A new aphid species arrived in Europe

As Carlos Hernández-Castellano and Nicolás Pérez Hidalgo reported in the journal Redia, it seems that a new invasive aphid species arrived in Europe. This aphid, called Sipha flava, is native to North America, although it has achieved to expand throughout South and Central America, where it is known as “the yellow sugarcane aphid,” and is an important pest of this crop leading to yield reduction (photo source here).

The yellow sugarcane aphid is usually lemon yellow, but under some conditions is pale green, it is 2 mm long, covered with short, black spines, and has two double rows of dark spots on its back. Cornicles are very short. Winged and wingless forms live in the colony.

Sugarcane is rather marginal in European continent, so Sipha flava is not expected to become a sugarcane pest in Europe, however, it could be important to map its spread since it may feed on several species from the same family and it is unknown to what extent this aphid species could represent a threat to some crops in Europe, such as rice or corn.

As stated by Authors:

We found four colonies of Sipha on four different plants of the common thatching grass Hyparrhenia hirta (L.) Stapf in Oliver in an organic citrus grove located in La Selva del Camp (Tarragona, NE Spain 41º13’07’’N, 01º08’35’’E) during a sampling campaign (17th June, 2014) in the context of a scientific project about arthropod trophic webs, and two colonies on the same host-plant in Blanes (Gerona 41º40’40’’N, 2º48’22’’E) on 28th June, 2014. The colonies of aphids were attended by the ants Pheidole pallidula Nylander and Plagiolepis pygmaea Latreille in La Selva del Camp and Blanes, respectively.

Interestingly, many predators feed on the yellow sugarcane aphid, but they are rarely parasitized so that ants could protect them from natural enemies.

Few days ago I was at a workshop about the defence of honey bees against the increasing threat of invasive species, a booming phenomenon caused by globalisation, leading not only to agricultural issues but also rising as the second cause of biodiversity loss in the world, just after habitat destruction in importance. It seems that also aphids would like to have a chance for invading new countries!!

The Scent of Fear – the aphid alarm pheromone

Originally posted on Don't Forget the Roundabouts:

We are all familiar with the effects of epinephrine (adrenaline) and norepinephrine (noradrenaline) on us when placed in a position of stress, such as public speaking or even worse danger.  We flush, shake, our heart rate accelerates and many of us we begin to sweat profusely, thus visibly advertising our distress; sometimes embarrassingly so

Sweating nervously

if we have an antiperspirant  fail and happen to be wearing a dark shirt.

man_giving_speech

http://www.silkyskin.co.uk/blog/wp-content/uploads/2012/07/man_giving_speech.jpg

Those seeing these symptoms may feel a degree of sympathy for the victim, but do not usually flee the scene, although they may sometimes feel tempted to do so.

The case with aphids is very different.   Aphids, when perceiving a threat to their neighbours by a predator or parasite, flee the scene rapidly, by flight, if winged, on foot if not, or even by leaping from their host-plant to the ground below.  The pea aphid, Acyrthosiphon pisum walks away or drops from their…

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Transferring primary symbionts: a missed link?

figure 1_Mandrioli_ManicardiOne of the most studied example of symbiotic interaction is related to aphids and their symbiont Buchnera aphidicola  that they host in specialized cells called bacteriocytes. Each aphid may host about 6 millions of Buchnera cells that are involved in the continuous overproduction of tryptophan and other amino acids.

In addition to obligate symbionts, exemplified by Buchnera, aphids contain several facultative bacterial symbionts that are mostly maternally transmitted and that may be readily transferred between hosts in laboratory experiments, since they can also have an independent life in view of their genome that still contain all the genes necessary for growth and replication. In contrast, exclusively maternally transmitted and obligate symbionts, such as Buchnera, have never faced with the challenge of invading new insects since they possess a reduced genome that lost several genes required, for example, for anaerobic respiration, synthesis of amino sugars, fatty acids, phospholipids and complex carbohydrates. As a consequence, Buchnera has resulted in one of the smallest known genomes of any living organism.

Up till now, an unanswered question is related to the extent of the reciprocal coadaptation of host and symbiont lineages and this why I found an intriguing reading the recent paper of Nancy Moran in PNAS related to the chance of transferring Buchnera symbionts between two different aphid lineages. Indeed as Moran and You wrote in their paper:

Potentially, the rapid evolution of symbiont genes and genomes is tracked by adaptive compensatory changes in host genes, accelerating the establishment of incompatibilities between symbiont and host genotypes from different host matrilines. If so, intimate symbioses might facilitate reproductive isolation and speciation. If beneficial symbionts such as Buchnera are generally interchangeable among related host species, this would suggest the lack of an evolutionary “arms race in these mutualistic relationships.

In order to better understand the state-of-art of this symbiotic relationship in aphids, Moran and Yun planned to exchange Buchnera bacteria between two aphid lineages. In parthenogenetic aphid generations, embryos begin development before their mothers are mature, so embryos ready for colonization are present in the injected juvenile aphids. The success of the transfers indicates an ability of the donor Buchnera to invade the posterior syncytium of these developing embryos and to be packaged normally during subsequent development and inherited stably in subsequent generations. In view of this intimate link, Buchnera and aphid matrilines are always strongly dependent.

Immagine 1In order to face this topics, Moran and Yun used two matrilines of the pea aphid Acyrthosiphon pisum, where the recipient line (the clone LSR1) contains a heat-sensitive Buchnera genotype, and the donor (clone 5AY) contains a heat-tolerant Buchnera genotype. In their study, they depleted native Buchnera by heat in the recipient line and donor Buchnera have been microinjected in the hemocoel of the recipient line in order to observe the results (as schematized at left in the image from the Moran paper in PNAS).

As reported in the paper, most embryos have been successfully colonized by the donor symbionts and in some cases, a complete replacement occurs in the progeny of injected females. In other cases, progeny have a mixed Buchnera population, which can be shifted completely to the donor type through further heat exposure. These results are very intriguing this Moran and Yun disrupted 100 million years (∼1 billion aphid generations) of continuous maternal transmission of Buchnera in its specific host aphid lineage showing that symbiotic relationship seems to occur between Buchnera and A. pisum as a whole without any specific genetic difference between aphid lineage genomes and/or in the Buchnera genomes. Lastly, aphids with the Buchnera replacement showed a strong increase in their heat tolerance demonstrating an effect of symbiont genotype on host ecology and the presence of a cross talk between aphids and their new Buchnera cells.

 

ResearchBlogging.orgMoran NA, & Yun Y (2015). Experimental replacement of an obligate insect symbiont. Proceedings of the National Academy of Sciences of the United States of America PMID: 25561531

 

Fertilizers and aphid growth

ResearchBlogging.org Plant vigour, health and growth rate are intensively managed by farmers to maximize production through the application of nitrogen fertilizers that also influence the growth of phytophagous insect populations because nitrogen is a macronutrient known to be limiting for them.

The effects of fertilizers reported in literature are quite controversial since fertilizers can make faster insect population growth, but they can also have detrimental effects to some phytophagous insects possibly as a result of nutrient imbalance. Indeed, most insects actively maintain close to optimal levels of macronutrients in their tissues, so that changing the ratio of macronutrients in plants can affect insect feeding and performance affecting herbivore abundance.

Aphids are important agricultural pests in many crops and, based on observations from both natural and agricultural systems, they seem to be positively affected by the nitrogen amount in the nutritionally poor phloem sap that they consume. Differently to what reported in aphids, chewing insects, such as grasshoppers and moths, are more likely to be negatively affected by increased defensive compounds that they ingest with leaf tissue. The diversity of insect responses to nitrogen that have been observed suggested that additional research in this area is needed to improve crop management.

B74gTdSCUAAhExRAn interesting set of data has been recently published by Kevi C. Mace and Nicholas J. Mills in the journal Agricultural and Forest Entomology studying the response of walnut aphid Chromaphis juglandicola populations to different levels of nitrogen application to walnut seedlings and the relationship between aphid density and chlorophyll content index (CCI), a non-invasive measure of foliar nitrogen.

Although added nitrogen significantly increased soluble nitrogen content, there was no effect on walnut aphid population growth suggesting that walnut aphid population growth on potted seedlings is limited by factors other than soluble nitrogen, such as amino acids. This suggestion is also supported by data assessing that there wasn’t any difference not only in the population growth, but also in aphid weights and in the age structure of the populations. This is an unusual finding because aphids, including some tree aphids, generally show a positive or dome-shaped response (measured either as increased life-history performance in laboratory studies or as greater abundance in field studies) to nitrogen fertilizer.

A relevant role in the regulation of the C. juglandicola growth could be played also by the production of plant defensive compounds so that it is possible that plant defence, rather than plant nutrition, was responsible for the apparent absence of bottom-up effects on walnut aphid populations in the present study because it has been shown that plant defence can vary with level of nitrogen fertilizer. Walnut foliage, for instance, is known to contain juglone, which is toxic to many insects. Being specialists, walnut aphids may even benefit from juglone or possess a sort of resistance so that a further study of defensive compounds and their relationship to fertilizer application and insect populations is needed to determine whether plant defences could drive the dynamics of walnut aphid populations.

The absence of beneficial effects of fertilizers in aphids is also accompanied by the absence of negative effects so that fertilizer management appears unlikely to effectively contribute to the control of C. juglandicola in commercial walnut orchards.

Mace, K., & Mills, N. (2015). Response of walnut aphid populations to increasing foliar nitrogen content Agricultural and Forest Entomology DOI: 10.1111/afe.12103

A Winter’s Tale – aphid overwintering

Originally posted on Don't Forget the Roundabouts:

Aphids that live in temperate or boreal regions have to be able to survive overwinter. Aphids, depending on species, are able to pass winter in two ways. If they are holocyclic i.e. possess an egg-laying stage, they usually overwinter as eggs. Aphid eggs are extremely cold-hardy; they have been reported to have super-cooling points of about -42oC (Somme ). If laid on a woody host, eggs are usually laid in the bud axils as in the case of the apple aphid, Aphis pomi, the black bean aphid Aphis fabae and the bird cherry aphid, Rhopaloishum padi.

aphid eggs

In some instances, such as the sycamore aphid, Drepanosiphum platanoidis, eggs are laid directly on the tree bark or in crevices in the bark or even in lichen growing on the bark.  See if you can spot the eggs in the picture below.

ovipsyc

If however, the aphid in question lives on an herbaceous host, the eggs…

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Symbiomes: when symbionts go to the congress

Symbiosis plays a very important role in the origin, organization and evolution of life and it is now evident that we live in a symbiotic world, where the key characteristic of the biological systems is to establish associations and connections with other organisms. The practical implications of this new approach are enormous, and largely unexplored and rwe have to dissect the different aspects of the interaction of the microbiota with the host in humans and other model systems through the interdisciplinary effort of experts from different fields including molecular and cellular biology, microbiology, immunology, computational biology, bioinformatics, genetics, population genetics, and epidemiology.

In order to facilitate interactions among scientists involved in the study of microbiomes, Claudio Donati (Fondazione Edmund Mach, Italy), Duccio Cavalieri (Fondazione Edmund Mach, Italy) and Marco Soriani (Novartis Vaccies and Diagnostics) organize a meeting to provide a discussion forum for experts in these different fields to foster new ideas for interdisciplinary research.

The conference, entitled Symbiomes: system biology of host-microbiome interactions, will cover different topics including:

  1. The healthy human commensal flora and its alterations
  2. Homeostasis between microbiota and immune system
  3. An ecological perspective of host-microbe interactions
  4. Environmental metagenonmics
  5. Soil and plant microbiome
  6. Therapeutic and prentive manipulation of microbiota
  7. Foresight

The conference will be held at Polonia Castle (Dom Polonii) in Pultusk, Poland in days 5-10 june 2015.

Defensive symbiosis

colemaniThe parasitoid wasps of aphids belong to the sub-family Aphidiinae (Hymenoptera: Ichneumonoidea: Braconidae) and to the genus Aphelinus (Hymenoptera: Chalcidoidea: Aphelinidae) and, as a whole, they consist in more than 400 species able to inject their eggs into the aphid body. The parasitoid larva hatches soon after oviposition and develops through three larval instars inside the still active host, which they kill prior to pupation.

Since the successful development of the parasitoid is always fatal to the parasitized host, aphids have evolved both behavioral defenses (such as kicking or dropping off the plant to avoid parasitoid oviposition) and physiological defenses to prevent parasitoid development after oviposition. According to literature data, behavioral resistance reduces parasitoid oviposition rate, whereas physiological resistance is fatal to the parasitoid’s egg or larva.

As never thought for several decades physiological resistance to parasitoids in aphids is due to the presence of defensive symbionts rather than to genes encoded by the aphid genome. The first evidences of the occurrence of defensive symbionts have by published by Oliver et al. (2003) reporting that experimental infection with the two species of facultative bacterial symbionts Hamiltonella defensa and Serratia symbiotica increased the resistance of pea aphids Acyrthosiphon pisum to the parasitoid Aphidius ervi. As recently revised by Christoph Vorburger in Insect Science, there is now a wide set of papers reporting that symbiont-conferred resistance to parasites and pathogens is an important and widespread phenomenon not only in aphids but also in other insects.

Interestingly, the protective mechanism of H. defensa is based is not due to the bacteria alone, but the major role is played by temperate bacteriophages called Acyrthosiphon pisum secondary endosymbionts (APSEs) that infect the aphid symbiont. according to different experimental evidences, APSEs encode toxins that kill the parasitoid egg or larva and thereby protect the aphid host (Moran et al 2005).
Recently, a third bacterial species able to confer a protection against the parasitoid Aphidius ervi has been identified and this species (named Regiella insecticola) seems to be involved also in the protection against  entomopathogenic fungi making defensive symbiosis more and more intriguing from an evolutionary point of view. Surprisingly, sequencing of R. insecticola genome revealed that the APSE phages were absent in this bacterial species and that the resistance was due to five categories of pathogenicity factors so that it appears that different symbionts have found different solutions to the same evolutionary challenge. At presence it is not clear how S. symbiotica protect aphids but it would be not surprising to discover that a third mechanism occurs.

Considering the strong selective advantage of an increased resistance to parasitoids, it is really surprising that most of the surveys found these bacteria to occur at low or intermediate frequencies so that aphids possessing defensive symbionts do not go to fixation in natural populations. As Vorgurger explained in his recent review, this result could be due to the balance of selective benefits and costs conferred by the symbionts as well as the balance between symbiont losses and gains that determine their frequency in a population. Different elements could be part of this balance and, for instance, it has been observed that symbiont-conferred resistance against parasitoids is reduced under heat stress suggesting that defensive symbionts of aphids may be suppressed or even eliminated during hot summer days. In view of the near-perfect fidelity of the vertical transmission of symbiotic bacteria in the parthenogenetic generations, the loss of defensive symbionts could be due to the cost of harboring these bacterial species. In particular, we can speculate a relevant cost since these bacteria provide aphids with a strong protection against parasitoid wasps. As reported in some papers defensive synbiosis shortened aphid lifespan probably as a consequence of the metabolic demands imposed by the presence of a large bacterial population in the host or in view of the costs of immune activation in the presence of symbionts, or it may be due to “collateral damages” to the host resulting from the symbiont’s production of toxins.

At the same time it is now quite clear that some parasitoid species, such as Aphidius ervi, are able to detect the presence of H. defensa (in the left photo from bacmap) in pea aphids and respond by laying two or more eggs in infected aphids to increase the chance of successful parasitism despite the defensive symbiont. Similarly A. ervi and Ephedrus plagiator  are able to distinguish infected from uninfected Sitobion avenae aphids and they may reduced attacks on aphids possessing H. defensa.

The establishment of defensive symbiosis is therefore an intriguing research field not only from an evolutionary point of view, but afor its implication for biological control. Indeed this symbiosis could reduce/affect the use of parasitoid waps for biological control of pest aphids so that we have to hope that parasitoid may have the ability to rapidly evolve counteradaptations to symbiont-conferred resistance.

 

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ResearchBlogging.orgVorburger C (2014). The evolutionary ecology of symbiont-conferred resistance to parasitoids in aphids. Insect science, 21 (3), 251-64 PMID: 24167113

Moran, N., Degnan, P., Santos, S., Dunbar, H., & Ochman, H. (2005). The players in a mutualistic symbiosis: Insects, bacteria, viruses, and virulence genes Proceedings of the National Academy of Sciences, 102 (47), 16919-16926 DOI: 10.1073/pnas.0507029102

Oliver KM, Russell JA, Moran NA, & Hunter MS (2003). Facultative bacterial symbionts in aphids confer resistance to parasitic wasps. Proceedings of the National Academy of Sciences of the United States of America, 100 (4), 1803-7 PMID: 12563031