Was Dan Janzen (1977) right about aphid clones being a "super-organism", i.e. a single "evolutionary individual"? : new insights from the use of molecular marker systems

Dan Janzen proposed in a paper in 1977 (loc. cit.), that a clone of aphids and for that matter dandelions consists, respectively, of one large ‘super-organism’. In effect a single evolutionary individual able to exploit resources over an expanded geographical range, and sometimes with aphids also, a wider range of resources (different kinds of host plants), much more than if the organism concerned were a single individual. Such a view is of course based on the notion that an asexual lineage (clone) has strict genetic fidelity, that is to say, is genetically identical over its entire genome between clone mates. This seems a highly unlikely scenario and indeed, modern molecular markers have revealed a plethora of mutational events within such so-called clones. Here in this talk I provide evidence from aphids that they are not ‘perfect forms’ but rather show a range of variations, including evidence of hybridization events, and that they can and do adapt to environmental circumstances, sometimes swiftly. Hence that even as asexual lineages, aphids are able to exploit new ecological circumstances and flourish, e.g. host adapted forms, whilst some species, notably the highly polyphagous peach-potato aphid (Myzus persicae), have also evolved resistance to a range of pesticides, and by so doing, have managed to survive in the face of these poisons. However, there are fitness costs associated with such adaptation, more especially in the highly resistant aphids. Because of the variation and adaptation shown by particular aphid species and asexual lineages, they cannot be described as a single evolutionary unit in a ‘Janzenian’ sense. What they show is ecological plasticity and an ability to adapt quickly, in large part enhanced by their incredible rate of reproduction and population expansion. Some migrating winged aphids are constrained in their exploitation of new habitats by environmental factors – geographical, climatic and ecological, especially lack of suitable hosts. In contrast, some other aphid species have seemingly colonized large areas of the world (probably aided by human agency) so that deciding what a population is exactly is a difficult task. It may even be that certain ‘super clones’ detected using molecular markers have indeed spread far and wide, clones which appear to fit the description of being ‘general purpose genotypes’ in that they can feed on a range of plant hosts under a range of different geographical-climatic conditions. As such, they are nearest to Dan Janzen’s views, although here again, strict genetic fidelity is not necessarily proven, only accepted from the application of a limited number of markers, e.g. multilocus genotypes in the case of microsatellite markers

Was Dan Janzen (1977) right about aphid clones being a "super-organism", i.e. a single "evolutionary individual"? : new insights from the use of molecular marker systems

Dan Janzen proposed in a paper in 1977 (loc. cit.), that a clone of aphids and for that matter dandelions consists, respectively, of one large ‘super-organism’. In effect a single evolutionary individual able to exploit resources over an expanded geographical range, and sometimes with aphids also, a wider range of resources (different kinds of host plants), much more than if the organism concerned were a single individual. Such a view is of course based on the notion that an asexual lineage (clone) has strict genetic fidelity, that is to say, is genetically identical over its entire genome between clone mates. This seems a highly unlikely scenario and indeed, modern molecular markers have revealed a plethora of mutational events within such so-called clones. Here in this talk I provide evidence from aphids that they are not ‘perfect forms’ but rather show a range of variations, including evidence of hybridization events, and that they can and do adapt to environmental circumstances, sometimes swiftly. Hence that even as asexual lineages, aphids are able to exploit new ecological circumstances and flourish, e.g. host adapted forms, whilst some species, notably the highly polyphagous peach-potato aphid (Myzus persicae), have also evolved resistance to a range of pesticides, and by so doing, have managed to survive in the face of these poisons. However, there are fitness costs associated with such adaptation, more especially in the highly resistant aphids. Because of the variation and adaptation shown by particular aphid species and asexual lineages, they cannot be described as a single evolutionary unit in a ‘Janzenian’ sense. What they show is ecological plasticity and an ability to adapt quickly, in large part enhanced by their incredible rate of reproduction and population expansion. Some migrating winged aphids are constrained in their exploitation of new habitats by environmental factors – geographical, climatic and ecological, especially lack of suitable hosts. In contrast, some other aphid species have seemingly colonized large areas of the world (probably aided by human agency) so that deciding what a population is exactly is a difficult task. It may even be that certain ‘super clones’ detected using molecular markers have indeed spread far and wide, clones which appear to fit the description of being ‘general purpose genotypes’ in that they can feed on a range of plant hosts under a range of different geographical-climatic conditions. As such, they are nearest to Dan Janzen’s views, although here again, strict genetic fidelity is not necessarily proven, only accepted from the application of a limited number of markers, e.g. multilocus genotypes in the case of microsatellite markers

The effect of landscape complexity and microclimate on the thermal tolerance of a pest insect

Landscape changes are known to exacerbate the impacts of climate change. As such, understanding the combined effect of climate and landscape on agroecosystems is vital if we are to maintain the function of agroecosystems. This study aimed to elucidate the effects of agricultural landscape complexity on the microclimate and thermal tolerance of an aphid pest to better understand how landscape and climate may interact to affect the thermal tolerance of pest species within the context of global climate change. Meteorological data were measured at the landscape level, and cereal aphids (Sitobion avenae, Metopolophium dirhodum and Rhopalosiphum padi) sampled, from contrasting landscapes (simple and complex) in winter 2013/2014 and spring 2014 in cereal fields of Brittany, France. Aphids were returned to the laboratory and the effect of landscape of origin on aphid cold tolerance (as determined by CTmin ) was investigated. Results revealed that local landscape complexity significantly affected microclimate, with simple homogenous landscapes being on average warmer, but with greater temperature variation. Landscape complexity was shown to impact aphid cold tolerance, with aphids from complex landscapes being more cold tolerant than those from simple landscapes in both winter and spring, but with differences among species. This study highlights that future changes to land use could have implications for the thermal tolerance and adaptability of insects. Furthermore, not all insect species respond in a similar way to microhabitat and microclimate, which could disrupt important predator-prey relationships and the ecosystem service they provide

Grain yield reductions in spring barley due to barley yellow dwarf virus and aphid feeding

peer-reviewedThe occurrence and control of barley yellow dwarf virus (BYDV) in spring barley was investigated, at Oak Park, in the periods 1990 to 1993 and 1996 to 2001. Barley was sown in March and April and treated with either organophosphorous or pyrethroid aphicide at various plant growth stages. The most common aphid encountered was Sitobion avenae and MAV the most common strain of BYDV. In untreated plots of March- and April-sown barley, 0.85% and 5.9%, respectively, of tillers had virus symptoms. Best control of symptoms, from a single aphicide in March- and April-sown crops, was a treatment at growth stage (g.s.) 14. This treatment contributed 77% of the reduction in symptoms recorded for multiple treatments in April-sown plots. The reduction in grain yield due to high, moderate and low BYDV infection in April-sown barley was 1.1 t/ha (20%), 0.65 t/ha (10%) and 0.36 t/ha (7%), respectively. In Marchsown barley, pyrethroid aphicide applied at g.s. 14 significantly improved grain yield by 0.26 t/ha (4%). In the season having the most severe BYDV outbreak, a pyrethroid aphicide at g.s. 14 was best in controlling yield loss. Pyrethroid aphicide gave better control of symptoms and better yields than organophosphorous aphicide. The estimated yield reductions in untreated April-sown barley due to feeding damage by Sitobion avenae was 0.71 t/ha and 0.83 t/ha (10.6% and 11.3%) in the two seasons in which this aphid was plentiful. In the three seasons in which Metopolophium dirhodum was recorded the estimated yield reductions were 0.32 t/ha, 0.48 t/ha and 0.43 t/ha (5.2%, 5.6% and 5.7%)

An overview of the functioning of Sitobion avenea populations at three spatial scales in France

In this paper we give 3 snapshots of our recent work on S. avenae at field, landscape and country scales. From April to June wheat fields are continuously colonized by winged S. avenae and previously established colonies experience high extinction rates. This leads to changes in spatial distribution of aggregates of aphids and to a progressive spatial homogenisation of the populations in fields, and highlights the role of spring immigrants in the field dynamics of S. avenae. In the surrounding landscape, the S. avenae populations on cultivated cereals (wheat, maize, barley) do not differ genetically. Conversely, aphids from weed margins and pastures (mostly on Poeae) clearly differ from those on cereals, indicating a low level of gene flow between the ‘uncultivated’ (mostly perennial) and cultivated system (annual). Consequently weeds and pasture grasses are probably poor sources of S. avenae for further infestation of cereal fields. The role of surrounding crops and weeds as a source of aphids infesting wheat was assessed by stable isotopic ratios and population genetic tools. In autumn, up to the beginning of October, most S. avenae landing on wheat originated from maize and after this from cereal volunteers. In spring the influx from surrounding cereal volunteers varied between years, but uncultivated Poeae played a minor role. At country scale on cereals, clonal reproduction and parthenogenetic overwintering prevails everywhere in France, but there is a trend toward increasing sexuality northward. The weak genetic differentiation of the regional populations on cereals and the presence of many identical genotypes in most of the regions sampled confirm the high dispersal ability of S. avenae. The high occurrence of widespread genotypes in multiple copies, belonging to the same genetic pool, which persist over several years in France and other countries in Europe, indicates a homogenising effect of selection by millions of hectares of cereal

Identifying aphid resistance in the ancestral wheat Triticum monococcum under field conditions.

Wheat is an economically, socially, and nutritionally important crop, however, aphid infestation can often reduce wheat yield through feeding and virus transmission. Through field phenotyping, we investigated aphid resistance in ancestral wheat Triticum monococcum (L.). Aphid (Rhopalosiphum padi (L.), Sitobion avenae (F.) and Metopolophium dirhodum (Wlk.)) populations and natural enemy presence (parasitised mummified aphids, ladybird adults and larvae and lacewing eggs and larvae) on two naturally susceptible wheat varieties, Triticum aestivum (L.) var. Solstice and T. monococcum MDR037, and three potentially resistant genotypes T. monococcum MDR657, MDR045 and MDR049 were monitored across three years of field trials. Triticum monococcum MDR045 and MDR049 had smaller aphid populations, whereas MDR657 showed no resistance. Overall, natural enemy presence was positively correlated with aphid populations; however, MDR049 had similar natural enemy presence to MDR037 which is susceptible to aphid infestation. It is hypothesised that alongside reducing aphid population growth, MDR049 also confers indirect resistance by attracting natural enemies. The observed resistance to aphids in MDR045 and MDR049 has strong potential for introgression into commercial wheat varieties, which could have an important role in Integrated Pest Management strategies to reduce aphid populations and virus transmission

Factors affecting the potential increase rate (e', PIR), as defined by Hughes, in populations of Sitobion αvenαe (F.) (Hemiptera: Aphididae

Σε πληθυσμούς της αφίδας Sitobion avenae (F.) κάτω από ελεγχόμενες συνθήκες και με παρουσία ή απουσία του παρασίτου Aphelinus abdominalis (Dalman) μελετήθηκε ο Δυνητικός Ρυθμός Αύξησης (PIR). Ο ρυθμός αυτός χρησιμεύει σαν πολλαπλασιαστικός παράγοντας για τον υπολογισμό της μεταβολής του πληθυσμού σε χρονική περίοδο ίση προς τη διάρκεια του μέσου όρου των τριών πρώτων σταδίων (instar-period) μιας αφίδας.Δύο παράγοντες που βρέθηκαν και που αφορούν, αφενός μεν την προτίμηση του ακμαίου παρασίτου ως αρπακτικού για το πρώτο νυμφικό στάδιο, αφετέρου δε την αύξηση της διάρκειας του τρίτου σταδίου των αφίδων που παρασιτίστηκαν στο πρώτο νυμφικό στάδιο. θεωρούνται υπεύθυνοι για την εύρεση μη αποδεκτών τιμών PIR κατά Hughes. Συγκεκριμένα, οι παράγοντες αυτοί οδήγησαν στον υπολογισμό μικρότερου αναμενόμενου πληθυσμού για την επομένη περίοδο (instar-period) όπως αυτή έχει αναφερθεί προηγουμένως.The potential rate of increase (PIR), used in Hughes’ time-specific life table analysis for aphid populations as a multiplication factor for the instar-period was studied in popula(ions of Sitobion avenae (F.) in the absence and presence of the parasite Aphelinus abdominalis (Dalman) under controlled conditions. Two factors were mainly found to alter PIR values in the presence of parasites. These were the feeding preference of adult A. abdominalis for the first instar aphids and the prolonged instar duration of the third instar of aphids which had been parasitized by an adult parasite at the first instar. These two factors contributed to lower values of PIR and as a consequence to an underestimation of the expected (potential) population for the next instar-perio

Contrasting population structure and demographic history of cereal aphids in different environmental and agricultural landscapes

Genetic diversity of populations has important ecological and evolutionary consequences, whose understanding is fundamental to improve the sustainability of agricultural production. Studies of how differences in agricultural management and environment influence the population structure of insect pests are central to predict outbreaks and optimise control programmes. Here, we have studied the population genetic diversity and evolution of Sitobion avenae and Sitobion miscanthi, which are among the most relevant aphid pests of cereals across Europe and Asia, respectively. We have used genotyping by sequencing (GBS) to identify genome-wide single nucleotide polymorphisms (SNPs) to infer the geographic structure and migration patterns. In the present study, we show that the population structure in present day populations are different from that described in previous studies, which suggests that they have evolved recently possibly as a response to human-induced changes in agriculture. This study shows that S. avenae in England is predominantly parthenogenetic and there has been a demographic and spatial expansion of a single genetic cluster, which could correspond with the insecticide-resistance superclone identified in previous studies. Conversely, in China, S. miscanthi populations are mostly cyclical parthenogenetic, with one sexual stage in autumn to produce overwintering eggs, and there are six genetically differentiated subpopulations and high genetic differentiation between geographic locations, which suggests that further taxonomical research is needed. Unlike S. avenae in England, there is no evidence for insecticide resistance and there is no predominance of a single lineage in S. miscanthi in China