Identification of a gene overexpressed in aphids reared under short photoperiod

Most aphids develop a cyclic parthenogenesis life-cycle. After several generations of viviparously produced parthenogenetic females, follows a single annual generation of sexual individuals, usually in autumn, that mate and lay the sexual eggs. Shortening of photoperiod at the end of the summer (together with temperature) is a key factor inducing the sexual response. Currently no genes involved in the cascade of events that lead to the appearance of sexual forms have been reported. After a Differential Display RT-PCR survey performed on Acyrthosiphon pisum aphids, we identified a gene that is overexpressed in aphids reared under short photoperiod conditions that induce sexuality in this species. This cDNA (called ApSDI-1) shows similarities with a protein involved in amino acid transport in GABAergic neurons. Since several studies implicate GABAergic transmission in the generation and modulation of circadian rhythmicity, we propose that ApSDI-1 could be involved in the transduction of the photoperiodic message and therefore be a candidate to participate at some point in processes that trigger the sexual response in aphids. This is the first gene identified in aphids whose expression is governed by the photoperiod.     

Melatonin in the seasonal response of the aphid Acyrthosiphon pisum

Aphids display life cycles largely determined by the photoperiod.During the warm long-day seasons.most aphid species reproduce by viviparous parthenogenesis.The shortening of the photoperiod in autumn induces a switch to sexual reproduction.Males and sexual females mate to produce overwintering resistant eggs.In addition to this full life cycle(holocycle),there are anholocyelic lineages that do not respond to changes in photoperiod and reproduce continuously by parthenogenesis.The molecular or hormonal events that trigger the scasonal response(i.c,induction of the sexual phenotypes)are still unknown.Although circadian synthesis of melatonin is known to play a key role in vertebrate photoperiodism,the involvement of the circadian clock and/or of the hor-mone melatonin in insect seasonal responses is not so well established.Here we show that melatonin levels in the aphid Acyrthosiphon pisum are significantly higher in holocyclice aphids reared under short days than under long days,while no differences were found between anholoeyelic aphids under the same conditions.We also found that melatonin is localized in the aphid suboesophageal ganglion(SOG)and in the thoracic ganglionic mass(TGM).In analogy to vertcbrates,insect-type arylalkxylamine N-acetyltransferases(i-AANATs)are thought to play a key role in melatonin synthesis.We measured the expression of four I-AANAT genes identified in A.pisum and localized two of them in situ in the insect central nervous systems(CNS).Levels of expression of these genes were compatible with the quantities of melatonin observed.Moreover,like melatonin,expression of these genes was found in the SOG and the TGM.     

Heritable genetic variation and potential for adaptive evolution in asexual aphids (Aphidoidea)

Aphid life cycles can encompass cyclical parthenogenesis, obligate parthenogenesis, obligate parthenogenesis with male production and an intermediate 'bet-hedging' strategy where an aphid genotype will over-winter by continuing to reproduce by parthenogenesis and by investment in sexually produced eggs. In this paper, we focus on aphid lineages that reproduce entirely parthenogenetically (asexual aphids), in contrast to those that have any sexual forms in the annual cycle. Using modern molecular techniques, aphid biologists have made many empirical observations showing that asexual lineages are widespread both geographically and temporally. Indeed, we are collectively beginning to gather data on the evolution and persistence of these lineages through time. Here we review aphid karyology and parthenogenesis, both essential for interpretation of the molecular and ecological evolution of aphid asexual lineages. We describe the growing list of studies that have identified aphid genotypes that are both temporally and geographically widespread. We then collate examples of molecular and chromosomal evolution in asexual aphids and review the literature pertaining to phenotypic evolution and ecological diversification of asexual aphid lineages. In addition, we briefly discuss the potential of bacterial endosymbionts and epigenetic effects to influence the evolution of asexual aphid lineages. Lastly we provide a list of aphid taxa believed to be obligately asexual. This will be a useful resource for those seeking parthenogenetic animals as study systems. In conclusion, we present guidelines for the use of the term clone in aphid biology and stress the need for well-designed and well-executed studies examining the potential of asexual aphid lineages for adaptive evolution.  © 2003 The Linnean Society of London. Biological Journal of the Linnean Society , 2003, 79 , 115–135.     

Inducing the sexual forms and hatching the eggs of pea aphids

In temperate climates, pea aphids (Acyrthosiphon pisum) produce a single sexual generation each year in response to declining photoperiod and temperature. Mating occurs in the fall and the eggs have an obligatory winter diapause. Genetic recombination during the sexual phase is thought to be an important source of genetic variability within cyclically parthenogenetic aphid populations. Methods for reliably producing sexual forms and hatching the eggs of aphids are therefore central not only to the study of evolutionary change in aphid populations, but also for a general understanding of the origin of agriculturally important variation in destructiveness within pest species.Here, sexual forms of six pea aphid clones were induced in the laboratory and eggs were successfully hatched by creating conditions that closely mimicked those found in field situations. A declining photoperiod was produced by controlling artificial lighting using a timer with variable cycle length. Using these conditions, sexual forms were successfully produced for all six clones tested, which were then mated in all combinations. Eggs were exposed to a daily cycle of freezing and thawing in an incubator under a short-day photoperiod. Egg hatch averaged 60%, but was as high as 89% for some crosses. These methods will permit testing of evolutionary hypotheses and execution of detailed genetic studies of sources of variability within pea aphid populations. They are thus important tools for both evolutionary and agricultural studies.     

Facultative symbiont infections affect aphid reproduction

Some bacterial symbionts alter their hosts reproduction through various mechanisms that enhance their transmission in the host population. In addition to its obligatory symbiont Buchnera aphidicola, the pea aphid Acyrthosiphon pisum harbors several facultative symbionts influencing several aspects of host ecology. Aphids reproduce by cyclical parthenogenesis whereby clonal and sexual reproduction alternate within the annual life cycle. Many species, including the pea aphid, also show variation in their reproductive mode at the population level, with some lineages reproducing by cyclical parthenogenesis and others by permanent parthenogenesis. While the role of facultative symbionts has been well studied during the parthenogenetic phase of their aphid hosts, very little is known on their possible influence during the sexual phase. Here we investigated whether facultative symbionts modulate the capacity to produce sexual forms in various genetic backgrounds of the pea aphid with controlled symbiont composition and also in different aphid genotypes from natural populations with previously characterized infection status and reproductive mode. We found that most facultative symbionts exhibited detrimental effects on their hosts fitness under sex-inducing conditions in comparison with the reference lines. We also showed that the loss of sexual phase in permanently parthenogenetic lineages of A. pisum was not explained by facultative symbionts. Finally, we demonstrated that Spiroplasma infection annihilated the production of males in the host progeny by inducing a male-killing phenotype, an unexpected result for organisms such as aphids that reproduce primarily through clonal reproduction.     

Stability and variation in aphid clonal lineages

Evidence in the literature purporting to show genetic variability within parthenogenetic lines of aphids is reviewed. The results of three experimental approaches to this problem are reported: (1) a long term study of fluctuation in alate morph production in six lines of Myzus persicae; (2) an attempt tp select for caudal hair form in Acyrthosiphon pisum; (3) a study of the inheritance of esterase variants of M. persicae in sexual and parthenogenetic reproduction. It is concluded that genetic recombination during parthenogenesis is not the general rule in diese aphids, and that aphid parthenogenesis should be regarded as of ameiotic or apomictic type. Possible alternative explanations for the different kinds of variation or apparent variation which occur within aphid clonal lineages are discussed.     

Differential gene expression of the honey bees Apis mellifera and A. cerana induced by Varroa destructor infection

Varroa destructor mite is currently the most serious threat to the world bee industry. Differences in mite tolerance are reported between two honey bee species Apis mellifera and Apis cerana. Differential gene expression of two honey bee species induced by V. destructor infection was investigated by constructing two suppression subtractive hybridization (SSH) libraries, as first steps toward elucidating molecular mechanisms of Varroa tolerance. From the SSH libraries, we obtained 289 high quality sequences which clustered into 132 unique sequences grouped in 26 contigs and 106 singlets where 49 consisted in A. cerana subtracted library and 83 in A. mellifera. Using BLAST, we found that 85% sequences had counterpart known genes whereas 15% were undescribed. A Gene Ontology analysis classified 51 unique sequences into different functional categories. Eight of these differentially expressed genes, representative of different regulation patterns, were confirmed by qRT-PCR. Upon the mite induction, the differentially expressed genes from both bee species were different, except hex 110 gene, which was up-regulated in A. cerana but down-regulated in A. mellifera, and Npy-r gene, which was down-regulated in both species. In general, most of the differential expression genes were involved in metabolic processes and nerve signaling. The results provide information on the molecular response of these two bee species to Varroa infection.     

Suppression of autumnal sexual morph production in spring by a seasonal timer in an aphid

Photoperiod is the major seasonal cue for phenotypic plasticity in the regulation of development and reproduction. Aphids have a peculiar mechanism in which sensitivity to the photoperiod is lost for a few months through successive generations after hatching from diapause eggs, and therefore, the mechanism responsible for the photorefractory period is termed “a seasonal timer”. However, whether the seasonal timer has adaptive significance has never been investigated. Here, we show that the photorefractory generation avoids an unseasonal sexual morph production under short days in spring in the pea aphid, Acyrthosiphon pisum (Harris). When the seasonal timer operated, the overwintered generation, stem mothers, produced only parthenogenetic progenies under natural photoperiods and temperatures in both warm and cold springs. After repeated parthenogenesis following diapause, however, aphids with an expired seasonal timer produced sexual females and males typical of autumn under natural photoperiods and temperatures in a warm spring. These sexual morphs are thought to have a reproductive disadvantage compared to parthenogenetic morphs in the following summer. We thus conclude that the seasonal timer is adaptive for avoiding sexual morph production in spring. The present study shows, for the first time, that a photorefractory period lasting over generations is a trait of ecological importance.     

The molecular bases of plant resistance and defense responses to aphid feeding: current status

Plant genes participating in the recognition of aphid herbivory in concert with plant genes involved in defense against herbivores mediate plant resistance to aphids. Several such genes involved in plant disease and nematode resistance have been characterized in detail, but their existence has only recently begun to be determined for arthropod resistance. Hundreds of different genes are typically involved and the disruption of plant cell wall tissues during aphid feeding has been shown to induce defense responses in Arabidopsis, Triticum, Sorghum, and Nicotiana species. Mi‐1.2, a tomato gene for resistance to the potato aphid, Macrosiphum euphorbiae (Thomas), is a member of the nucleotide‐binding site and leucine‐rich region Class II family of disease, nematode, and arthropod resistance genes. Recent studies into the differential expression of Pto‐ and Pti1‐like kinase genes in wheat plants resistant to the Russian wheat aphid, Diuraphis noxia (Mordvilko), provide evidence of the involvement of the Pto class of resistance genes in arthropod resistance. An analysis of available data suggests that aphid feeding may trigger multiple signaling pathways in plants. Early signaling includes gene‐for‐gene recognition and defense signaling in aphid‐resistant plants, and recognition of aphid‐inflicted cell damage in both resistant and susceptible plants. Furthermore, signaling is mediated by several compounds, including jasmonic acid, salicylic acid, ethylene, abscisic acid, giberellic acid, nitric oxide, and auxin. These signals lead to the development of direct chemical defenses against aphids and general stress‐related responses that are well characterized for a number of abiotic and biotic stresses. In spite of major plant taxonomic differences, similarities exist in the types of plant genes expressed in response to feeding by different species of aphids. However, numerous differences in plant signaling and defense responses unique to specific aphid–plant interactions have been identified and warrant further investigation.     

The genetic outcomes of sex and recombination in long-term functionally parthenogenetic lineages of Australian Sitobion aphids

The typical life cycle of an aphid is cyclical parthenogenesis which involves the alternation of sexual and asexual reproduction. However, aphid life cycles, even within a species, can encompass everything on a continuum from obligate sexuality, through facultative sexuality to obligate asexuality. Loss of the sexual cycle in aphids is frequently associated with the introduction of a new pest and can occur for a number of environmental and genetic reasons. Here we investigate loss of sexual function in Sitobion aphids in Australia. Specifically, we aimed to determine whether an absence of sexual reproduction in Australian Sitobion results from genetic loss of sexual function or environmental constraints in the introduced range. We addressed our aims by performing a series of breeding experiments. We found that some lineages have genetically lost sexual function while others retain sexual function and appear environmentally constrained to asexuality. Further, in our crosses, using autosomal and X-linked microsatellite markers, we identified processes deviating from normal Mendelian segregation. We observed strong deviations in X chromosome transmission through the sexual cycle. Additionally, when progeny genotypes were examined across multiple loci simultaneously we found that some multilocus genotypes are significantly over-represented in the sample and that levels of heterozygosity were much higher than expected at almost all loci. This study demonstrates that strong biases in the transmission of X chromosomes through the sexual cycle are likely to be widespread in aphids. The mechanisms underlying these patterns are not clear. We discuss several possible alternatives, including mutation accumulation during periods of functional asexuality and genetic imprinting.     

Morphological and ecological traits promoting aphid colonization of the Hawaiian Islands

Species introductions into novel habitats, especially island ecosystems, can have devastating effects on ecosystem function and stability. Though none are native, at least 96 aphid species can now be found on one or more of the Hawaiian Islands. As aphids cause direct feeding damage and transmit plant viruses, it is important to identify the traits that have enabled these particular species to successfully colonize the archipelago. To address this question, nine morphological and ecological traits that may contribute to successful colonization were assessed for aphids present in Hawaii. As a comparative null model, we assessed the same traits for heterospecific congeners which are not present in the archipelago, but are present elsewhere in the world. Here we report that traits with higher frequencies among colonizing aphid species are: small apterae size, broad host range, anholocycly (i.e., permanent parthenogenesis), and presence in continental USA. Small aphids arriving from the mainland US and capable of feeding on numerous plant species may be intercepted less often by plant protection agents. It is also likely that asexually reproducing species are well suited to the Hawaiian subtropical climate, thereby eliminating the need for sexual phases and egg-laying for overwintering. By understanding the traits that enable aphids to successfully colonize remote islands, it is our hope that plant protection efforts may be enhanced, thereby reducing damage to native ecosystems.     

Evolutionary and functional insights into reproductive strategies of aphids

Aphids are among the few organisms capable of reproducing either sexually or asexually. This plasticity in reproductive mode is viewed as an adaptive response to cope with seasonal changes. Clonal reproduction occurs during the growing season allowing rapid population increase, while sexual reproduction occurs during late summer and leads to frost-resistant eggs that can survive winter conditions. This shift between these two extreme reproductive modes is achieved by using the same genotype, i.e. within the same genetic clone, and is triggered by photoperiodic changes perceived by the aphid brain or visual system. Advances have been made recently to depict genetic programs that relate to the regulation of reproductive modes in aphids. These studies have benefited from the rapid development of genomic and post-genomic resources obtained through the International Aphid Genomics Consortium. Here, we underline the importance of several candidate genes in the switch from clonal to sexual reproduction in aphids and whose roles await full validation. Besides reproductive mode variation expressed at the genotypic level, aphid species also frequently encompass lineages which have lost the sexual phase and hence the alternating clonal and sexual reproductive phases of the life cycle. This coexistence of sex and asexual reproduction within the same species raises questions on its evolutionary and ecological significance. We summarize the knowledge accumulated to date on the maintenance of sex as well as on the origin and evolution of asexuality in aphids. By combining functional genomics, genetic and ecological approaches on reproductive plasticity and polymorphism, we hope to obtain an integrative view of the evolutionary forces shaping aphid reproductive strategies, from gene to population and species levels.     

Microarray analysis of the interaction between the aphid <Emphasis Type="Italic">Rhopalosiphum padi</Emphasis> and host plants reveals both differences and similarities between susceptible and partially resistant barley lines

The bird cherry-oat aphid (Rhopalosiphum padi L.) is an important pest on cereals causing plant growth reduction without specific leaf symptoms. Breeding of barley (Hordeum vulgare L.) for R. padi resistance shows that there are several resistance genes, reducing aphid growth. To identify candidate sequences for resistance-related genes, we performed microarray analysis of gene expression after aphid infestation in two susceptible and two partially resistant barley genotypes. One of the four lines is a descendant of two of the other genotypes. There were large differences in gene induction between the four lines, indicating substantial variation in response even between closely related genotypes. Genes induced in aphid-infested tissue were mainly related to defence, primary metabolism and signalling. Only 24 genes were induced in all lines, none of them related to oxidative stress or secondary metabolism. Few genes were down-regulated, with none being common to all four lines. There were differences in aphid-induced gene regulation between resistant and susceptible lines. Results from control plants without aphids also revealed differences in constitutive gene expression between the two types of lines. Candidate sequences for induced and constitutive resistance factors have been identified, among them a proteinase inhibitor, a serine/threonine kinase and several thionins. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

棉花型和瓜型棉蚜产生有性世代能力的分化

为了明确寄主专化性是否会影响棉蚜的繁殖策略或生活史对策,采用低温和短光照组合（18℃,L∶D=8∶16）分别对棉花型和瓜型棉蚜进行有性世代的诱导,比较两寄主专化型棉蚜产生有翅蚜、雄性母、雌性母及无翅孤雌蚜的能力。结果表明：两寄主专化型棉蚜在产生有性世代能力上存在显著差异,表现为棉花型棉蚜在诱导后代中产生有翅蚜、雄性母和雌性母的比率显著高于瓜型棉蚜,并且雌、雄性母产生的时间明显早于瓜型棉蚜。瓜型棉蚜在诱导条件下产生无翅孤雌蚜的比率显著高于棉花型棉蚜,并且发现有不产生有性世代的营专性孤雌生殖个体,而在棉花型棉蚜中没有发现。产生性母蚜的棉花型和瓜型棉蚜均可同时产生孤雌胎生后代。寄主型与诱导时间长短对棉蚜有性世代的产生存在交互作用。采集于田间棉花和黄瓜上的棉蚜,也表现为棉花上的易产生有性世代,在诱导的第2代中产生性母蚜的比率显著高于黄瓜上的棉蚜,并且性母蚜比率在诱导的第2代与第3代间无显著差异; 但黄瓜上的棉蚜性母蚜产生比率第3代显著高于第2代。由此推测,棉蚜寄主专化性的形成与生活史特性的分化有关。