Sexual and asexual oogenesis require the expression of unique and shared sets of genes in the insect Acyrthosiphon pisum

Reproduction-associated pathways in females of gibel carp (Carassius gibelio) shed light on the molecular mechanisms of the coexistence of asexual and sexual reproduction

Gibel carp (Carassius gibelio) is a cyprinid fish that originated in eastern Eurasia and is considered as invasive in European freshwater ecosystems. The populations of gibel carp in Europe are mostly composed of asexually reproducing triploid females (i.e., reproducing by gynogenesis) and sexually reproducing diploid females and males. Although some cases of coexisting sexual and asexual reproductive forms are known in vertebrates, the molecular mechanisms maintaining such coexistence are still in question. Both reproduction modes are supposed to exhibit evolutionary and ecological advantages and disadvantages. To better understand the coexistence of these two reproduction strategies, we performed transcriptome profile analysis of gonad tissues (ovaries) and studied the differentially expressed reproduction-associated genes in sexual and asexual females. We used high-throughput RNA sequencing to generate transcriptomic profiles of gonadal tissues of triploid asexual females and males, diploid sexual males and females of gibel carp, as well as diploid individuals from two closely-related species, C. auratus and Cyprinus carpio. Using SNP clustering, we showed the close similarity of C. gibelio and C. auratus with a basal position of C. carpio to both Carassius species. Using transcriptome profile analyses, we showed that many genes and pathways are involved in both gynogenetic and sexual reproduction in C. gibelio; however, we also found that 1500 genes, including 100 genes involved in cell cycle control, meiosis, oogenesis, embryogenesis, fertilization, steroid hormone signaling, and biosynthesis were differently expressed in the ovaries of asexual and sexual females. We suggest that the overall downregulation of reproduction-associated pathways in asexual females, and their maintenance in sexual ones, allows the populations of C. gibelio to combine the evolutionary and ecological advantages of the two reproductive strategies. However, we showed that many sexual-reproduction-related genes are maintained and expressed in asexual females, suggesting that gynogenetic gibel carp retains the genetic toolkits for meiosis and sexual reproduction. These findings shed new light on the evolution of this asexual and sexual complex.

Transcriptomic basis of sex loss in the pea aphid

BACKGROUND

Transitions from sexual to asexual reproduction are common in eukaryotes, but the underlying mechanisms remain poorly known. The pea aphid-Acyrthosiphon pisum-exhibits reproductive polymorphism, with cyclical parthenogenetic and obligate parthenogenetic lineages, offering an opportunity to decipher the genetic basis of sex loss. Previous work on this species identified a single 840 kb region controlling reproductive polymorphism and carrying 32 genes. With the aim of identifying the gene(s) responsible for sex loss and the resulting consequences on the genetic programs controlling sexual or asexual embryogenesis, we compared the transcriptomic response to photoperiod shortening-the main sex-inducing cue-of a sexual and an obligate asexual lineage of the pea aphid, focusing on heads (where the photoperiodic cue is detected) and embryos (the final target of the cue).

RESULTS

Our analyses revealed that four genes (one expressed in the head, and three in the embryos) of the region responded differently to photoperiod in the two lineages. We also found that the downstream genetic programs expressed during embryonic development of a future sexual female encompass ∼1600 genes, among which miRNAs, piRNAs and histone modification pathways are overrepresented. These genes mainly co-localize in two genomic regions enriched in transposable elements (TEs).

CONCLUSIONS

Our results suggest that the causal polymorphism(s) in the 840 kb region somehow impair downstream epigenetic and post-transcriptional regulations in obligate asexual lineages, thereby sustaining asexual reproduction.

Spatio-temporal patterns of ovarian development and VgR gene silencing reduced fecundity in parthenogenetic Artemia

The halophilic zooplankton brine shrimp Artemia has been used as an experimental animal in multidisciplinary studies. However, the reproductive patterns and its regulatory mechanisms in Artemia remain unclear. In this study, the ovarian development process of parthenogenetic Artemia (A. parthenogenetica) was divided into five stages, and oogenesis or egg formation was identified in six phases. The oogenesis mode was assumed to be polytrophic. We also traced the dynamic translocation of candidate germline stem cells (cGSCs) using EdU labelling and elucidated several key cytological events in oogenesis through haematoxylin and eosin staining and fluorescence imaging. Distinguished from the ovary structure of insects and crustaceans, Artemia germarium originated from ovariole buds and are located at the base of the ovarioles. RNA-seq based on five stages of ovarian development identified 2657 upregulated genes related to reproduction by pair-to-pair comparison. Gbb, Dpp, piwi, vasa, nanos, VgA and VgR genes associated with cGSCs recognition and reproductive development were screened and verified using qPCR. Silencing of the VgR gene in A. parthenogenetica (Ap-VgR) at ovarian development Stage II led to a low level of gene expression (less than 10%) within 5 days, which resulted in variations in oogenesis-related gene expression and significantly inhibited vitellogenesis, impeded oocyte maturation, and eventually decreased the number of offspring. In conclusion, we have illustrated the patterns of ovarian development, outlined the key spatio-temporal features of oogenesis and identified the negative impacts of VgR gene knockdown on oogenesis using A. parthenogenetica as an experimental animal. The findings of this study also lay a foundation for the further study of reproductive biology of invertebrates.

Contrasting Evolutionary Patterns Between Sexual and Asexual Lineages in a Genomic Region Linked to Reproductive Mode Variation in the pea aphid

Although asexual lineages evolved from sexual lineages in many different taxa, the genetics of sex loss remains poorly understood. We addressed this issue in the pea aphid Acyrthosiphon pisum, whose natural populations encompass lineages performing cyclical parthenogenesis (CP) and producing one sexual generation per year, as well as obligate parthenogenetic (OP) lineages that can no longer produce sexual females but can still produce males. An SNP-based, whole-genome scan of CP and OP populations sequenced in pools (103 individuals from 6 populations) revealed that an X-linked region is associated with the variation in reproductive mode. This 840-kb region is highly divergent between CP and OP populations (FST = 34.9%), with >2,000 SNPs or short Indels showing a high degree of association with the phenotypic trait. In OP populations specifically, this region also shows reduced diversity and Tajima's D, consistent with the OP phenotype being a derived trait in aphids. Interestingly, the low genetic differentiation between CP and OP populations at the rest of the genome (FST = 2.5%) suggests gene flow between them. Males from OP lineages thus likely transmit their op allele to new genomic backgrounds. These genetic exchanges, combined with the selection of the OP and CP reproductive modes under different climates, probably contribute to the long-term persistence of the cp and op alleles.

What Are the Functional Roles of Piwi Proteins and piRNAs in Insects?

Research on Piwi proteins and piRNAs in insects has focused on three experimental models: oogenesis and spermatogenesis in Drosophila melanogaster, the antiviral response in Aedes mosquitoes and the molecular analysis of primary and secondary piRNA biogenesis in Bombyx mori-derived BmN4 cells. Significant unique and complementary information has been acquired and has led to a greater appreciation of the complexity of piRNA biogenesis and Piwi protein function. Studies performed in other insect species are emerging and promise to add to the current state of the art on the roles of piRNAs and Piwi proteins. Although the primary role of the piRNA pathway is genome defense against transposons, particularly in the germline, recent findings also indicate an expansion of its functions. In this review, an extensive overview is presented of the knowledge of the piRNA pathway that so far has accumulated in insects. Following a presentation of the three major models, data from other insects were also discussed. Finally, the mechanisms for the expansion of the function of the piRNA pathway from transposon control to gene regulation were considered.

Patterns of gene expression in ovaries of sexual vs. asexual lineages of a freshwater snail

Why sexual reproduction is so common when asexual reproduction should be much more efficient and less costly remains an open question in evolutionary biology. Comparisons between otherwise similar sexual and asexual taxa allow us to characterize the genetic architecture underlying asexuality, which can, in turn, illuminate how this reproductive mode transition occurred and the mechanisms by which it is maintained or disrupted. Here, we used transcriptome sequencing to compare patterns of ovarian gene expression between actively reproducing obligately sexual and obligately asexual females from multiple lineages of Potamopyrgus antipodarum, a freshwater New Zealand snail characterized by frequent separate transitions to asexuality and coexistence of otherwise similar sexual and asexual lineages. We also used these sequence data to evaluate whether population history accounts for variation in patterns of gene expression. We found that source population was a major source of gene expression variation, and likely more influential than reproductive mode. This outcome for these common garden-raised snails is strikingly similar to earlier results from field-collected snails. While we did not identify a likely set of candidate genes from expression profiles that could plausibly explain how transitions to asexuality occurred, we identified around 1,000 genes with evidence of differential expression between sexual and asexual reproductive modes, and 21 genes that appear to exhibit consistent expression differences between sexuals and asexuals across genetic backgrounds. This second smaller set of genes provides a good starting point for further exploration regarding a potential role in the transition to asexual reproduction. These results mark the first effort to characterize the causes of asexuality in P. antipodarum, demonstrate the apparently high heritability of gene expression patterns in this species, and hint that for P. antipodarum, transitions to asexuality might not necessarily be strongly associated with broad changes in gene expression.

The transcriptomic signature of obligate parthenogenesis

Investigating the origin of parthenogenesis through interspecific hybridization can provide insight into how meiosis may be altered by genetic incompatibilities, which is fundamental for our understanding of the formation of reproductive barriers. Yet the genetic mechanisms giving rise to obligate parthenogenesis in eukaryotes remain understudied. In the microcrustacean Daphnia pulex species complex, obligately parthenogenetic (OP) isolates emerged as backcrosses of two cyclically parthenogenetic (CP) parental species, D. pulex and D. pulicaria, two closely related but ecologically distinct species. We examine the genome-wide expression in OP females at the early resting egg production stage, a life-history stage distinguishing OP and CP reproductive strategies, in comparison to CP females of the same stage from the two parental species. Our analyses of the expression data reveal that underdominant and overdominant genes are abundant in OP isolates, suggesting widespread regulatory incompatibilities between the parental species. More importantly, underdominant genes (i.e., genes with expression lower than both parentals) in the OP isolates are enriched in meiosis and cell-cycle pathways, indicating an important role of underdominance in the origin of obligate parthenogenesis. Furthermore, metabolic and biosynthesis pathways enriched with overdominant genes (i.e., expression higher than both parentals) are another genomic signature of OP isolates.

Contribution of Epigenetic Mechanisms in the Regulation of Environmentally-Induced Polyphenism in Insects

Simple Summary

Polyphenism is a widespread phenomenon in insects that allows organisms to produce alternative and discrete phenotypes in response to environmental conditions. Epigenetic mechanisms, including histone post-translational modifications, DNA methylation and non-coding RNAs, are essential mechanisms that can promote rapid and flexible changes in the expression of transcriptional programs associated with the production of alternative phenotypes. This review summarizes knowledge regarding the contribution of those mechanisms in the regulation of the most-studied examples of polyphenism in insects.

Abstract

Many insect species display a remarkable ability to produce discrete phenotypes in response to changes in environmental conditions. Such phenotypic plasticity is referred to as polyphenism. Seasonal, dispersal and caste polyphenisms correspond to the most-studied examples that are environmentally-induced in insects. Cues that induce such dramatic phenotypic changes are very diverse, ranging from seasonal cues, habitat quality changes or differential larval nutrition. Once these signals are perceived, they are transduced by the neuroendocrine system towards their target tissues where gene expression reprogramming underlying phenotypic changes occur. Epigenetic mechanisms are key regulators that allow for genome expression plasticity associated with such developmental switches. These mechanisms include DNA methylation, chromatin remodelling and histone post-transcriptional modifications (PTMs) as well as non-coding RNAs and have been studied to various extents in insect polyphenism. Differential patterns of DNA methylation between phenotypes are usually correlated with changes in gene expression and alternative splicing events, especially in the cases of dispersal and caste polyphenism. Combinatorial patterns of histone PTMs provide phenotype-specific epigenomic landscape associated with the expression of specific transcriptional programs, as revealed during caste determination in honeybees and ants. Alternative phenotypes are also usually associated with specific non-coding RNA profiles. This review will provide a summary of the current knowledge of the epigenetic changes associated with polyphenism in insects and highlights the potential for these mechanisms to be key regulators of developmental transitions triggered by environmental cues.

Aphid estrogen-related receptor controls glycolytic gene expression and fecundity

Aphids, the major insect pests of agricultural crops, reproduce sexually and asexually depending upon environmental factors such as the photoperiod and temperature. Nuclear receptors, a unique family of ligand-dependent transcription factors, control insect development and growth including morphogenesis, molting, and metamorphosis. However, the structural features and biological functions of the aphid estrogen-related receptor (ERR) are largely unknown. Here, we cloned full-length cDNA encoding the ERR in the green peach aphid, Myzus persicae, (Sulzer) (Hemiptera: Aphididae) (MpERR) and demonstrated that the MpERR modulated glycolytic gene expression and aphid fecundity. The phylogenetic analysis revealed that the MpERR originated in a unique evolutionary lineage distinct from those of hemipteran insects. Moreover, the AF-2 domain of the MpERR conferred nuclear localization and transcriptional activity. The overexpression of the MpERR significantly upregulated the gene expression of rate-limiting enzymes involved in glycolysis such as phosphofructokinase and pyruvate kinase by directly binding to ERR-response elements in their promoters. Moreover, ERR-deficient viviparous female aphids showed decreased glycolytic gene expression and produced fewer offspring. These results suggest that the aphid ERR plays a pivotal role in glycolytic transcriptional control and fecundity.

Epigenetic Molecular Mechanisms in Insects

Insects are the largest animal group on Earth both in biomass and diversity. Their outstanding success has inspired genetics and developmental research, allowing the discovery of dynamic process explaining extreme phenotypic plasticity and canalization. Epigenetic molecular mechanisms (EMMs) are vital for several housekeeping functions in multicellular organisms, regulating developmental, ontogenetic trajectories and environmental adaptations. In Insecta, EMMs are involved in the development of extreme phenotypic divergences such as polyphenisms and eusocial castes. Here, we review the history of this research field and how the main EMMs found in insects help to understand their biological processes and diversity. EMMs in insects confer them rapid response capacity allowing insect either to change with plastic divergence or to keep constant when facing different stressors or stimuli. EMMs function both at intra as well as transgenerational scales, playing important roles in insect ecology and evolution. We discuss on how EMMs pervasive influences in Insecta require not only the control of gene expression but also the dynamic interplay of EMMs with further regulatory levels, including genetic, physiological, behavioral, and environmental among others, as was earlier proposed by the Probabilistic Epigenesis model and Developmental System Theory.

Behavioural and chronic toxicity of fullerene to Daphnia magna: Mechanisms revealed by transcriptomic analysis

Extensive application of fullerene nanoparticles (nC₆₀) leads to potential environmental pollution. The acute toxic effects of nC₆₀ have been largely investigated, but studies of behavioural and chronic toxicity at sublethal doses are still rare and the underlying molecular mechanisms remain unknown. The present study investigated behavioural and chronic toxicity of nC₆₀ to Daphnia magna. The results showed that, in response to nC₆₀ exposure, hopping, heartbeat frequencies and feeding ability of D. magna decreased significantly, displaying negative relationship with exposure time and dose. Chronic treatments with 0.1 mg/L or 1 mg/L nC₆₀ for 21 days significantly reduced survival and reproduction of D. magna. These harmful effects suggested negative impacts of nC₆₀ on aquatic ecosystems. Moreover, transcriptome analysis showed that the behavioural and chronic toxicity of nC₆₀ to D. magna might be related to physiological functions such as cell structural repair, protein degradation, energy metabolism and reproduction. We found that nC₆₀ accumulated in guts of D. magna, which should be responsible for the decrease of food ingestion and consequently inhibiting energy intake. Deficiency of energy not only affects behaviours but also declines reproduction in D. magna. Overall, this is the first study comprehensively considered the behavioral and chronic toxicity of nC₆₀ to aquatic organism. The results should be helpful to better understand the ecological consequences of C₆₀ released into water environments.

The reproductive system of the male and oviparous female of a model organism-the pea aphid, Acyrthosiphon pisum (Hemiptera, Aphididae)

The structure of the reproductive system of the sexual generation-males and oviparous females-of the pea aphid Acyrthosiphon pisum (Harris) (Hemiptera, Aphididae), a serious pest of cultivated plants of Fabaceae, was investigated. For the first time we describe the morphology, histology and ultrastructure of the reproductive system in both morphs of the sexual generation of aphids within one species, using light and fluorescent microscopy, as well as transmission and scanning electron microscopy. The results revealed that males have testes composed of three follicles fused by the upper ends of the vasa efferentia, the vasa deferentia run independently, the accessory glands are asymmetric and the ejaculatory duct shortened. Oviparous females have ovaries composed of seven ovarioles each. The lateral oviducts join to a short common oviduct connected with the unpaired spermatheca and paired accessory glands. Yolky eggs with an aggregation of symbiotic bacteria at the posterior pole are produced. Histologically, the components of genital tracts are broadly similar: the epithelial cells of the walls of the vasa deferentia and accessory glands of the male and oviparous female have secretory functions which correlate with the age of the studied morphs. We also found symbiotic bacteria within the vasa deferentia epithelial cells in males and within the cells of the lateral oviducts of females. Because the pea aphid is listed among the 14 species that are of the greatest economic importance, our results will be useful for managing aphid populations, protecting plants and ensuring global food security.

The Effects of Different Diets and Transgenerational Stress on Acyrthosiphon pisum Development

Despite the fact that sap-feeding hemipterans are major agricultural pests, little is known about the pea aphid's (Acyrthosiphon pisum) nymphal development, compared to other insect models. Given our limited understanding of A. pisum nymphal development and variability in the naming/timing of its developmental events between different environmental conditions and studies, here, we address developmental knowledge gaps by elucidating how diet impacts A. pisum nymphal development for the LSR1 strain when it develops on its universal host plant (Vicia faba), isolated leaves, and artificial diet. Moreover, we test how plant age and transgenerational stressors, such as overcrowding and low plant vigor, can affect nymphal development. We also validate a morphological method to quickly confirm the life stage of each nymphal instar within a mixed population. Overall, we found extremely high variation in the timing of developmental events and a significant delay in nymphal (~5-25-h/instar) and pre-reproductive adult (~40-h) development when reared on isolated leaves and artificial diets, compared to intact host plants. Also, delays in development were observed when reared on older host plants (~9-17-h/event, post 2nd instar) or when previous generations were exposed to overcrowding on host plants (~20-h delay in nymph laying) compared to controls.

The right tools for the job: Regulating polyphenic morph development in insects

Polyphenism is a form of developmental plasticity in which organisms respond to environmental cues by producing adaptive, discrete, alternative phenotypes known as morphs. The phenomenon is common and important as both a form of adaptation and a source of variation for natural selection. Understanding the evolution of polyphenism will require understanding the proximate factors that regulate alternative morph production. Renewed interest and technological advances have fueled multiple approaches to the latter, including hormone manipulation studies, targeted transcriptomic studies, and epigenetic profiling. We review these studies and suggest that integration of multilayered approaches will be necessary to understand the complex mechanisms involved in regulating alternative morphologies.

NMDA receptor activation upstream of methyl farnesoate signaling for short day-induced male offspring production in the water flea, Daphnia pulex

BACKGROUND

The cladoceran crustacean Daphnia pulex produces female offspring by parthenogenesis under favorable conditions, but in response to various unfavorable external stimuli, it produces male offspring (environmental sex determination: ESD). We recently established an innovative system for ESD studies using D. pulex WTN6 strain, in which the sex of the offspring can be controlled simply by changes in the photoperiod: the long-day and short-day conditions can induce female and male offspring, respectively. Taking advantage of this system, we demonstrated that de novo methyl farnesoate (MF) synthesis is necessary for male offspring production. These results indicate the key role of innate MF signaling as a conductor between external environmental stimuli and the endogenous male developmental pathway. Despite these findings, the molecular mechanisms underlying up- and downstream signaling of MF have not yet been well elucidated in D. pulex.

RESULTS

To elucidate up- and downstream events of MF signaling during sex determination processes, we compared the transcriptomes of daphnids reared under the long-day (female) condition with short-day (male) and MF-treated (male) conditions. We found that genes involved in ionotropic glutamate receptors, known to mediate the vast majority of excitatory neurotransmitting processes in various organisms, were significantly activated in daphnids by the short-day condition but not by MF treatment. Administration of specific agonists and antagonists, especially for the N-methyl-D-aspartic acid (NMDA) receptor, strongly increased or decreased, respectively, the proportion of male-producing mothers. Moreover, we also identified genes responsible for male production (e.g., protein kinase C pathway-related genes). Such genes were generally shared between the short-day reared and MF-treated daphnids.

CONCLUSIONS

We identified several candidate genes regulating ESD which strongly suggests that these genes may be essential factors for male offspring production as an upstream regulator of MF signaling in D. pulex. This study provides new insight into the fundamental mechanisms underlying how living organisms alter their phenotypes in response to various external environments.

Genetic control of contagious asexuality in the pea aphid

Although evolutionary transitions from sexual to asexual reproduction are frequent in eukaryotes, the genetic bases of such shifts toward asexuality remain largely unknown. We addressed this issue in an aphid species where both sexual and obligate asexual lineages coexist in natural populations. These sexual and asexual lineages may occasionally interbreed because some asexual lineages maintain a residual production of males potentially able to mate with the females produced by sexual lineages. Hence, this species is an ideal model to study the genetic basis of the loss of sexual reproduction with quantitative genetic and population genomic approaches. Our analysis of the co-segregation of ∼ 300 molecular markers and reproductive phenotype in experimental crosses pinpointed an X-linked region controlling obligate asexuality, this state of character being recessive. A population genetic analysis (>400-marker genome scan) on wild sexual and asexual genotypes from geographically distant populations under divergent selection for reproductive strategies detected a strong signature of divergent selection in the genomic region identified by the experimental crosses. These population genetic data confirm the implication of the candidate region in the control of reproductive mode in wild populations originating from 700 km apart. Patterns of genetic differentiation along chromosomes suggest bidirectional gene flow between populations with distinct reproductive modes, supporting contagious asexuality as a prevailing route to permanent parthenogenesis in pea aphids. This genetic system provides new insights into the mechanisms of coexistence of sexual and asexual aphid lineages.

Comparative transcriptional analysis of asexual and sexual morphs reveals possible mechanisms in reproductive polyphenism of the cotton aphid

Aphids, the destructive insect pests in the agriculture, horticulture and forestry, are capable of reproducing asexually and sexually upon environmental change. However, the molecular basis of aphid reproductive mode switch remains an enigma. Here we report a comparative analysis of differential gene expression profiling among parthenogenetic females, gynoparae and sexual females of the cotton aphid Aphis gossypii, using the RNA-seq approach with next-generation sequencing platforms, followed by RT-qPCR. At the cutoff criteria of fold change ≥2 and P<0.01, we identified 741 up- and 879 down-regulated genes in gynoparae versus parthenogenetic females, 2,101 up- and 2,210 down-regulated genes in sexual females compared to gynoparae, and 1,614 up- and 2,238 down-regulated genes in sexual females relative to parthenogenetic females. Gene ontology category and KEGG pathway analysis suggest the involvement of differentially expressed genes in multiple cellular signaling pathways into the reproductive mode transition, including phototransduction, cuticle composition, progesterone-mediated oocyte maturation and endocrine regulation. This study forms a basis for deciphering the molecular mechanisms underlying the shift from asexual to sexual reproduction in the cotton aphid. It also provides valuable resources for future studies on this host-alternating aphid species, and the insight into the understanding of reproductive mode plasticity in different aphid species.

Posterior localization of ApVas1 positions the preformed germ plasm in the sexual oviparous pea aphid Acyrthosiphon pisum

Background

Germline specification in some animals is driven by the maternally inherited germ plasm during early embryogenesis (inheritance mode), whereas in others it is induced by signals from neighboring cells in mid or late development (induction mode). In the Metazoa, the induction mode appears as a more prevalent and ancestral condition; the inheritance mode is therefore derived. However, regarding germline specification in organisms with asexual and sexual reproduction it has not been clear whether both strategies are used, one for each reproductive phase, or if just one strategy is used for both phases. Previously we have demonstrated that specification of germ cells in the asexual viviparous pea aphid depends on a preformed germ plasm. In this study, we extended this work to investigate how germ cells were specified in the sexual oviparous embryos, aiming to understand whether or not developmental plasticity of germline specification exists in the pea aphid.

Results

We employed Apvas1, a Drosophila vasa ortholog in the pea aphid, as a germline marker to examine whether germ plasm is preformed during oviparous development, as has already been seen in the viviparous embryos. During oogenesis, Apvas1 mRNA and ApVas1 protein were both evenly distributed. After fertilization, uniform expression of Apvas1 remained in the egg but posterior localization of ApVas1 occurred from the fifth nuclear cycle onward. Posterior co-localization of Apvas1/ApVas1 was first identified in the syncytial blastoderm undergoing cellularization, and later we could detect specific expression of Apvas1/ApVas1 in the morphologically identifiable germ cells of mature embryos. This suggests that Apvas1/ApVas1-positive cells are primordial germ cells and posterior localization of ApVas1 prior to cellularization positions the preformed germ plasm.

Conclusions

We conclude that both asexual and sexual pea aphids rely on the preformed germ plasm to specify germ cells and that developmental plasticity of germline specification, unlike axis patterning, occurs in neither of the two aphid reproductive phases. Consequently, the maternal inheritance mode implicated by a preformed germ plasm in the oviparous pea aphid becomes a non-canonical case in the Hemimetabola, where so far the zygotic induction mode prevails in most other studied insects.

The pea aphid (Acyrthosiphon pisum) genome encodes two divergent early developmental programs

The pea aphid (Acyrthosiphon pisum) can reproduce either sexually or asexually (parthenogenetically), giving rise, in each case, to almost identical adults. These two modes of reproduction are accompanied by differences in ovarian morphology and the developmental environment of the offspring, with sexual forms producing eggs that are laid, whereas asexual development occurs within the mother. Here we examine the effect each mode of reproduction has on the expression of key maternal and axis patterning genes; orthodenticle (otd), hunchback (hb), caudal (cad) and nanos (nos). We show that three of these genes (Ap-hb, Ap-otd and Ap-cad) are expressed differently between the sexually and asexually produced oocytes and embryos of the pea aphid. We also show, using immunohistochemistry and cytoskeletal inhibitors, that Ap-hb RNA is localized differently between sexually and asexually produced oocytes, and that this is likely due to differences in the 3' untranslated regions of the RNA. Furthermore, Ap-hb and Ap-otd have extensive expression domains in early sexually produced embryos, but are not expressed at equivalent stages in asexually produced embryos. These differences in expression likely correspond with substantial changes in the gene regulatory networks controlling early development in the pea aphid. These data imply that in the evolution of parthenogenesis a new program has evolved to control the development of asexually produced embryos, whilst retaining the existing, sexual, developmental program. The patterns of modification of these developmental processes mirror the changes that we see in developmental processes between species, in that early acting pathways in development are less constrained, and evolve faster, than later ones. We suggest that the evolution of the novel asexual development pathway in aphids is not a simple modification of an ancestral system, but the evolution of two very different developmental mechanisms occurring within a single species.

Inventory and phylogenetic analysis of meiotic genes in monogonont rotifers

A long-standing question in evolutionary biology is how sexual reproduction has persisted in eukaryotic lineages. As cyclical parthenogens, monogonont rotifers are a powerful model for examining this question, yet the molecular nature of sexual reproduction in this lineage is currently understudied. To examine genes involved in meiosis, we generated partial genome assemblies for 2 distantly related monogonont species, Brachionus calyciflorus and B. manjavacas. Here we present an inventory of 89 meiotic genes, of which 80 homologs were identified and annotated from these assemblies. Using phylogenetic analysis, we show that several meiotic genes have undergone relatively recent duplication events that appear to be specific to the monogonont lineage. Further, we compare the expression of "meiosis-specific" genes involved in recombination and all annotated copies of the cell cycle regulatory gene CDC20 between obligate parthenogenetic (OP) and cyclical parthenogenetic (CP) strains of B. calyciflorus. We show that "meiosis-specific" genes are expressed in both CP and OP strains, whereas the expression of one of the CDC20 genes is specific to cyclical parthenogenesis. The data presented here provide insights into mechanisms of cyclical parthenogenesis and establish expectations for studies of obligate asexual relatives of monogononts, the bdelloid rotifer lineage.

Tyrosine pathway regulation is host-mediated in the pea aphid symbiosis during late embryonic and early larval development

BACKGROUND

Nutritional symbioses play a central role in insects' adaptation to specialized diets and in their evolutionary success. The obligatory symbiosis between the pea aphid, Acyrthosiphon pisum, and the bacterium, Buchnera aphidicola, is no exception as it enables this important agricultural pest insect to develop on a diet exclusively based on plant phloem sap. The symbiotic bacteria provide the host with essential amino acids lacking in its diet but necessary for the rapid embryonic growth seen in the parthenogenetic viviparous reproduction of aphids. The aphid furnishes, in exchange, non-essential amino acids and other important metabolites. Understanding the regulations acting on this integrated metabolic system during the development of this insect is essential in elucidating aphid biology.

RESULTS

We used a microarray-based approach to analyse gene expression in the late embryonic and the early larval stages of the pea aphid, characterizing, for the first time, the transcriptional profiles in these developmental phases. Our analyses allowed us to identify key genes in the phenylalanine, tyrosine and dopamine pathways and we identified ACYPI004243, one of the four genes encoding for the aspartate transaminase (E.C. 2.6.1.1), as specifically regulated during development. Indeed, the tyrosine biosynthetic pathway is crucial for the symbiotic metabolism as it is shared between the two partners, all the precursors being produced by B. aphidicola. Our microarray data are supported by HPLC amino acid analyses demonstrating an accumulation of tyrosine at the same developmental stages, with an up-regulation of the tyrosine biosynthetic genes. Tyrosine is also essential for the synthesis of cuticular proteins and it is an important precursor for cuticle maturation: together with the up-regulation of tyrosine biosynthesis, we observed an up-regulation of cuticular genes expression. We were also able to identify some amino acid transporter genes which are essential for the switch over to the late embryonic stages in pea aphid development.

CONCLUSIONS

Our data show that, in the development of A. pisum, a specific host gene set regulates the biosynthetic pathways of amino acids, demonstrating how the regulation of gene expression enables an insect to control the production of metabolites crucial for its own development and symbiotic metabolism.

The pea aphid uses a version of the terminal system during oviparous, but not viviparous, development

Background

In most species of aphid, female nymphs develop into either sexual or asexual adults depending on the length of the photoperiod to which their mothers were exposed. The progeny of these sexual and asexual females, in turn, develop in dramatically different ways. The fertilized oocytes of sexual females begin embryogenesis after being deposited on leaves (oviparous development) while the oocytes of asexual females complete embryogenesis within the mother (viviparous development). Compared with oviparous development, viviparous development involves a smaller transient oocyte surrounded by fewer somatic epithelial cells and a smaller early embryo that comprises fewer cells. To investigate whether patterning mechanisms differ between the earliest stages of the oviparous and viviparous modes of pea aphid development, we examined the expression of pea aphid orthologs of genes known to specify embryonic termini in other insects.

Results

Here we show that pea aphid oviparous ovaries express torso-like in somatic posterior follicle cells and activate ERK MAP kinase at the posterior of the oocyte. In addition to suggesting that some posterior features of the terminal system are evolutionarily conserved, our detection of activated ERK in the oocyte, rather than in the embryo, suggests that pea aphids may transduce the terminal signal using a mechanism distinct from the one used in Drosophila. In contrast with oviparous development, the pea aphid version of the terminal system does not appear to be used during viviparous development, since we did not detect expression of torso-like in the somatic epithelial cells that surround either the oocyte or the blastoderm embryo and we did not observe restricted activated ERK in the oocyte.

Conclusions

We suggest that while oviparous oocytes and embryos may specify posterior fate through an aphid terminal system, viviparous oocytes and embryos employ a different mechanism, perhaps one that does not rely on an interaction between the oocyte and surrounding somatic cells. Together, these observations provide a striking example of a difference in the fundamental events of early development that is both environmentally induced and encoded by the same genome.

Characterization of transcriptomes from sexual and asexual lineages of a New Zealand snail (Potamopyrgus antipodarum)

Understanding the evolution and maintenance of sexual reproduction is one of the central challenges of evolutionary biology, yet we know very little about how sex influences molecular evolution. The New Zealand freshwater snail Potamopyrgus antipodarum is ideally suited to address this knowledge gap because obligately sexual individuals often coexist with multiple independently derived obligately asexual lineages. This unusual situation allows direct comparisons both between sexual and asexual P. antipodarum and across populations that differ in the relative frequency of sexual individuals. As such, P. antipodarum has received a great deal of attention as a model system for the maintenance of sex in nature and is also used as a model for environmental toxicology and biological invasions. Molecular genetic resources for P. antipodarum will thus be useful to investigators in a variety of biological fields. We used 454 sequencing of cDNA libraries to generate transcriptomes from two sexual and two asexual P. antipodarum lineages. A de novo assembly of 116.7 Mb of sequence reads produced 41 396 contigs, and sequence similarity-based Gene Ontology annotations were obtained for 3740 contigs. We detected 408 315 SNP loci and 7315 microsatellite loci, which together represent the first genome-scale resource available for P. antipodarum. Raw 454 read sequences, contig sequences, annotation data and polymorphism data are publicly available in a searchable online database and for download at http://www.biology.uiowa.edu/neiman/transcriptome.php.