Life on the Edge: Ecological Genetics of a High Arctic Insect Species and Its Circumpolar Counterpart.

Arctic ecosystems are subjected to strong environmental constraints that prevent both the colonization and development of many organisms. In Svalbard, few aphid species have established permanent populations. These high arctic aphid species have developed peculiar life-history traits such as shortened life cycles and reduced dispersal capacities. Here, we present data on the distribution and population genetics of Acyrthosiphon svalbardicum in Spitsbergen, the main island of the Svalbard archipelago, and compared its genetic structure with that of its close relative Acyrthosiphon brevicorne, sampled in the top of Scandinavian mainland. We found that A. svalbardicum is common but heterogeneously distributed along the west coast of Spitsbergen. We recorded this species up to 79°12', which constitutes the northernmost location for any aphid. Genetic structure examined using microsatellite markers showed more pronounced spatial differentiation in A. svalbardicum than in A. brevicorne populations, presumably due to reduced dispersal capacities in the former species. Although populations of A. brevicorne and A. svalbardicum were well-delineated at nuclear loci, they shared similar cytoplasmic DNA haplotypes as revealed by sequence analysis of two DNA barcodes. These results raise questions about whether these two taxa are different species, and the colonization sources and history of the Svalbard archipelago by A. svalbardicum.

Differences in egg hatching time between cyclical and obligate parthenogenetic lineages of aphids.

Many aphid species exhibit a variation in reproductive mode which is influenced by winter climate regimes, with cyclical parthenogenetic (CP) lines dominating in cold winter areas (because they produce cold-resistant eggs) and obligate parthenogenetic (OP) ones in mild winter regions (because of their parthenogenetic overwintering). Genetic studies on several aphid species have shown that the OP trait can be transmitted during sexual events involving the 2 types of lines. This genetic system could be considered as a local safeguarding mechanism for OP alleles in case severe frost would have killed all parthenogenetically overwintering individuals. However, this strategy would only be efficient in restoring local polymorphism in breeding systems if the newly hatched OP recombinants remain competitive over their CP counterparts. In this study we compared egg hatching sequences of CP and OP F1 clones from several crosses obtained for 2 cereal aphid species, Sitobion avenae (constant 5 °C, 8 h of light) and Rhopalosiphum padi (winter outdoor conditions). For S. avenae, we obtained F1 offspring from 6 crosses, involving 4 clones while in R. padi F1 were obtained from 11 crosses involving 14 clones. We showed that in both species proportions of OP clones were higher in the first half of the progeny relative to the second half. In addition, F1 OP clones hatched in the mean about a week earlier than their CP sibs, which gives them a demographic advantage at the start of the growth season. We then discussed the consequences of this fitness advantage for the maintenance and spread of the OP trait in aphid populations.

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.

Widespread host-dependent hybrid unfitness in the pea aphid species complex.

Linking adaptive divergence to hybrid unfitness is necessary to understand the ecological factors contributing to reproductive isolation and speciation. To date, this link has been demonstrated in few model systems, most of which encompass ecotypes that occupy relatively early stages in the speciation process. Here we extend these studies by assessing how host-plant adaptation conditions hybrid fitness in the pea aphid, Acyrthosiphon pisum. We made crosses between and within five pea aphid biotypes adapted to different host plants and representing various stages of divergence within the complex. Performance of F1 hybrids and nonhybrids was assessed on a "universal" host that is favorable to all pea aphid biotypes in laboratory conditions. Although hybrids performed equally well as nonhybrids on the universal host, their performance was much lower than nonhybrids on the natural hosts of their parental populations. Hence, hybrids, rather than being intrinsically deficient, are maladapted to their parents' hosts. Interestingly, the impact of this maladaptation was stronger in certain hybrids from crosses involving the most divergent biotype, suggesting that host-dependent postzygotic isolation has continued to evolve late in divergence. Even though host-independent deficiencies are not excluded, hybrid maladaptation to parental hosts supports the hypothesis of ecological speciation in this complex.

Inheritance patterns of secondary symbionts during sexual reproduction of pea aphid biotypes.

Herbivorous insects frequently harbor bacterial symbionts that affect their ecology and evolution. Aphids host the obligatory endosymbiont Buchnera, which is required for reproduction, together with facultative symbionts whose frequencies vary across aphid populations. These maternally transmitted secondary symbionts have been particularly studied in the pea aphid, Acyrthosiphon pisum, which harbors at least 8 distinct bacterial species (not counting Buchnera) having environmentally dependent effects on host fitness. In particular, these symbiont species are associated with pea aphid populations feeding on specific plants. Although they are maternally inherited, these bacteria are occasionally transferred across insect lineages. One mechanism of such nonmaternal transfer is paternal transmission to the progeny during sexual reproduction. To date, transmission of secondary symbionts during sexual reproduction of aphids has been investigated in only a handful of aphid lineages and 3 symbiont species. To better characterize this process, we investigated inheritance patterns of 7 symbiont species during sexual reproduction of pea aphids through a crossing experiment involving 49 clones belonging to 9 host-specialized biotypes, and 117 crosses. Symbiont species in the progeny were detected with diagnostic qualitative PCR at the fundatrix stage hatching from eggs and in later parthenogenetic generations. We found no confirmed case of paternal transmission of symbionts to the progeny, and we observed that maternal transmission of a particular symbiont species (Serratia symbiotica) was quite inefficient. We discuss these observations in respect to the ecology of the pea aphid.

Transcriptomic profiling of the reproductive mode switch in the pea aphid in response to natural autumnal photoperiod.

Aphids are among the rare organisms that can change their reproductive mode across their life cycle. During spring and summer they reproduce clonally and efficiently by parthenogenesis. At the end of summer aphids perceive the shortening of day length which triggers the production of sexual individuals - males and oviparous females - that will mate and lay overwintering cold-resistant eggs. Recent large scale transcriptomic studies allowed the discovery of transcripts and functions such as nervous and hormonal signaling involved in the early steps of detection and transduction of the photoperiodic signal. Nevertheless these experiments were performed under controlled conditions when the photoperiod was the only varying parameter. To characterize the response of aphids under natural conditions, aphids were reared outdoor both in summer and autumn and material was collected to compare their transcriptomic profile using a cDNA microarray containing around 7000 transcripts. Statistical analyses revealed that close to 5% of these transcripts (367) were differentially expressed at two developmental stages of the process in response to the autumnal environmental conditions. Functional classification of regulated transcripts confirmed the putative contribution of the neuro-endocrine system in the process. Furthermore, these experiments revealed the regulation of transcripts involved in juvenile hormone synthesis and signaling pathway, confirming the key role played by these molecules in the reproductive mode switch. Aphids placed under outdoor conditions were confronted to a range of abiotic factors such as temperature fluctuations which was confirmed by the differential expression of an important proportion of heat shock protein transcripts between the two seasons. Finally, this original approach completed the understanding of genetic programs involved in aphid phenotypic plasticity.

Revisiting the anatomy of the central nervous system of a hemimetabolous model insect species: the pea aphid Acyrthosiphon pisum.

Aphids show a marked phenotypic plasticity, producing asexual or sexual and winged or wingless morphs depending on environmental conditions and season. We describe here the general structure of the brain of various morphs of the pea aphid Acyrthosiphon pisum. This is the first detailed anatomical study of the central nervous system of an aphid by immunocytochemistry (synapsin, serotonin, and several neuropeptides), ethyl-gallate staining, confocal laser scanning microscopy, and three-dimensional reconstructions. The study has revealed well-developed optic lobes composed of lamina, medulla, and lobula complex. Ocelli are only present in males and winged parthenogenetic females. The central complex is well-defined, with a central body divided into two parts, a protocerebral bridge, and affiliated lateral accessory lobes. The mushroom bodies are ill-defined, lacking calyces, and only being visualized by using an antiserum against the neuropeptide orcokinin. The antennal lobes contain poorly delineated glomeruli but can be clearly visualized by performing antennal backfills. On the basis of our detailed description of the brain of winged and wingless parthenogenetic A. pisum females, an anatomical map is now available that should improve our knowledge of the way that these structures are involved in the regulation of phenotypic plasticity.

Gene knockdown by RNAi in the pea aphid Acyrthosiphon pisum.

BACKGROUND: RNA interference (RNAi) is a powerful method to inhibit gene expression in a sequence specific manner. RESULTS: Here, we described the development of RNAi by micro-injection of double-stranded RNA (dsRNA) in the pea aphid Acyrthosiphon pisum. Injection of dsRNA into whole aphid body induced the silencing of two marker genes with different expression patterns: the ubiquitously expressed Ap-crt genes encoding a calreticulin and the gut specific Ap-cath-L gene encoding a cathepsin-L. Time-course analysis of the silencing showed similar temporal patterns for both genes: inhibition started at 1 day after injection, reached its maximum at 5 days and stopped at 7 days. A comparable 40% decrease of gene expression was observed for Ap-crt and Ap-cath-L. CONCLUSION: The pea aphid is the first Hemipteran insect for which genome sequence will be available soon. The gene knockdown technique developed in this study will be an essential post-genomic tool for further investigations in aphidology.

Large-scale gene discovery in the pea aphid Acyrthosiphon pisum (Hemiptera).

Aphids are the leading pests in agricultural crops. A large-scale sequencing of 40,904 ESTs from the pea aphid Acyrthosiphon pisum was carried out to define a catalog of 12,082 unique transcripts. A strong AT bias was found, indicating a compositional shift between Drosophila melanogaster and A. pisum. An in silico profiling analysis characterized 135 transcripts specific to pea-aphid tissues (relating to bacteriocytes and parthenogenetic embryos). This project is the first to address the genetics of the Hemiptera and of a hemimetabolous insect.

Strong biases in the transmission of sex chromosomes in the aphid Rhopalosiphum padi.

The typical life cycle of aphids involves several parthenogenetic generations followed by a single sexual one in autumn, i.e. cyclical parthenogenesis. Sexual females are genetically identical to their parthenogenetic mothers and carry two sex chromosomes (XX). Male production involves the elimination of one sex chromosome (to produce X0) that could give rise to genetic conflicts between X-chromosomes. In addition, deleterious recessive mutations could accumulate on sex chromosomes during the parthenogenetic phase and affect males differentially depending on the X-chromosome they inherit. Genetic conflicts and deleterious mutations thus may induce transmission bias that could be exaggerated in males. Here, the transmission of X-chromosomes has been studied in the laboratory in two cyclically parthenogenetic lineages of the bird cherry-oat aphid Rhopalosiphum padi . X-chromosome transmission was followed, using X-linked microsatellite loci, at male production in the two lineages and in their hybrids deriving from reciprocal crosses. Genetic analyses revealed non-Mendelian inheritance of X-chromosomes in both parental and hybrid lineages at different steps of male function. Putative mechanisms and evolutionary consequences of non-Mendelian transmission of X-chromosomes to males are discussed.

Aster self-organization at meiosis: a conserved mechanism in insect parthenogenesis?

Unfertilized eggs usually lack maternal centrosomes and cannot develop without sperm contribution. However, several insect species lay eggs that develop to adulthood as unfertilized in the absence of a preexisting centrosome. We report that the oocyte of the parthenogenetic viviparous pea aphid Acyrthosiphon pisum is able to self-organize microtubule-based asters, which in turn interact with the female chromatin to form the first mitotic spindle. This mode of reproduction provides a good system to investigate how the oocyte can assemble new centrosomes and how their number can be exactly monitored. We propose that the cooperative interaction of motor proteins and randomly nucleated surface microtubules could lead to the formation of aster-like structures in the absence of pre-existing centrosomes. Recruitment of material along the microtubules might contribute to the accumulation of pericentriolar material and centriole precursors at the focus of the asters, thus leading to the formation of true centrosomes. The appearance of microtubule asters at the surface of activated oocytes could represent a possible common mechanism for centrosome formation during insect parthenogenesis.