Aphids were the first animals reported as photoperiodic as their life cycles are strongly determined by the photoperiod. During the favourable seasons (characterised by long days) aphid populations ...consist exclusively of viviparous parthenogenetic females (known as virginoparae). Shortening of the photoperiod in autumn is perceived by aphids as the signal that anticipates the harsh season, leading to a switch in the reproductive mode giving place to the sexual morphs (oviparae females and males) that mate and lay winter-resistant (diapause-like) eggs. The molecular and cellular basis governing the switch between the two reproductive modes are far from being understood. Classical experiments identified a group of neurosecretory cells in the pars intercerebralis of the aphid brain (the so called group I of neurosecretory cells) that were essential for the development of embryos as parthenogenetic females and were thus proposed to synthesise a parthenogenesis promoting substance that was termed “virginoparin”. Since insulin-like peptides (ILPs) have been implicated in the control of diapause in other insects, we investigated their involvement in aphid photoperiodism. We compared the expression of two ILPs (ILP1 and ILP4) and an Insulin receptor coding genes in A. pisum aphids reared under long- and short-day conditions. The three genes showed higher expression in long-day reared aphids. In addition, we localised the site of expression of the two ILP genes in the aphid brain. Both genes were found to be expressed in the group I of neurosecretory cells. Altogether, our results suggest that ILP1 and ILP4 play an important role in the control of the aphid life-cycle by promoting the parthenogenetic development during long-day seasons while their repression by short days would activate the sexual development. Thus we propose these ILPs correspond to the so called “virginoparin” by early bibliography. A possible connection with the circadian system is also discussed.
Display omitted
•ILP1 and ILP4 expression is higher under parthenogenesis promoting conditions (long days) in holocyclic aphids.•No difference in levels of expression of ILPs in anholocyclic aphids reared under LD and SD photoperiods.•ILP1/4 transcripts are localised in the group I of neurosecretory cells in the pars intercerebralis of the protocerebrum.•Our results point to ILP1 and ILP4 being the parthenogenesis promoting factor called virginoparin in early bibliography.
Resistance to endoparasitoids in aphids involves complex interactions between insect and microbial players. It is now generally accepted that the facultative bacterial symbiont
Hamiltonella defensa ...of the pea aphid
Acyrthosiphon pisum is implicated in its resistance to the parasitoid
Aphidius ervi. It has also been shown that heat negatively affects pea aphid resistance, suggesting the thermosensitivity of its defensive symbiosis. Here we examined the effects of heat and UV-B on the resistance of
A. pisum to
A. ervi and we relate its stability under heat stress to different facultative bacterial symbionts hosted by the aphid. For six
A. pisum clones harboring four different facultative symbiont associations, the impact of heat and UV-B was measured on their ability to resist
A. ervi parasitism under controlled conditions. The results revealed that temperature strongly affected resistance, while UV-B did not. As previously shown, highly resistant
A. pisum clones singly infected with
H. defensa became more susceptible to parasitism after exposure to heat. Interestingly, clones that were superinfected with
H. defensa in association with a newly discovered facultative symbiont, referred to as PAXS (pea aphid X-type symbiont), not only remained highly resistant under heat stress, but also expressed previously unknown, very precocious resistance to
A. ervi compared to clones with
H. defensa alone. The prevalence of dual symbiosis involving PAXS and
H. defensa in local aphid populations suggests its importance in protecting aphid immunity to parasitoids under abiotic stress.
The wing polyphenism of pea aphids is a compelling laboratory model with which to study the molecular mechanisms underlying phenotypic plasticity. In this polyphenism, environmental stressors such as ...high aphid density cause asexual, viviparous adult female aphids to alter the developmental fate of their embryos from wingless to winged morphs. This polyphenism is transgenerational, in that the pea aphid mother experiences the environmental signals, but it is her offspring that are affected. Previous research suggested that the steroid hormone ecdysone may play a role in this polyphenism. Here, we analyzed ecdysone-related gene expression patterns and found that they were consistent with a down-regulation of the ecdysone pathway being involved in the production of winged offspring. We therefore predicted that reduced ecdysone signaling would result in more winged offspring. Experimental injections of ecdysone or its analog resulted in a decreased production of winged offspring. Conversely, interfering with ecdysone signaling using an ecdysone receptor antagonist or knocking down the ecdysone receptor gene with RNAi resulted in an increased production of winged offspring. Our results are therefore consistent with the idea that ecdysone plays a causative role in the regulation of the proportion of winged offspring produced in response to crowding in this polyphenism. Our results also show that an environmentally regulated maternal hormone can mediate phenotype production in the next generation, as well as provide significant insight into the molecular mechanisms underlying the functioning of transgenerational phenotypic plasticity.
Aphids feed from sieve tubes deep inside the host plant. Therefore, aphids must be able to recognize their host plant(s) and to direct their stylets which must be long and thin enough to reach and ...puncture the sieve tubes at a particular site. Sieve tubes in angiosperms are longitudinal arrays of sieve element/companion cell modules which are highly sensitive to disturbance of any kind. The sieve tubes dispose of elaborate sealing mechanisms such as protein plugging and callose sealing which are triggered by a rise in calcium in the sieve tubes. Aphids seem to have developed a range of physical and chemical measures to limit the amount of calcium influx in response to stylet puncturing. Loss of sieve-element turgor pressure induced by stylet insertion is minimized by the minute stylet volume. Turgor-dependent Ca2+ influx, possibly mediated by mechano sensitive Ca2+ channels, must therefore be limited. The components of the sheath and watery saliva play a pivotal role in establishing the physical and chemical constraints on the rise of calcium. Most likely, sheath saliva prevents the influx of calcium from the apoplast by sealing the stylet puncture site while watery saliva may prevent plugging and sealing of sieve plates by potential interaction with SE sap ingredients.
Engytatus varians (Distant) (Hemiptera: Miridae) es una especie zoofitófaga debido a que se alimenta de las plantas hospederas y de los insectos que viven en ellas como áfidos, moscas blancas, ...pseudocóccidos y lepidópteros. Actualmente, esta especie presenta una amplia pero imprecisa distribución. En este trabajo se reporta por primera vez la presencia de E. varians en México depredando ninfas del psílido del tomate, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae) bajo condiciones de invernadero. En promedio, una ninfa de cuarto estadio de E. varians se alimentó de 46% de las ninfas de tercer estadio de B. cockerelli en un periodo de 24 h. Este resultado muestra que este depredador podría representar un candidato potencial para el manejo del psílido del tomate. También se incluye información de su distribución, plantas hospederas y presas. Palabras clave: Psílido de la papa o tomate, enemigos naturales nativos, míridos zoofitófagos. Engytatus varians (Distant) (Hemiptera: Miridae) is a zoophytophagous species that feed on plants as well as of the phytophagous insects living on them such as aphids, whiteflies, pseudococcids, and lepidopterans. Currently, this species has a wide but not precise distribution. In this work is reported for first time the presence of E. varians in Mexico and the predation on nymphs of tomato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae) under greenhouse conditions. A fourth instar of E. varians fed 46% of third instar of B. cockerelli in a period of 24 h. This result showed that this predator could be a promising agent for management for tomato psyllid. Data of its distribution, host plants, and preys are also included. Key words: Potato or tomato psyllid, native natural enemies, mirids as zoophytophagous.
Phenotypic plasticity results in a diversity of phenotypes from a single genotype in response to environmental cues. To understand the molecular basis of phenotypic plasticity, studies have focused ...on differential gene expression levels between environmentally determined phenotypes. The extent of alternative splicing differences among environmentally determined phenotypes has largely been understudied. Here, we study alternative splicing differences among plastically produced morphs of the pea aphid using RNA-sequence data. Pea aphids express two separate polyphenisms (plasticity with discrete phenotypes): a wing polyphenism consisting of winged and wingless females and a reproduction polyphenism consisting of asexual and sexual females. We find that pea aphids alternatively splice 34% of their genes, a high percentage for invertebrates. We also find that there is extensive use of differential spliced events between genetically identical, polyphenic females. These differentially spliced events are enriched for exon skipping and mutually exclusive exon events that maintain the open reading frame, suggesting that polyphenic morphs use alternative splicing to produce phenotype-biased proteins. Many genes that are differentially spliced between polyphenic morphs have putative functions associated with their respective phenotypes. We find that the majority of differentially spliced genes is not differentially expressed genes. Our results provide a rich candidate gene list for future functional studies that would not have been previously considered based solely on gene expression studies, such as ensconsin in the reproductive polyphenism, and CAKI in the wing polyphenism. Overall, this study suggests an important role for alternative splicing in the expression of environmentally determined phenotypes.
Sitobion miscanthi is an ideal model for studying host plant specificity, parthenogenesis-based phenotypic plasticity, and interactions between insects and other species of various trophic levels, ...such as viruses, bacteria, plants, and natural enemies. However, the genome information for this species has not yet to be sequenced and published. Here, we analyzed the entire genome of a parthenogenetic female aphid colony using Pacific Biosciences long-read sequencing and Hi-C data to generate chromosome-length scaffolds and a highly contiguous genome assembly.
The final draft genome assembly from 33.88 Gb of raw data was ∼397.90 Mb in size, with a 2.05 Mb contig N50. Nine chromosomes were further assembled based on Hi-C data to a 377.19 Mb final size with a 36.26 Mb scaffold N50. The identified repeat sequences accounted for 26.41% of the genome, and 16,006 protein-coding genes were annotated. According to the phylogenetic analysis, S. miscanthi is closely related to Acyrthosiphon pisum, with S. miscanthi diverging from their common ancestor ∼25.0-44.9 million years ago.
We generated a high-quality draft of the S. miscanthi genome. This genome assembly should help promote research on the lifestyle and feeding specificity of aphids and their interactions with each other and species at other trophic levels. It can serve as a resource for accelerating genome-assisted improvements in insecticide-resistant management and environmentally safe aphid management.
Bacteriophages are known to carry key virulence factors for pathogenic bacteria, but their roles in symbiotic bacteria are less well understood. The heritable symbiont Hamiltonella defensa protects ...the aphid Acyrthosiphon pisum from attack by the parasitoid Aphidius ervi by killing developing wasp larvae. In a controlled genetic background, we show that a toxin-encoding bacteriophage is required to produce the protective phenotype. Phage loss occurs repeatedly in laboratory-held H. defensa-infected aphid clonal lines, resulting in increased susceptibility to parasitism in each instance. Our results show that these mobile genetic elements can endow a bacterial symbiont with benefits that extend to the animal host. Thus, phages vector ecologically important traits, such as defense against parasitoids, within and among symbiont and animal host lineages.
Many insects harbour facultative symbiotic bacteria, some of which have been shown to provide resistance against natural enemies. One of the best-known protective symbionts is Hamiltonella defensa, ...which in pea aphid (Acyrthosiphon pisum) confers resistance against attack by parasitoid wasps in the genus Aphidius (Braconidae). We asked (i) whether this symbiont also confers protection against a phylogenetically distant group of parasitoids (Aphelinidae) and (ii) whether there are consistent differences in the effects of bacteria found in pea aphid biotypes adapted to different host plants. We found that some H. defensa strains do provide protection against an aphelinid parasitoid Aphelinus abdominalis. Hamiltonella defensa from the Lotus biotype provided high resistance to A. abdominalis and moderate to low resistance to Aphidius ervi, while the reverse was seen from Medicago biotype isolates. Aphids from Ononis showed no evidence of symbiont-mediated protection against either wasp species and were relatively vulnerable to both. Our results may reflect the different selection pressures exerted by the parasitoid community on aphids feeding on different host plants, and could help explain the maintenance of genetic diversity in bacterial symbionts.
Plant-mediated RNA interference (RNAi) has been successfully used as a tool to study gene function in aphids. The persistence and transgenerational effects of plant-mediated RNAi in the green peach ...aphid (GPA) Myzus persicae were investigated, with a focus on three genes with different functions in the aphid. Rack1 is a key component of various cellular processes inside aphids, while candidate effector genes MpC002 and MpPIntO2 (Mp2) modulate aphid–plant interactions. The gene sequences and functions did not affect RNAi-mediated down-regulation and persistence levels in the aphids. Maximal reduction of gene expression was ~70% and this was achieved at between 4 d and 8 d of exposure of the aphids to double-stranded RNA (dsRNA)-producing transgenic Arabidopsis thaliana. Moreover, gene expression levels returned to wild-type levels within ~6 d after removal of the aphids from the transgenic plants, indicating that a continuous supply of dsRNA is required to maintain the RNAi effect. Target genes were also down-regulated in nymphs born from mothers exposed to dsRNA-producing transgenic plants, and the RNAi effect lasted twice as long (12–14 d) in these nymphs. Investigations of the impact of RNAi over three generations of aphids revealed that aphids reared on dsMpC002 transgenic plants experienced a 60% decline in aphid reproduction levels compared with a 40% decline of aphids reared on dsRack1 and dsMpPIntO2 plants. In a field setting, a reduction of the aphid reproduction by 40–60% would dramatically decrease aphid population growth, contributing to a substantial reduction in agricultural losses.
PDF
