiquitous Nv-Hb protein expression. We hypothesized that Nv-smaug performs an early general repressive function on Nv-nos, and tested this idea by pRNAi against Nv-smaug. If our hypothesis were correct, we would expect to see premature translational repression of Nv-hb. This is not observed, and the distribution of Nv-Hb remains at normal levels and ubiquitous, up to the pre-pole cell stages, in Nv-smaug RNAi embryos (Fig. 3E). However, after blastoderm formation, during approximately nuclear cycles 10-12 (the same time period where translational repression of posterior Nv-hb is observed), levels of Nv-Hb protein are significantly reduced throughout the embryo (Fig. 3F) (45%, 31), suggesting a general increase in translation of Nv-Nos.
To further confirm that the reduction of Nv-Hb translation in Nv-smaug pRNAi is caused by derepression of Nv-nos translation, we knocked down Nv-nos and smaug simultaneously with pRNAi. Nv-Hb protein is expressed at the early blastoderm stages (approximately cycles 10-12) in most of these embryos (Fig. 3G), and its expression domain expands towards the posterior pole (Fig. 3H) (55%, n=44), similar to what is seen in single Nv-nos pRNAi embryos. These results suggest that the effects of Nv-smaug pRNAi on Nv-Hb protein production are carried through regulation of Nv-nos.
Maternal expression of Nv-Hb protein
One of the most striking differences between the mode of hb translational repression between Nasonia and Drosophila is the apparent long delay of the onset of posterior translational repression in the wasp, which is only evident after blastoderm formation and appears to reflect a delay in the production of Nv-Nos protein. In the fly, hb repression is detectable soon after egg activation (Tautz, 1988). One hypothesis to explain this delay is that perhaps Nv-nos is not translated in the early embryo. This would be consistent with the migratory nature of the oosome containing Nv-nos, which makes it a poor source of positional information in the early embryo. In the absence of an antibody against Nv-Nos, we could not test this hypothesis directly.
An alternative hypothesis is that the Nv-Hb protein observed in the early embryo is provided maternally, i.e. is derived from mRNA translated during oogenesis. If this hypothesis were correct, then the issue of whether Nv-Nos is produced in the early stages would be irrelevant, as Nv-Nos protein should not affect the levels of Nv-Hb protein already produced. We examined Nasonia ovaries using an antibody against Nv-Hb, and found that, indeed, Nv-Hb is found in the oocyte nucleus, as well as in the posterior nurse cells starting at very early stages of oogenesis (Fig. 4A).
Interestingly, the pattern of Nv-Hb accumulation changes as follicles mature: Nv-Hb is no longer restricted to the oocyte nucleus and Nv-Hb-bearing particles become visible initially around the nuclear periphery, and are later distributed throughout the oocyte (Fig. 4B). The behavior of these particles appears similar to that of structures called accessory nuclei that were recently described in Nasonia. These structures result from a budding of membrane from the oocyte nucleus and have been found to be important for distributing components of centrosomes around the cortex of the oocyte where they will be crucial for fertilization and the activation of embryonic development (Ferree et al., 2006). To determine whether Nv-Hb protein is contained within accessory nuclei, we stained ovaries simultaneously with an antibody against nuclear pore components, and with anti-Nv-Hb. The particles of Nv-Hb appear to be surrounded by nuclear membrane, indicating that maternal Nv-Hb is contained within accessory nuclei, which are distributed throughout the oocyte at the late stages of oogenesis (Fig. 4C,C′) and are presumably the source of Nv-Hb protein observed in the early embryo.
Nv-nos and embryonic patterning
In Drosophila, larvae resulting from embryos depleted of nos function lack all abdominal segments, which results from ectopic posterior Hb (Hülskamp et al., 1990). We sought to understand the role of Nv-nos in patterning the embryo by examining the effect of loss- or gain-of-function Nos through pRNAi against Nv-nos or its regulators (Nv-vas and smaug) on the expression of the gap genes Nv-giant (Nv-gt; Fig. 5A) and Nv-Krüppel (Nv-Kr) (Fig. 5B), and the cuticular structures of first instar larvae (Fig. 5C). Nv-gt is expressed in two gap domains, one at the anterior and one at the posterior (Fig. 5A) (Brent et al., 2007). In Nv-nos pRNAi embryos, the anterior domain of Nv-gt is unaffected, whereas the posterior domain is shifted and expands posteriorly (Fig. 5D).
Similar to Drosophila, Nv-Kr is expressed in a band towards the middle of the embryo, just posterior to the edge of the zygotic hb domain (Fig. 5B) (Brent et al., 2007). After Nv-nos pRNAi, the domain of Nv-Kr expands significantly towards the posterior, while the anterior boundary appears unaffected (Fig. 5E), which is similar to the behavior of Kr in Drosophila mutant for nos (Cinnamon et al., 2004).
In the late blastoderm stages (nuclear cycles 13-14), Nv-hb is expressed in a broad anterior domain covering the anterior 50% of the embryo, and a narrower domain at the extreme posterior (Fig. 5A) (Pultz et al., 2005). In Nv-nos pRNAi, the anterior domain of zygotic Nv-Hb protein (and mRNA; data not shown) expression does not appear to change (Fig. 5D,E). This pattern indicates that the posterior border is set independently of the extent of translational repression of the ubiquitous early maternal hb mRNA by Nos. However, the posterior domain of Nv-Hb (and Nv-hb mRNA, data not shown) is usually lost completely (Fig. 5D,E), again reflecting a shift in the posterior fate map.
The patterning effects of knocking down Nv-nos can also be seen in first instar larval cuticles. As with other genes knocked down by pRNAi in Nasonia, variable phenotypes are observed (Lynch and Desplan, 2006). In the most severe cases, up to seven of the posterior abdominal segments are lost in Nv-nos pRNAi embryos (Fig. 5F). In many cases, the remaining segments are also highly compressed, with very thin denticle belts visible on each remaining segment (Fig. 5F). Although the loss of posterior segments can be explained by the observed misregulation of posterior gap genes (Fig. 5D,E), the origin of the compression phenotype, which is not observed in flies, is not clear.
These results show that Nv-nos function is required for proper patterning of the posterior half of the Nasonia embryo, similar to the situation observed for Drosophila nos. Because we also see ectopic persistence of Nv-Hb protein in the posterior during the early blastoderm stages (Fig. 3C), it is likely that the patterning function of Nv-Nos is mediated through its ability to translationally repress Nv-hb mRNA. Thus, in Nv-nos pRNAi embryos, we propose that the ectopic Nv-Hb protein changes the expression of posterior gap genes, for example, by activating Nv-Kr expression further toward the posterior, leading to the repression of other posterior gap genes, such as Nv-gt. In principle, it would be possible to test whether the Nv-nos pRNAi phenotype is mediated by Nv-hb by eliminating both factors simultaneously. However, the fact that pRNAi for both Nv-nos and Nv-hb leads to phenotypes of variable strengths makes the interpretation of such an experiment difficult.
We next asked whether the loss of Nv-vasa leads to patterning defects similar to those of Nv-nos. The posterior domain of Nv-gt is often shifted posteriorly or lost, the posterior domain of Nv-Kr expands posteriorly, and most of the abdominal segments are missing in Nv-vasa pRNAi embryos. These posterior phenotypes are often more severe than those seen with Nv-nos mRNA, which could be due to the simultaneous loss of posterior localization of Nv-otd1 observed in Nv-vasa pRNAi ovaries (Fig. 1D). We have previously shown that posteriorly localized Nv-otd1 is an activator of posterior Nv-gt and Nv-hb (Lynch et al., 2006b) and a posterior repressor of Nv-Kr (Brent et al., 2007). In addition, other phenotypes are sometimes observed in Nv-vas pRNAi cuticles, including apparent dorsoventral defects, empty eggshells and, in some cases, head defects, indicating other roles for Nv-vasa besides allowing for Nv-nos and Nv-otd1 localization and function.
Nv-smaug pRNAi appears to lead to an increase in Nv-Nos function and thus reduced Nv-hb translation in early blastoderm embryos (Fig. 3E). Late embryos lacking Nv-smaug function show dramatic anterior fate map shifts. The zygotic Nv-Hb domain is reduced to a small region at the extreme anterior (Fig. 5J,K), but is never completely lost. The anterior Nv-gt domain also shows a reduction and shift of its expression, but is also never completely lost. Nv-Kr shows a dramatic anterior expansion, the extent of which is correlated with the reduction of the Nv-Hb domain.
The maintenance of anterior caps of zygotic Nv-gt and Nv-Hb expression indicates that a small amount of anterior patterning capacity remains after Nv-smaug pRNAi, and the resulting reduction of Nv-Hb at the early blastoderm stage. This could arise as a result of residual patterning capacity of Nv-Otd1 in the absence of synergy with Nv-Hb. Alternatively, or in parallel, localized maternal Nv-gt mRNA (Brent et al., 2007) could be responsible for preventing the expansion of the Nv-Kr domain all the way to the anterior pole.
Nv-smaug, like its Drosophila ortholog (Dahanukar et al., 1999), appears to have functions not directly related to repression of Nv-nos, as pRNAi against this gene causes defects in blastodermal mitoses (see Fig. S1 in the supplementary material), and prevents the production of larval cuticle. Smaug has recently been shown to be crucial for mediating the maternal-to-zygotic transition in Drosophila, and many of its additional targets are involved in regulating the cell cycle (Benoit et al., 2009). Our results with Nv-smaug indicate that many of the targets of Smaug regulation may have been conserved across holometabolan insect phylogeny.
Conserved function of Nv-nos and Nv-pumilio in maintaining the germline
In Drosophila, nos and its partner pumilio, also have crucial roles in maintaining the germline stem cells, a requirement for continuous production of egg chambers (Forbes and Lehman, 1998; Wang and Lin, 2004). A similar role seems to be conserved in other animal groups (Draper et al., 2007). This function is conserved Nasonia, where egg chamber production becomes strongly reduced over time (Fig. 6B) compared with wild type (Fig. 6A), and defects in the polarity of these truncated ovarioles are also observed (Fig. 6C) after Nv-nos pRNAi. Similar effects are observed with Nv-pumilio pRNAi (Fig. 6D,E), although this gene is much more sensitive to pRNAi knockdown, as no conditions were found between those that gave complete sterility and those where only wild-type embryos were produced.
We have shown that Nv-nos has a conserved role in repressing translation of hb in the posterior region of the embryo, and that the loss of Nv-nos function leads to misregulation of posterior gap genes and loss of posterior abdominal segments, similar to what is seen in Drosophila. In addition, the regulatory logic upstream of Nv-Nos function appears quite similar to that in the fly: Nv-Vas, a component of the germ plasm, promotes Nv-Nos function, while Nv-Smaug serves to restrict its production to the posterior. These results, in combination with the basal phylogenetic position of Nasonia among holometabolous insects (Savard et al., 2006; Wiegmann et al., 2009) indicate that the general strategy to restrict Nos function to the posterior was probably present in a common ancestor of all holometabolous insects.
Despite these similarities, there are a number of unique properties of the Nasonia system, including its mode of localization during oogenesis, the time at which acts during embryogenesis, and the nature of the functional mRNA population that patterns the embryo.
Evolution of Nv-nos localization mechanisms and germ plasm assembly
In Drosophila, posterior localization of nos mRNA occurs very late in oogenesis, relying on both the streaming movements of the oocyte cytoplasm (Forrest and Gavis, 2003), and the earlier assembly of the pole plasm by factors, including Osk, Vas and Tud (Wang et al., 1994; Bergsten and Gavis, 1999). By contrast, Nv-nos mRNA is localized posteriorly from a very early stage (Fig. 1A) and this early localization does not appear to depend on the assembly of the germ plasm, as knockdown of Nv-vas does not disrupt targeting of Nv-nos to the posterior pole (Fig. 1C,D). In addition, the timing of Nv-nos localization is not consistent with cytoplasmic streaming playing a crucial role, as this process only occurs at the latest stages of oogenesis in the fly. Thus, the mode of early localization of Nv-nos appears to depend on molecular factors and cellular processes that differ from those in the fly.
Nv-vas pRNAi leads to looser posterior localization of Nv-nos mRNA and posterior Nv-otd1 mRNA becomes delocalized and eventually moves to the anterior pole (Fig. 1C′,D′). As this loosened localization at late stages of oogenesis correlates with the loss of organized germ plasm (oosome) in freshly laid eggs, Nv-Vas, although not required to recruit Nv-nos and Nv-otd1 mRNA to the posterior, might be required for its tight posterior anchoring and proper integration into the germ plasm late in oogenesis. This anchoring is particularly crucial for Nv-otd1, probably because, in the absence of the anchoring function of Nv-Vas, Nv-otd1 mRNA follows its normal anterior localization at late stages of oogenesis (Fig. 1C′,D′), a process that depends on microtubule polarity in Nasonia (Olesnicky and Desplan, 2007).
The timing of Nv-nos function in the embryo is significantly delayed compared with Drosophila nos
In Drosophila, Nos-dependent translational repression of maternal hb mRNA is evident almost immediately after egg activation. By contrast, the Nv-Nos dependent translational repression of Nv-hb is not detected until well after the migration of the syncytial nuclei to the embryo surface and the formation of the pole cells (Fig. 3A,B). In addition, the effects of derepression of Nv-nos translation after Nv-smaug pRNAi are also delayed until this later time point (Fig. 3E,F).
This indicates that the delay in clearing Nv-Hb protein from the posterior of the wasp embryo is due to the presence of maternally provided Nv-Hb in the oocyte (Fig. 4), which persists in the early embryo (Fig. 3A′) and is apparently degraded only after the formation of the syncytial blastoderm. This is in contrast to Drosophila, where maternal Hb protein has not been detected. These observations raise the question: what is the significance of maternal Nv-Hb protein and the consequent delay in the formation of the Nv-Hb gradient?
The maternal provision of Hb protein is conserved in at least one other insect, the locust Schistocerca (Patel et al., 2001). Furthermore, in both Schistocerca and Nasonia, maternal Hb protein is initially highly concentrated in the oocyte nucleus, then enters the cytoplasm at later stages, which suggests a conserved mode of maternal Hb function in these organisms. Nv-Hb seems to have an important role in oogenesis as Nv-hb pRNAi causes a severe reduction in egg production (Lynch et al., 2006a). In Schistocerca, it was proposed that the maternally provided Hb protein plays a role in specifying the region of the egg that will give rise to the embryonic primordium (Patel et al., 2001). In any case, as Hb protein is not observed in Drosophila oocytes, the requirement for maternal Hb protein appears to have been lost in the fly.
From these observations, we propose that the loss of maternal Hb in Drosophila is related to one of the remarkable features of this organism: its extremely rapid early development [approximately four times faster than Nasonia (Pultz and Leaf, 2003)]. As there is no population of maternal Hb protein in need of specific degradation in Drosophila, exclusion of Hb protein from the posterior pole by translational repression can begin directly after egg laying. This could then allow for more rapid progression of syncytial nuclear divisions by obviating the danger of any ectopic Hb protein at the posterior persisting into the crucial embryonic patterning stages and disrupting abdominal patterning.
An additional interesting feature of maternal Nv-Hb is that it is taken up into the accessory nuclei. Accessory nuclei have been proposed to perform a number of functions in hymenopteran oocytes (Bilinski and Kloc, 2002; Jaglarz et al., 2005), and have recently been shown to distribute centrosomal components throughout the oocyte and embryo (Ferree et al., 2006). However, a potential role in embryonic patterning had until now not been proposed. Thus, further analysis of Nv-Hb function, particularly during oogenesis, may provide insights into additional ancestral functions of Hb in insects, and possibly into functions of the accessory nuclei among the Hymenoptera.
Two populations of Nv-nos mRNA with distinct functions and fates in the Nasonia embryo
In Drosophila, only the nos mRNA incorporated into the posterior polar granules is translated in the early embryo. In addition, as the polar granules are completely incorporated into the pole cells, only the protein produced from this mRNA population prior to pole cell formation has a role in embryonic patterning. A very different pattern is observed in Nasonia: while a large portion of Nv-nos mRNA is taken up into the pole cells, a significant amount remains in the embryo proper, forming a posterior to anterior gradient (Fig. 2D,E). As Nv-Nos dependent translational repression of Nv-hb mRNA is not evident until well after the pole cells have budded, and rather coincides with the formation of a posterior-to-anterior gradient of the non-oosomal mRNA population, we propose that it is this gradient of Nv-nos mRNA that carries out the patterning function.
It is not yet clear how the distinction arises between mRNAs localized to the oosome and those that will remain in the posterior pole cytoplasm. We propose that the oosome is a dynamic structure, and that mRNA and protein shuttle between it and the surrounding the posterior cytoplasm. In this case, the mRNA molecules that will remain in the embryo once the oosome buds would not be pre-determined. Rather, the mRNAs that remain in the embryo to perform the posterior patterning function would be those that just happened by chance to be outside of the oosome at the time of pole cell budding.
Consistent with the second hypothesis, Nv-vas seems to be required for the regulation of both populations of Nv-nos mRNA. It is required for the assembly of the oosome, and the production of pole cells. Furthermore, Nv-vas pRNAi gives an Nv-nos-like phenotype, despite the presence of high levels of Nv-nos mRNA remaining at the posterior, implying that another function for Nv-Vas is to promote translation of Nv-nos mRNA. In addition, because all mRNAs known so far from Nasonia that are localized and maintained in the oosome and pole cells also show the same perinuclear gradient in the early blastoderm stages as seen for Nv-nos (J.A.L., unpublished), the posterior pole cytoplasm appears to have a similar molecular composition to the oosome, indicating a common origin for both populations of mRNA.
Classical embryological manipulations of the Ichneumonid wasp Pimpla turionellae (Achtelig and Krause, 1971) fit well with our proposed distinction in function of the two populations of Nv-nos mRNA. Two distinct cytoplasmic regions could be discriminated at the posterior pole of this wasp embryo: the oosome, and the posterior pole plasm, which surrounds the oosome. Complete removal of the oosome prevented the formation of pole cells, but had little effect on posterior abdominal segment formation, as long as the posterior pole plasm was not disturbed. Furthermore, ligations of the egg resulted in increasing loss of abdominal segments from the posterior, but these losses could be rescued by translocation of posterior cytoplasm anterior to the restriction point. The position of the oosome itself in regard to the ligations did not matter (Achtelig and Krause, 1971). This indicates that the posterior cytoplasm in Pimpla contains a factor with patterning capabilities similar to those of Nos. Indeed, we propose that Pimpla nos mRNA, like Nv-nos, also has dual, functionally distinct, aspects of localization.
nos expression in the honeybee Apis mellifera, is somewhat different from what is seen in Nasonia. In Apis, there is no oosome, and no pole cells are formed, which is unlike Nasonia, Pimpla and other hymenopterans (Dearden, 2006). However, Apis nos mRNA is loosely localized at the posterior during oogenesis, and forms a posterior to anterior gradient of mRNA in the early embryo (Dearden, 2006). We speculate that this gradient of Apis nos is the equivalent to the gradient formed by Nv-nos after pole cell formation. If this were the case, we would expect Apis Nos to have a posterior patterning function and Apis Vasa to be required for this function to be performed.
In summary, we have shown that Nasonia nos has a function in embryonic patterning that is conserved with its Drosophila ortholog. This patterning function appears to be carried out through the regulation of hb translation in the posterior of the embryo in both species. On the other hand, the mechanisms of mRNA localization and translational regulation used to establish the gradient of Nos activity seem to diverge significantly between Drosophila and Nasonia. Thus, the conserved patterning function of Nos probably reflects a shared ancestral condition that cannot easily be displaced, whereas the divergent means of localization and function of nos probably reflect divergent evolutionary pressures on the mechanisms of germ plasm formation and maternal provisioning experienced by the separate lineages leading to Nasonia and Drosophila.
We thank Daniel Jackson and Ava Brent for performing early Nv-nos in situs, and Arzu Celik for providing the basis of the two-color FISH technique. We thank Mary Anne Pultz and David Leaf for providing the Nv-Hb antibody. We thank Ava Brent, Eugenia Olesnicky, Veit Riechmann, Siegfried Roth, Brent Wells and Zhenqing Chen for helpful comments on this article. We thank the Desplan and Roth labs for their constant support. This work was supported by grants from NIH GM-64864 to C.D. J.A.L. was supported by NIH Training Grant T32 HD007520, by a Dean's Dissertation Fellowship from NYU, by NIH Fellowship F32 GM078832 and by the SFB 680 at the University of Cologne. J.A.L. designed and carried out experiments. J.A.L. and C.D. wrote the paper. Deposited in PMC for release after 12 months.
The authors declare no competing financial interests.
Competing interests statement