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PLoS One. 2011; 6(12): e28051.
Published online Dec 5, 2011. doi:  10.1371/journal.pone.0028051
PMCID: PMC3230593

Expansion of Genes Encoding piRNA-Associated Argonaute Proteins in the Pea Aphid: Diversification of Expression Profiles in Different Plastic Morphs

Guy Smagghe, Editor

Abstract

Piwi-interacting RNAs (piRNAs) are known to regulate transposon activity in germ cells of several animal models that propagate sexually. However, the role of piRNAs during asexual reproduction remains almost unknown. Aphids that can alternate sexual and asexual reproduction cycles in response to seasonal changes of photoperiod provide a unique opportunity to study piRNAs and the piRNA pathway in both reproductive modes. Taking advantage of the recently sequenced genome of the pea aphid Acyrthosiphon pisum, we found an unusually large lineage-specific expansion of genes encoding the Piwi sub-clade of Argonaute proteins. In situ hybridisation showed differential expressions between the duplicated piwi copies: while Api-piwi2 and Api-piwi6 are “specialised” in germ cells their most closely related copy, respectively Api-piwi5 and Api-piwi3, are expressed in the somatic cells. The differential expression was also identified in duplicated ago3: Api-ago3a in germ cells and Api-ago3b in somatic cells. Moreover, analyses of expression profiles of the expanded piwi and ago3 genes by semi-quantitative RT-PCR showed that expressions varied according to the reproductive types. These specific expression patterns suggest that expanded aphid piwi and ago3 genes have distinct roles in asexual and sexual reproduction.

Introduction

Small non-coding RNAs play key roles in the regulation of gene expression and control of transposon activity in eukaryotes. These RNAs include microRNAs (miRNAs) that regulate gene expression, endogenous small interfering RNAs (endo siRNAs) that regulate gene expression and transposition in somatic cells, and Piwi-interacting RNAs (piRNAs) that silence transposons in germ cells. They are key components of the various small RNA silencing pathways and act within protein complexes including the Argonaute proteins, which represent a large protein family with different small RNA specificities [1]. All members of the Argonaute family share three structural motifs: the Piwi-Argonaute-Zwille (PAZ) domain, the Middle (MID) domain, and the PIWI domain. The function of the PAZ domain is not known but it can bind to the characteristic 2-base 3′ overhangs of small RNAs (sRNAs) digested by RNase III. The MID and PIWI domains, on the other hand, are associated with the 5′ ends of the small RNAs. Among these three domains, PIWI contains the catalytic DDH triad composed of two Aspartic Acid residues (D) and one Histidine (H) residue and responsible for the RNAseH-like endonuclease activity. It can trigger the double-stranded RNA (dsRNA) guided hydrolysis of single-stranded RNA (ssRNA) and it possesses the endonuclease “slicer” activity [2], [3].

Argonaute genes are highly conserved but their numbers vary between organisms ranging from one copy in the fission yeast Schizosaccharomyces pombe [4], to five in the fruit fly Drosophila melanogaster [5], and 27 in the nematode Caenorhabditis elegans [6]. The Argonaute protein family is subdivided into two subgroups, the Ago subfamily and the Piwi subfamily, according to their distinct functions and small RNA specificities. The Ago subfamily binds to small interfering RNAs (siRNAs) as well as miRNAs, playing a central role in siRNA and miRNA silencing pathway; the Piwi subfamily is involved in the piRNA pathway, participating in germline development, meiosis and gametogenesis [7] but their role in somatic tissues remains unclear [7], [8]. In Drosophila, the Ago subfamily is composed of the miRNA-related Argonaute-1 (Ago1) and the siRNA-related Argonaute-2 (Ago2) whilst the Piwi subfamily, which includes Aubergine (Aub), Piwi and Argonaute-3 (Ago3), is involved in the piRNA pathways [9]. Usually expression of the Piwi subfamily is restricted to the germ cells while that of the Ago subfamily is ubiquitous. However, this scenario has been questioned since Drosophila Piwi has been recently identified in both the germline and the ovarian somatic support cells [10][12], in contrast to Ago3 and Aub that are restricted to the germline [13], [14].

To date, piRNAs in Drosophila can be categorized as two distinct types: the germline piRNAs and the somatic piRNAs. The germline piRNAs, which are associated with Aub, Piwi and Ago3, are processed by the “ping-pong mechanism”. The somatic piRNAs, which are usually associated only with Piwi, are generated via the “primary piRNA model” [7], [15]. It has been demonstrated that the germline piRNAs are involved in germline specification, maintenance of germ cells, meiosis and transposon movement [7]. In addition, roles for piRNAs in early embryogenesis have been described for the Piwi subfamily and piRNAs in Drosophila [16], [17] and zebrafish [18][21]. In Drosophila, maternally inherited piRNAs are present in early embryos where they provide resistance to transposition [16]. Moreover, Aub and Ago3, both of which are components of the piRNA pathway, are found to associate with Smaug and the CCR4 deadenylase complex to regulate the decay of the maternal nanos mRNA, a posterior determinant in Drosophila [17]. In zebrafish, both ziwi and zili encode Piwi proteins that can bind piRNAs of opposite polarity and suppress the transposon activity in germ cells [18], [19]. In addition to these conserved roles in transposon defense and germline development, recent reports show that zili is involve in axis patterning via transforming growth factor (TGF)-β and fibroblast growth factor (FGF) signaling pathways [20], [21].

Here, we provide information on the piRNA pathway for the pea aphid Acyrthosiphon pisum, an emerging genomic model organism with a recently sequenced and annotated genome [22]. This phloem-sucking insect has an unusual capacity for phenotypic plasticity, displaying an ability to adapt its phenotype according to environmental conditions [23]. Aphids switch their reproductive mode in response to seasonal changes. During spring and summer, aphids reproduce by clonal viviparous parthenogenesis: a single female can give birth to approximately 80 larvae in 10 days, all of which are genetically identical to each other and to their mother. This reproductive mode allows a rapid and effective colonisation of host plants during the growing seasons. In autumn, the decrease of day length induces parthenogenetic viviparous females (named sexuparae) that produce sexual males and sexual oviparous females (named oviparae). Embryos in both virginoparae and sexuparae viviparous females complete development within the mother, and prior to larviposition germ cells within the embryos are specified and migrate to coalesce with the somatic gonads [24] – resulting in a phenomenon known as “telescoping of generations”. The sexual males and oviparous sexual females then mate and produce overwintering eggs [25]. The pea aphid thus offers a great opportunity to analyse the molecular mechanisms of sexual and asexual reproduction, including the roles of small non-coding RNAs in the alternation of reproductive modes and in the development of the different sexual and asexual morphs. We describe here an expansion of the piRNA associated Argonaute family in the pea aphid. This expansion correlates to a diversification of developmental and spatial expression profiles in different reproductive morphs. Diversified expressions of these expanded copies suggest an adaptation of the piRNA pathways to contribute to the regulation of reproductive plasticity in aphids.

Results

Piwi-type Argonautes in the pea aphid genome

Aphid homologues of genes from the Piwi subgroup of the Argonaute family (Piwi and Ago3) were identified in the A. pisum genome. All of the eight Api-Piwi and the two Api-Ago3 deduced proteins possess the functional PAZ, MID and PIWI domains (Data S1) characteristic of the Argonaute family. The main residues involved in the binding of the 5′ end of small RNAs and the DDH catalytic triad are conserved in most of the A. pisum proteins, suggesting that these proteins are functional Argonautes. However, the mutation of the DDH triad into DDR in Api-Piwi8 indicates that this protein may have lost slicer activity (Data S1).

In addition, we identified piwi homologues in the genome of another hemipteran, the blood-sucking bug Rhodnius prolixus. The genome of R. prolixus has been recently sequenced (http://genome.wustl.edu/genomes/view/rhodnius_prolixus/) and it is to date the only sequenced genome in the same order (Hemiptera) as the pea aphid. Different phylogenetic methods were tested (neighbor-joining (NJ) and maximum likelihood (ML) with different substitution models) and gave the same results (Figure 1 represents the ML tree with the BLOSUM62 model). First, a group of Ago3-like sequences was clearly identified, as it consists of a robust group with 100% bootstrap support. Two complete copies (Api-ago3a and Api-ago3b) were identified in the pea aphid genome. Interestingly, all other insect genomes surveyed contain a single ago3 gene. In fact, the pea aphid was even found to contain a third copy Api-ago3c very close to Api-ago3b, but this third copy appears to be incomplete and contains in-frame stop codons, suggesting that it is a pseudogene (this sequence was thus discarded and not included in the analysis). When compared to Api-ago3a, Api-ago3b appears to evolve more rapidly as shown by its longer branch. For piwi-like genes, the A. pisum genome was found to contain eight complete copies (Api-piwi1 to 8) -three further piwi-like genes were detected; however, like Api-ago3c these were incomplete, having in-frame stop codons and frameshifts, and were thus considered as likely pseudogenes. Piwi-like genes from A. pisum made a robust phylogenetic group, showing that the different copies arose though lineage-specific duplication. This gene family is divided in two subgroups (Api-piwi2, 3, 5, 6 versus Api-piwi1, 4, 7, 8), which appear to correspond to a relatively ancient duplication event (Figure 1). More recent duplications would have followed, defining more recent groups of paralogues (Api-piwi2 and 5, Api-piwi3 and 6, and Api-piwi 1,4,7). These closely related paralogues are particularly interesting to compare in terms of their expression profile, to determine the pace of specialisation or sub-functionalisation processes.

Figure 1
ML phylogenetic tree including protein Piwi-like and Ago3-like sequences from insects.

Gene amplifications can also be seen in representatives of the insect orders Hymenoptera (for Nasonia vitripennis and Harpegnathos saltator) and Diptera (for D. melanogaster, Anopheles gambiae, Culex quinquefasciatus, and Aedes aegypti); nevertheless, A. pisum shows the largest piwi expansion for any insect investigated to date. Within the Culicidae (mosquitoes), the copies can be divided in two subgroups that probably correspond to a relatively ancient duplication preceding speciation. In R. prolixus, the other hemipteran insect included in this analysis, genomic amplification was also detected, resulting in 3 copies (Rpr-piwi1 to 3). However this amplification was found to be independent of the A. pisum duplications. Interestingly, Rpr-piwi1 and Rpr-piwi2 are intronless and might have arisen through one retroposition event (followed by duplication), and the longer branches (in particular for Rpr-piwi2) suggest accelerated evolutionary rates for these two copies. By contrast, none of the A. pisum copies are intronless.

Expression profiles in the reproductive morphs

Aphids display the amazing ability to switch from asexual parthenogenesis to sexual reproduction in response to seasonal modification of photoperiods. Expression level of the eight Api-piwi and two Api-ago3 genes was analysed by semi-quantitative RT-PCR in the different reproductive morphs of the pea aphid (Figure S1): adult parthenogenetic virginoparae females (that produce parthenogenetic embryos), adult parthenogenetic sexuparae females (that produce sexual embryos), sexual oviparae females, and males. For each morph, RT-PCR was performed on RNA extracted from three independent batches of 10 adult whole bodies collected 48 h after adult moult. The ratio between the expression of the amplified gene and that of the Api-rpl7 reference gene was calculated to normalise the variation in experimental conditions. An average expression level was calculated for each reproductive morph based on the expression level obtained from the three independent batches (Figure 2). ANOVA tests were performed on the normalised values for each reproductive morph.

Figure 2
Expression profiles of Api-piwi and Api-ago3 genes of A. pisum in the four reproductive morphs.

All the eight Api-piwi and the two Api-ago3 genes were expressed in females but showed distinct expression profiles that varied between the morphs (Figure 2). Most Api-piwi genes and Api-ago3a genes were expressed at a lower level in males than in the three female morphs. For example, Api-piwi2, 3, 4 and 6 were almost undetectable in males. However, Api-piwi5 was strongly expressed in males, and Api-piwi7 expression level is stronger in males than in virginoparae and oviparae females. As for genes that were preferentially expressed in some morphs, the following specific expression patterns were noteworthy: (1) The expression profiles of Api-piwi2 and Api-piwi8 discriminated between parthenogenetic females (virginoparae and sexuparae) and sexual females (oviparae), with Api-piwi2 clearly overexpressed in sexual females and Api-piwi8 in asexuals; (2) Api-piwi5 was preferentially expressed in males while it was only weakly expressed in the female morphs; (3) Api-piwi6 and Api-ago3b were preferentially overexpressed in sexuparae females. Taken together, these results indicate that the expansion of the piRNA-related piwi and ago3 genes in the pea aphid appears to be linked to its reproductive plasticity.

Developmental expression of Api-piwi and Api-ago3 genes

The temporal and spatial distribution of the eight Api-piwi and two Api-ago3 transcripts were analysed by in situ hybridisation on dissected ovarioles containing adult germaria, oocytes, and developing embryos. For each gene, specific antisense riboprobe was designed to detect the distribution of the corresponding transcripts; sense riboprobes were applied in negative controls. Because of the high sequence identity between Api-piwi1, 4 and 7 (>92%), we were not able to synthesise specific probes that could discriminate individual expressions of these three genes. Nevertheless, in situ signals detected by antisense riboprobes synthesised from different template regions of Api-piwi1, 4, and 7 reflected consensus expression patterns.

In virginoparae, combinational expression of Api-piwi1, 4 and 7 was detected in germaria, oocytes, and early embryos before gastrulation (Figure 3 A–C). Preferential expression was identified in the follicle cells located between germaria and oocytes (Figure 3A, B) but newly-segregated germ cells were almost devoid of staining (Figure 3C). During mid-embryogenesis, expression patterns appeared dynamic (Figure 3D, E), but in situ signals were preferentially detected in germ cells residing in the gonads of late embryos (Figure 3F). Expression of Api-piwi8, a gene within the same phylogenetic subgroup of Api-piwi1, 4, and 7, was almost undetected during oogenesis and embryogenesis (Figure 3 G–I) except in early embryos before gastrulation (Figure 3I). During mid and late embryogenesis, Api-piwi8 was weakly expressed (Figure 3 J–L). Gene expression was not detected in ovarioles hybridised with sense riboprobes (Figure 3M, N), indicating that the in situ signals identified with antisense probes accurately represented the distributions of Api-piwi1, 4 and/or 7 and Api-piwi8. The developmental characteristics of embryos hybridised with probes of Api-piwi1, 4, 7, 8 and those stained with other gene probes are highlighted in the cartoon illustration shown in Figure 3O.

Figure 3
Comparison of developmental expressions of Api-piwi1,4,7 and Api-piwi8 in virginoparous embryos.

Coding regions with more than 25% sequence difference were used as templates to synthesise probes to distinguish expression of the closely related Api-piwi2/Api-piwi5 and Api-piwi3/Api-piwi6 genes in virginoparae embryos. During early development, transcripts of Api-piwi2 were restricted to the germarial lumen and the anterior region of the oocytes (Figure 4A). Weak expression of Api-piwi2 was visible in the newly-segregated germ cells (Figure 4B), but strong intensity of the germline-specific signals of Api-piwi2 were identified from gastrulation onward (Figure 4 C–E). In comparison with Api-piwi2, a lower level of Api-piwi5 transcript abundance was identified in germaria and oocytes (Figure 4F). Expression of Api-piwi5 was evenly distributed in embryos throughout development and no preferential expression was identified in germ cells (Figure 4 G–J). Expression of Api-piwi3 was undetected in germaria and gastrulating embryos (Figure 5 A–C), and transcripts of Api-piwi3 appeared not specifically restricted to germ cells (Figure 5D, E). In contrast, the transcripts of its closely related copy Api-piwi6 were identified specifically in germ cells from mid embryogenesis onward (Figure 5 H–J) as was also observed for Api-piwi2 (Figure 4 C–E), another germline-specific piwi gene in the pea aphid. However, preferential expression of Api-piwi6 was detected neither in the oocyte anterior (Figure 5F) nor in the newly-segregated germ cells (Figure 5G), which distinguished its expression pattern from that of Api-piwi2 (Figure 4A, B) during early embryogenesis.

Figure 4
Comparison of expressions of Api-piwi2 and Api-piwi5 in the virginoparous embryos.
Figure 5
Comparison of expressions of Api-piwi3 and Api-piwi6 in virginoparous embryos.

Localisation of these transcripts was also investigated in ovarioles containing the embryonic stages of parthenogenetic sexuparae and sexual oviparae. The expression patterns of Api-piwi genes in sexuparae and oviparae were similar to those obtained from virginoparae: Api-piwi2 and Api-piwi6 were germline-specific while their closely related copies Api-piwi5 and Api-piwi3 respectively were not (Figure 6 and Figure S2). However, Api-piwi6 showed a distinct localisation of expression between asexual and sexual embryos: it was preferentially restricted to germ cells in the virginoparae (Figure 5 H–J) and sexuparae (Figure S2K, L), but it was evenly expressed in the oviparae (Figure S2O, P).

Figure 6
Comparison of expressions of Api-piwi2 and Api-piwi5 in sexuparous and oviparous embryos.

In addition to the piwi genes, we also analysed the expression of ago3 genes (Api-ago3a and Api-ago3b) in the pea aphid. In virginoparae, Api-ago3a was expressed in the germarial lumen as well as in the cytoplasm of oocytes (Figure 7A). During embryogenesis, the transcripts of Api-ago3a were preferentially restricted to the primordial germ cells (Figure 7B), migrating germ cells (Figure 7C, D), and germ cells in the gonads (Figure 7E). Expression of Api-ago3b was not detected in the germ cells of developing virginoparae, nor was it detected in any other cells throughout development (Figure 7 F–J). In strong contrast to the virginoparae, expression of Api-ago3a in sexuparae and oviparae was not restricted to the germ cells (Figure S3 A–H), but expression of Api-ago3b remained almost undetected (Figure S3 I–P).

Figure 7
Comparison of expressions of Api-ago3a and Api-ago3b in virginoparous embryos.

In order to verify the expression of Api-piwi2, Api-piwi6 and Api-ago3a are located in the embryonic germ cells, we performed double in situ hybridisation of these three genes with the germline marker gene vasa [24]. Experimental results show that transcripts of Api-piwi2, Api-piwi6 and Api-ago3a are colocalised with vasa to the germ cells (Figures S4, S5, S6), confirming that their preferential expressions are restricted to the germline.

Discussion

Evolutionary diversification of Api-piwi genes

We describe here an expansion of the genes encoding Piwi sub-clade of Argonaute proteins in the pea aphid A. pisum. Annotation of the A. pisum genome identified eight copies of the piwi genes and two copies of the Api-ago3. Conservation of the key residues of PAZ, MID and PIWI domains suggest that Api-Piwi1-7 proteins are functional Argonautes while Api-Piwi8 presented mutation within the DDH catalytic triad and may have lost slicer activity. Since Api-piwi8 results from an ancient duplication, this mutation appears to be evolutionary conserved. From the comparison of complete genomes in different insect orders, the duplication of ago3 in aphids appears to be unique (Figure 1). By contrast, duplications of piwi have been reported in several organisms: at least six piwi genes have been identified in the mosquitoes Culex pipiens and A. aegypti respectively [26]; six piwi genes have been annotated in the genome of the crustacean Daphnia pulex [27]; 12 and 15 piwi genes have been characterized in the ciliate protozoa Tetrahymena thermophila and Paramecium tetraurelia respectively [28][30]. The biological significance of the expansions in mosquitoes and Daphnia has not yet been examined in detail. However, in the protozoan T. thermophila and P. tetraurelia, the expansion of piwi genes has been associated with neo- or sub-functionalisation of these proteins [28][30], as has been observed previously for Argonaute proteins in plants [31] and in the nematode C. elegans [6]. In most cases, including the pea aphid, piwi expansions appear to be independent events and lineage-specific, as shown by the monophyletic grouping of copies from each species. Indeed, the pea aphid expansion is not only independent of the expansion in mosquitoes, but also of the expansion we identified in its closer relative R. prolixus. Thus the expansions of the Piwi family in different animal groups appear to represent the repeated unfolding of a similar scenario.

Diversification of developmental expression profiles in germline and in soma

In situ hybridisation on whole mounted ovarioles showed differential expression among some pea aphid Api-piwi and Api-ago3 genes (piwi expression profiles are summarised in Figure 8). In animals, expression of Piwi and Ago3 is mainly restricted to the germline [19], [32], [33] and to the gonadal somatic cells in Drosophila [13]. In pea aphid virginoparae, we observed differential expression of the closely-related piwi genes Api-piwi2/Api-piwi5 (Figure 4) and Api-piwi3/Api-piwi6 (Figure 5), suggesting that duplication of piwi genes in this instance has led to the differentiation of piwi function between germline (Api-piwi2, Api-piwi6) and soma (Api-piwi3, Api-piwi5). Similar diversification of expression patterns of piwi paralogs has been reported in other organisms such as the ciliate protozoan T. thermophila or the crustacean water flea D. pulex [27], [29].

Figure 8
Summary of piwi expressions in the pea aphid.

In Drosophila, the piwi gene family is represented by two copies, named respectively piwi and aub, also resulting from a duplication event. Like expanded piwi genes in aphids, piwi and aub show somatic/germline diversification of expression profiles in Drosophila: Piwi is expressed in somatic follicle cells as well as in germline while Aub and Ago3 are restricted to the germline [10], [13], [14]. These two spatial expression patterns are associated with two distinct functions. In ovarian somatic cells Piwi alone is involved in primary linear piRNA biogenesis, but in germline Piwi, Aub and Ago3 are involved in a more complex piRNA biogenesis pathway - the “ping-pong mechanism” that generates repeat-associated piRNAs in germ cells [7], [13], [15]. Taken together, it suggests that a common scenario occurs repetitively following duplication of piwi genes in distantly related organisms including the aphids: some copies are “specialised” in germ cells whereas others are expressed in the somatic cells.

Somatic expression was reported for Api-piwi3 and Api-piwi5 in the embryonic stages of the three female morphs, Api-piwi6 in embryonic oviparae, and Api-ago3a in embryonic oviparae and sexuparae. Ubiquitous expression of piwi, aub and ago3 has also been observed during Drosophila embryogenesis [5]. However this somatic expression in Drosophila was restricted to early embryos, in which a significant proportion of mRNA is maternally inherited [10]. In aphid embryos, somatic signals were observed for Api-piwi5, Api-ago3a and Api-piwi6 in early and late embryo stages and specifically in late embryo stages for Api-piwi6. While signal observed until embryo blastula stage 6 can be imputed to maternal mRNAs [34], clear signal observed for these genes in late somatic stage (stage 12–18) reflects de novo somatic transcription (Table S1). In adult Drosophila, until recently, somatic expression of Piwi and piRNAs has only been reported in a specific type of somatic tissues: ovarian follicle cells, where they are responsible for the maintenance of germline stem cells (GSCs) in ovaries [15], [35]. Restriction of the Api-piwi1,4,7 transcripts to the follicle cells in the posterior margin of the germarium (Figure 3A, B), from which the GSCs are derived [36], hence appears to suggest a potential role in the development of GSCs, like that in Drosophila. Apart from the ovarian follicle cells, recent findings show that piRNA-like molecules have now been identified in head and imaginal discs of Drosophila [8], but the biogenesis of the “non-ovarian follicle” somatic piRNAs still needs to be understood. Similarly, it remains to be solved whether the somatic ubiquitous zygotic expression of Api-piwi3, Api-piwi5, Api-piwi6 in oviparae and Api-ago3a in sexuparae and oviparae reflects somatic expression in adults, as has been reported for several piwi genes in D. pulex [27]. Semi-quantitative RT-PCR realised on adult virginoparae heads did not evidence clear somatic expression of Api-piwi3 and Api-piwi5 (S. Jaubert-Possamai unpublished results). The role of such somatic piwi genes therefore requires further investigation.

Diversification of expression profiles is correlated with the reproductive mode

In response to seasonal modifications of photoperiod, aphids can switch from asexual parthenogenesis to sexual reproduction. Analyses of expression profiles of the expanded Api-piwi and Api-ago3 genes by semi-quantitative RT-PCR showed that these genes are differentially expressed in the various adult reproductive morphs of the pea aphid (piwi expression profiles are summarised in Figure 8). Every Api-piwi and Api-ago3 gene exhibits a distinct expression profile, including the closely-related genes Api-piwi2/Api-piwi5, Api-piwi3/Api-piwi6, and Api-ago3a/Api-ago3b. These specific expression patterns of mRNAs suggest that expanded aphid Piwi proteins may have distinct roles in asexual or sexual reproductive phases.

In situ hybridisation also provided evidence for differentiation among reproductive morphs in the spatial expression of duplicated Api-piwi and Api-ago3 genes. Interestingly, among all studied argonaute genes in the pea aphid, Api-piwi2 is the only one that remains germline-specific in three different reproduction morphs (Figures 4 and and6).6). The germline-specific expression of other argonaute genes, however, varies between morphs. For example, the association of two genes, Api-piwi6 and Api-ago3a, with germline tissue varied substantially among female morphs. Api-piwi6 expression was germline specific in virginoparae and sexuparae (Figure 5 H–J and Figure S2K, L) but was not specific to germ cells in oviparae (Figure S2O, P). Similarly, Api-ago3a was specifically expressed in germ cells in virginoparae (Figure 7 B–E), but showed no germline-specific localisation in either sexuparae or oviparae (Figure S3 A–H). This difference in expression location between the morphs strongly suggests that expressions of Api-piwi6 and Api-ago3a are regulated by the change of photoperiods.

We did observe some inconsistencies between the results of our RT-PCR and in situ hybridisation analyses for Api-ago3b and Api-piwi8. These differences in the detection of expression between the two methods could have been caused by the increased sensitivity of the RT-PCR approach, or because the target tissue for RT-PCR was the whole adult body whilst the in situ hybridisation targeted only the dissected ovarioles. This latter explanation would require that these genes be expressed only in non-ovarian tissues.

To our knowledge, this is the first report implicating Piwi proteins in asexual reproduction in a metazoan. However, our results do parallel previous findings obtained for the protozoans T. thermophila and P. tetraurelia, which resemble aphids in that they alternate between sexual and asexual phases during their life cycle. Like in aphids, protozoan piwi genes showed a diversification of expression pattern during sexual and asexual phases of their life cycle. Despite having highly similar structures, these protozoan Piwi proteins show different specificity to distinct classes of small non-coding RNAs [28][30]. Functional studies of piwi genes must now be conducted in the pea aphid in order to understand the piRNA binding specificities.

Our results indicate that the expansion of genes encoding the Piwi sub-clade of Argonaute proteins in the pea aphid is associated with a diversification of expression profiles that correlates with the different reproductive modes displayed by this insect. Expanded Piwi and Ago3 of the pea aphid belong to the large Argonaute protein family, which also includes the microRNA specific Ago1 and the siRNA specific Ago2 in insects. We previously described a duplication of ago1 but no duplication of ago2 in the pea aphid genome [37]. Altogether our results show a global expansion of the Argonaute protein family with the exception of Ago2 in the pea aphid. Beside Argonaute proteins, other components of the microRNA and piRNA pathways are expanded in the pea aphid, such as Dicer-1 (2 copies) and Pasha (4 copies) for the microRNA pathway [37] and Pimet/Hen1 (2 copies) for the piRNA pathway (S. Jaubert-Possamai personal communication). This duplication appears to be specific to the microRNA and piRNA pathways since no duplication of the genes of the siRNA pathway has been identified, suggesting an expansion of a part of several small RNA pathways in the pea aphid genome. The significance of this expansion remains to be understood. A crucial role for Piwi together with Dicer-1 and a protein named Fragile X Mental Retardation Protein (FMRP) has been proposed in germline fate determination during Drosophila embryogenesis [38]. In the pea aphid we have identified five copies of fmr, suggesting that duplication of Api-fmr, together with the duplicated Api-piwi and Api-dicer-1, may be critical to the specification of germ cells (S. Jaubert-Possamai unpublished data). Further functional analyses will be necessary to clarify the role of these expanded small RNAs machineries in the pea aphid A. pisum.

Materials and Methods

Pea aphid clones and induction of sexual reproduction

The LSR1-A1-G1 clone of the pea aphid A. pisum [22] was reared on broad bean (Vicia fabae) at 18°C. Parthenogenetic reproduction was maintained at 16 hours (h) of light and aphids were reared at low density (1 to 5 individuals per plant). Production of sexual morphs was obtained by rearing aphids at 12 h light and 18°C for two generations [39]. Total RNA was extracted by using the RNeasy kit (QIAGEN) from each of the A. pisum reproductive morphs: adult parthenogenetic females reared under long day photoperiod (called virginoparae) and producing parthenogenetic clones, adult parthenogenetic females reared under short day photoperiod (called sexuparae) and producing males and sexual oviparous female clones, adult sexual female oviparae, and adult sexual males. Parthenogenetic and viviparous pea aphid morphs used for in situ hybridisation were from an obligate parthenogenetic clone reared in the laboratory at the National Taiwan University [40].

Annotation of genes encoding Piwi-type Argonaute Proteins

Homologues of Piwi/Aub and Ago3 were identified by mining the genomic data in the A. pisum LSR1 genome (Acyr 1.2 version of the assembly) at AphidBase (www.aphidbase.com). This was performed using the corresponding D. melanogaster sequences as bait and A. pisum predicted proteins (program BlastP) and genomic scaffolds (program TBlastn) as targets. The hits were included in a preliminary phylogenetic analysis by using Neighbour-Joining (NJ), which allowed an unambiguous identification of homologues. Gene models from prediction programs were checked, resulting in only a few manual curations. All annotated genes are listed in Table S3. Amino acid sequences were then deduced from the curated pea aphid gene models for all the candidate genes. The domain distribution of the deduced A. pisum proteins was predicted by using Pfam [41] and Interproscan [42] software. Because R. prolixus (another hemipteran) is to date the closest evolutionary parent with a complete genome sequence (which is not yet fully annotated), we performed a similar research of piwi-like and ago3-like genes in the scaffolds of that species (Blast at NCBI).

Phylogenetic analysis

Homologues of Piwi/Aub and Ago3 were also identified in other insects using Blast against GenBank; hit sequences were collected to form a file in which we included all similar A. pisum genes. Given the relatively large evolutionary distance between the different insect species represented in the data set (and even among copies of A. pisum), analysing protein rather than DNA sequences seemed to be most appropriate. The sequences were aligned using T-Coffee [43]. A few of the sequences (from some Diptera species), being incomplete, were discarded. The N-terminal end of the protein sequence, which is poorly conserved among the different piwi-like genes and ago3-like genes, was removed. An NJ analysis was performed using MEGA, with the pairwise deletion option and Poisson correction [44]. A Maximum Likelihood (ML) phylogeny analysis was also performed with PHYML on the same alignment, testing different models of substitution. Bootstrap support for the nodes was calculated using 100 replicates.

Gene expression profiling in different reproductive morphs

The expression level of the eight Api-piwi and the two Api-ago3 genes in the various reproductive morphs was compared by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). Expression of Api-piwi and Api-ago3 genes in the different morphs was normalised against the gene encoding ribosomal protein 7 (Ap-rpl7) as a reference gene [45]. For each morph, three batches of 10 individuals from three independent biological experiments were frozen in liquid nitrogen 48 h after adult moult and used for RNA extraction. The concentration and quality of the extracted RNA was estimated with a NanoDrop (Thermo Scientific). First strand cDNAs were produced from 500 ng total RNA using the SuperscriptIII reverse transcriptase (Invitrogen) and random nonamers (Promega) following the supplier's instructions. DNA contamination was removed by treating RNA extraction products with RNase-free DNAse (Promega). As a negative control, RT-PCR experiments were realised on total RNA without SuperscriptIII reverse transcriptase.

The standard PCR program comprised an initial step of 4 minutes (min) at 94°C, then multiple cycles composed of 2 min at 94°C, 30 seconds (sec) at the annealing temperature and 1 min 30 sec at 72°C, and a final elongation step of 5 min at 72°C. For quantification of the amplification products, the appropriate number of cycles corresponding to the exponential range was defined for every gene. The sequences of PCR primers, the corresponding annealing temperatures and the appropriate number of cycles used to measure expression level of the different Api-piwi and Api-ago3 genes are listed in Table S3. Images of the RT-PCR Sybr-safe (Invitrogen) stained agarose gels were acquired with a G:BOX Imager (Syngene) and quantification of the bands was performed using Image-J (http://rsbweb.nih.gov/ij/). For each batch of each reproductive morph, the ratio between the band intensity of the amplified gene and Api-rpl7 reference gene was calculated to normalise for initial variations in sample concentration. The data were subjected to an arc-sine transformation for analysis of variance (ANOVA) in order to test for the significance of variation of expression level (p-value >0.05).

Cloning, amplification, and synthesising riboprobes of Api-piwi and Api-ago3 genes

We cloned, then amplified, partial sequences of Api-piwi and Api-ago3 genes through RT-PCR. PCR cloning of template sequences for synthesising riboprobes were performed for 40 thermal cycles. Complementary DNA (cDNA) templates were reversely transcribed using HiScript I reverse transcriptase (Bionovas) and poly dT18 primers from total RNA extracted from dissected ovaries of the adult virginoparae. Gene-specific primers were designed based upon sequences of putative homologues of Api-piwi and Api-ago3 in AphidBase. PCR fragments containing verified sequences were then cloned into the pGEM-T Easy Vector (Promega) for subsequent in vitro transcription. Sense and antisense digoxigenin (DIG)-labelled riboprobes for in situ hybridisation were synthesised from linearised plasmids containing target sequences using the DIG RNA Labeling Kit (SP6/T7) (Roche). Probes of vasa subjected to double in situ hybridisation were labelled with fluorescein (FL) using the DIG/FL labeling mix (Roche). Primer sequences, lengths of PCR amplicons, and annealing temperatures of PCR are summarized in Table S4.

Whole-mount in situ hybridisation and microscopy

Dissected ovarioles, which contain developing oocytes and embryos, were fixed in 3.8% formaldehyde in 1× phosphate-buffered saline (PBS) at 4°C overnight. The steps for single and double in situ hybridisation followed the protocol described in [40]. The working concentration of each probe, including sense and antisense strands, ranged from 1.5 to 2.0 ng/µl. We optimised the hybridisation temperatures according to the best signal intensity that could be discriminated between experimental (antisense probes) and control (sense probes) groups. The hybridisation temperature of each probe varied (Table S3). In single in situ hybridisation, we applied Nitroblue tetrazolium (NBT)/5-bromo-4-chloro-3-indolyl phosphate (BCIP) (Roche) as the substrates for developing Api-piwi and Api-ago3 signals. For double in situ hybridisation, we used 4-benzoylamino-2,5 diethoxybenzenediazonium chloride hemi[zinc chloride] (Fast Blue BB) salt/naphthol-AS-MXphosphate (NAMP) (Sigma) to develop signals of Api-piwi2 but remained NBT/BCIP as the substrates for Api-piwi6 and Api-ago3a. After signals of piwi and ago3a were developed, vasa signals were then generated using the Fast Red substrate (Roche). In order to increase the signal specificity of Api-piwi6 and Api-ago3a, we briefly rinsed the embryos in methanol after signal development with NBT/BCIP. After in situ hybridisation, samples were mounted in 70% of glycerol/1× PBS and photographed with a Leica DMR connected to Canon EOS 5D MarkII digital camera (Canon). Morphological characteristics of developmental stages were described according to Miura et al. [46] and locations of germ cells were described according to Chang et al. [24]

Supporting Information

Figure S1

Expression levels of the Api-piwi and Api-ago3 genes in the four reproductive morphs. Expressions of the eight Api-piwi and the two Api-ago3 genes were quantified in the four reproductive morphs of the pea aphid by semi-quantitative RT-PCR. The figure shows agarose gels after electrophoresis of RT-PCR products. Quantification of gene expression was analysed by semi-quantitative PCR with copy specific primers in the four reproductive morphs of A. pisum: parthenogenetic virginoparae, parthenogenetic sexuparae, oviparae sexual females and sexual males. The expression of the ribosomal Api-rpl7 gene was analysed as a reference gene [45]. For each morph, RT-PCR were realised on total RNA extracted from batches (R1, R2, R3) of 10 pooled adults resulting from three independent biological replicates. As a negative control, RT-PCR experiments were realised on each RNA sample without SuperscriptIII reverse transcriptase (-). Primers used to investigate the expression of Api-piwi4 co-amplified Api-piwi1, so only the amplification product corresponding to Api-piwi4 (*) was considered for gene expression analysis. Abbreviations, R: replicates.

(TIF)

Figure S2

Comparison of expressions of Api-piwi3 and Api-piwi6 in sexuparous and oviparous embryos. Dissected ovarioles were hybridised with DIG-labelled antisense riboprobes of Api-piwi3 (A–H) and Api-piwi6 (I–P), respectively. For orientations and morphological characteristics of embryos refer to Figure 3O. Early development: A, B, E, F, I, J, M, N; Mid development: C, G, K, O; Late development: D, H, L, P. Germ cells stained with antisense Api-piwi3 and Api-piwi6 riboprobes are highlighted with arrows (with preferential expression) and double arrows (without preferential expression). (A, E, I, M) Germaria and segregated oocytes (stage 1). Transcripts of Api-piwi3 were weakly expressed in the germaria and the oocyte of sexuparae (arrowheads) whilst transcripts of Api-piwi3 were not detected in oviparae. Api-piwi6 expression was distributed in germaria and the oocytes of sexuparae and oviparae (arrowheads). (B, F, J, N) Embryos with newly-segregated germ cells (stage 6). Transcripts of Api-piwi3 were almost not identified in the embryos of sexuparae and oviparae. Api-piwi6 expression was not detected in the embryos of sexuparae but was evenly distributed in those of the oviparae. (C, G, K, O) Extension of the germband and limb bud formation (stage 12–13). Transcripts of Api-piwi3 were not identified in the embryos of sexuparae and oviparae but expression of Api-piwi6 in sexuparous embryos was preferentially identified in germ cells. Api-piwi6 transcripts were evenly distributed in the embryos of oviparae. In panel (G), germ cells are not presented in the shown focal plane. (D, H, L, P) Germband retraction (stage 17) and completion of germband retraction (stage 18). Transcripts of Api-piwi3 were almost not identified in the embryos of sexuparae and oviparae. In sexuparae specific expression of Api-piwi6 was identified in germ cells located in the dorsal region of the embryo. Universal expression of Api-piwi6 was detected in the embryos of oviparae. Abbreviations: Bc, endosymbiotic bacteria; Dev, development; Hd, head; St, stage. Scale bar, 20 µm.

(TIF)

Figure S3

Comparison of expressions of Api-ago3a and Api-ago3b in sexuparous and oviparous embryos. Dissected ovarioles were hybridised with DIG-labelled antisense riboprobes of Api-ago3a (A–H) and Api-ago3b (I–P), respectively. For orientation and morphological characteristics of embryos refer to Figure 3O. Early development: A, B, E, F, I, J, M, N; Mid development: C, G, K, O; Late development: D, H, L, P. Germ cells stained with antisense Api-ago3a and Api-ago3b riboprobes are highlighted with arrows (with preferential expression) and double arrows (without preferential expression). (A, E, I, M) Germaria and segregated oocytes (stage 1). Transcripts of Api-ago3a were identified in the germaria and the oocytes of sexuparae and oviparae embryos (arrowheads) but transcripts of Api-ago3b were not detected. (B, F, J, N) Embryos with newly-segregated germ cells (stage 6). Expression of Api-ago3a was evenly distributed in the embryos of sexuparae and oviparae. Transcripts of Api-ago3b were not identified in the embryo of both sexuparae and oviparae. (C, G, K, O) Extension of the germband and limb bud formation (stage 12–14). Transcripts of Api-ago3a were evenly distributed in the embryos of sexuparae and oviparae. By contrast, transcripts of Api-ago3b were not almost identified in both morphs. (D, H, L, P) Germband retraction (stage 17) and completion of germband retraction (stage 18). Expression of Api-ago3a was evenly distributed in embryos of sexupare and oviparae, but transcripts of Api-ago3b were almost undetected in the embryos of both morphs. Abbreviations: Bc, endosymbiotic bacteria; Dev, development; Hd, head; St, stage. Scale bar, 20 µm.

(TIF)

Figure S4

Double in situ hybridisation of Api-piwi2 and vasa in virginoparous embryos. (A–D) Ovarioles hybridised with DIG-labelled antisense riboprobe of Api-piwi2; (E–H) Ovarioles hybridised with FL-labelled antisense riboprobes of vasa; (I–L) Ovarioles hybridised with both DIG-labelled Api-piwi2 and FL-labelled vasa riboprobes. Color keys indicating single and double in situ signals are highlighted below the figures. Orientation and developmental stages of embryos refer to Figure 3O. Single in situ hybridisations show that both Api-piwi2 and vasa were expressed in germaria and oocytes during early development (panels A and E). From mid development onward, Api-piwi2 (B–D) and vasa (F–H) marked germ cells specifically (arrows). Co-localised signals of Api-piwi2 and vasa (I–L) were identified in germaria, oocytes and embryonic germ cells (arrows). Abbreviations: Bc, endosymbiotic bacteria; Dev, development; Hd, head; St, stage. Scale bar, 20 µm.

(TIF)

Figure S5

Double in situ hybridisation of Api-piwi6 and vasa in virginoparous embryos. (A–D) Ovarioles hybridised with DIG-labelled antisense riboprobe of Api-piwi6; (E–H) Ovarioles hybridised with FL-labelled antisense riboprobes of vasa; (I–L) Ovarioles hybridised with both DIG-labelled Api-piwi6 and FL-labelled vasa riboprobes. Color keys indicating single and double in situ signals are highlighted below the figures. Orientation and developmental stages of embryos refer to Figure 3O. Locations of germ cells are indicated with arrows. Single in situ hybridisations show that both Api-piwi6 and vasa were expressed in germaria and oocytes during early development (panels A and E). From mid development onward, transcripts of Api-piwi6 were preferentially expressed in germ cells but universal expression of Api-piwi6 could be identified in somatic cells (B–D). Expression of vasa remained specific in germ cells (F–H). Panels (I) to (L) show co-localised signals of Api-piwi6 and vasa in germaria, oocytes and embryonic germ cells. Abbreviations: Bc, endosymbiotic bacteria; Dev, development; Hd, head; St, stage. Scale bar, 20 µm.

(TIF)

Figure S6

Double in situ hybridisation of Api-ago3a and vasa in virginoparous embryos. (A–D) Ovarioles hybridised with DIG-labelled antisense riboprobe of Api-ago3a; (E–H) Ovarioles hybridised with FL-labelled antisense riboprobes of vasa; (I–L) Ovarioles hybridised with both DIG-labelled Api-ago3a and FL-labelled vasa riboprobes. Color keys indicating single and double in situ signals are highlighted below the figures. Orientation and developmental stages of embryos refer to Figure 3O. Locations of germ cells are indicated with arrows. Single in situ hybridisations show that both Api-ago3a and vasa were expressed in germaria and oocytes during early development (panels A and E). From mid development onward, expression of Api-ago3a was specifically identified in germ cells (B–D); however, in late embryos background staining was also detected (D). As mentioned in Figures S4 and S5, specific expression of vasa was used to mark germ cells. Panels (I) to (L) show co-localised signals of Api-ago3a and vasa in germaria, oocytes and embryonic germ cells. Abbreviations: Bc, endosymbiotic bacteria; Dev, development; Hd, head; St, stage. Scale bar, 20 µm.

(TIF)

Table S1

Summary of in situ hybridisation of Api-piwi and Api-ago3 genes during embryogenesis.

(DOC)

Table S3

Primers used for semi quantitative RT-PCR.

(DOC)

Table S4

Primer pairs used for the synthesis of sense and antisense riboprobes.

(DOC)

Data S1

Alignment of deduced protein sequences of the eight Api-piwi genes and the two Api-ago3 genes. Deduced protein sequence of the eight Api-Piwi and the two Api-Ago3 proteins were aligned with their D. melanogaster orthologues Dme-Piwi, Dme-Aub and Dme-Ago3 by using ClustalW2 (http://www.ebi.ac.uk/Tools/msa/clustalw2/). The PAZ and PIWI domains [47] of each protein were deduced from protein sequence by Interproscan [42]. Amino acids (AA) that belong to the PAZ domain are underlined in yellow, AA that belong to the MID domain are underlined in blue and those that belong to the PIWI domain are underlined in gray. Key residues predicted in the PAZ domain to be involved in the binding of sRNA are indicated in blue, those in the MID domain predicted to be involved in anchoring the 5′ phosphate are indicated in pink. The DDH triad of the PIWI domain involved in slicer activity is indicated in red.

(DOC)

Acknowledgments

The authors would like to acknowledge Sylvie Hudaverdian (UMR BiO3P, Rennes) for her technical assistance. CC and HLL would like to thank Chuck Cook for valuable reading of the manuscript, Greg Davis and Shuji Shigenobu for initiating the collaboration of this project.

Footnotes

Competing Interests: The authors have declared that no competing interests exist.

Funding: This work was supported by the National Science Council (97-2313-B-002-035-MY3, web site: http://web1.nsc.gov.tw/mp.aspx?mp=7) and the Bureau of Animal and Plant Health Inspection and Quarantine (BAPHIQ) of the Agricultural Council (97-1.1.1-B1(6); 98-1.1.1-B1(2); 100 -9.2.1- B1(6)), web site: http://www.baphiq.gov.tw/mp.asp?mp=2). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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