Maintenance of aphid clonal lineages: images of immortality?

Telomeres in ecology and evolution: A review and classification of hypotheses

Research on telomeres in the fields of ecology and evolution has been rapidly expanding over the last two decades. This has resulted in the formulation of a multitude of, often name-given, hypotheses related to the associations between telomeres and life-history traits or fitness-facilitating processes (and the mechanisms underlying them). However, the differences (or similarities) between the various hypotheses, which can originate from different research fields, are often not obvious. Our aim here is therefore to give an overview of the hypotheses that are of interest in ecology and evolution and to provide two frameworks that help discriminate among them. We group the hypotheses (i) based on their association with different research questions, and (ii) using a hierarchical approach that builds on the assumptions they make, such as about causality of telomere length/shortening and/or the proposed functional consequences of telomere shortening on organism performance. Both our frameworks show that there exist parallel lines of thoughts in different research fields. Moreover, they also clearly illustrate that there are in many cases competing hypotheses within clusters, and that some of these even have contradictory assumptions and/or predictions. We also touch upon two topics in telomere research that would benefit from further conceptualization. This review should help researchers, both those familiar with and those new to the subject, to identify future avenues of research.

Aphids in focus: unravelling their complex ecology and evolution using genetic and molecular approaches

Aphids are renowned plant parasites of agriculture, horticulture and forestry, causing direct physical damage by sucking phloem and especially by transmission of plant pathogenic viruses. The huge yield loss they cause amounts to hundreds of millions of dollars globally, and because of this damage and the intense efforts expended on control, some 20 species are now resistant to pesticides worldwide. Aphids represent an ancient, mainly northern temperate group, although some species occur in the tropics, often as obligate asexual lineages or even asexual ‘species’. However, besides their notoriety as enemies of plant growers, aphids are also extremely interesting scientifically, especially at the molecular and genetic levels. They reproduce mainly asexually, one female producing 10–90 offspring in 7–10 days and therefore, theoretically, could produce billions of offspring in one growing season in the absence of mortality factors (i.e. climate/weather and antagonists). In this overview, we provide examples of what molecular and genetic studies of aphids have revealed concerning a range of topics, especially fine-grained ecological processes. Aphids, despite their apparently limited behavioural repertoire, are in fact masters (or, perhaps more accurately, mistresses) of adaptation and evolutionary flexibility and continue to flourish in a variety of ecosystems, including the agro-ecosystem, regardless of our best efforts to combat them.

Evidence of indirect symbiont conferred protection against the predatory lady beetle Harmonia axyridis in the pea aphid

Background

Defensive symbionts can provide significant fitness advantages to their hosts. Facultative symbionts can protect several species of aphid from fungal pathogens, heat shock, and parasitism by parasitoid wasps. Previous work found that two of these facultative symbionts can also indirectly protect pea aphids from predation by the lady beetle Hippocampus convergens. When aphids reproduce asexually, there is extremely high relatedness among aphid clone-mates and often very limited dispersal. Under these conditions, symbionts may indirectly protect aphid clone-mates from predation by negatively affecting the survival of a predator after the consumption of aphids harboring the same vertically transmitted facultative symbionts. In this study, we wanted to determine whether this indirect protection extended to another lady beetle species, Harmonia axyridis.

Results

We fed Ha. axyridis larvae aphids from one of four aphid sub-clonal symbiont lines which all originated from the same naturally symbiont free clonal aphid lineage. Three of the sub-clonal lines harbor different facultative symbionts that were introduced to the lines via microinjection. Therefore these sub-clonal lineages vary primarily in their symbiont composition, not their genetic background. We found that aphid facultative symbionts affected larval survival as well as pupal survival in their predator Ha. axyridis. Additionally, Ha. axyridis larvae fed aphids with the Regiella symbiont had significantly longer larval developmental times than beetle larvae fed other aphids, and females fed aphids with the Regiella symbiont as larvae weighed less as adults. These fitness effects were different from those previously found in another aphid predator Hi. convergens suggesting that the fitness effects may not be the same in different aphid predators.

Conclusions

Overall, our findings suggest that some aphid symbionts may indirectly benefit their clonal aphid hosts by negatively impacting the development and survival of a lady beetle aphid predator Ha. axyridis. By directly affecting the survival of predatory lady beetles, aphid facultative symbionts may increase the survival of their clone-mates that are clustered nearby and have significant impacts across multiple trophic levels. We have now found evidence for multiple aphid facultative symbionts negatively impacting the survival of a second species of aphid predatory lady beetle. These same symbionts also protect their hosts from parasitism and fungal infections, though these fitness effects seem to depend on the aphid species, predator or parasitoid species, and symbiont type. This work further demonstrates that beneficial mutualisms depend upon complex interactions between a variety of players and should be studied in multiple ecologically relevant contexts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-017-0136-x) contains supplementary material, which is available to authorized users.

Genetic Structure of the Aphis craccivora (Hemiptera: Aphididae) From Thailand Inferred From Mitochondrial COI Gene Sequence

The cowpea aphid, Aphis craccivora Koch (Hemiptera: Aphididae), is one of the most destructive insect pests of legume plants worldwide. Although outbreaks of this pest occur annually in Thailand causing heavy damage, its genetic structure and demographic history are poorly understood. In order to determine genetic structure and genetic relationship of the geographic populations of this species, we examined sequences of mitochondrial cytochrome c oxidase subunit I (COI) gene of 51 individuals collected from 32 localities throughout Thailand. Within the sequences of these geographic populations, 32 polymorphic sites defined 17 haplotypes, ranging in sequence divergence from 0.2% (1 nucleotide) to 2.7% (16 nucleotides). A relatively high haplotype diversity but low nucleotide diversity was detected in the populations of A. craccivora, a finding that is typical for migratory species. Phylogenetic analysis revealed a weak phylogeographic structuring among the geographic populations and among the haplotypes, indicating their close relationship. Considering the distance between the sampling sites, the occurrence of identical haplotypes over wide areas is noteworthy. Moreover, the low genetic distance (FST ranging from -0.0460 to 0.3263) and high rate of per-generation female migration (Nm ranging from 1.0323 to 20.3333) suggested population exchange and gene flow between the A. craccivora populations in Thailand.

Genetic diversity of Sitobion avenae (Homoptera: Aphididae) populations from different geographic regions in China

Sitobion avenae is a major agricultural pest of wheat in China. Using microsatellite markers, we studied the potential gene flow, genetic diversity, genetic differentiation, and genetic structure of seven S. avenae populations from different regions of China (Beijing, Hebei, Henan, Hubei, Jiangsu, Shandong, and Shanxi provinces). The populations from Henan, Shandong, and Jiangsu showed high levels of genic and genotypic diversity. By contrast, the genic diversity in the Beijing and Hebei populations was much lower. Despite this low genic diversity, the genotypic diversity of the Beijing population was higher than that of all of the other populations, except those from Jiangsu and Shandong. Overall, the genetic divergence among the seven S. avenae populations tested was high, though there was almost no differentiation between the Shandong and Henan populations. We observed significant negative correlation between the strength of gene flow and the geographic distances among populations. Based on genetic analysis, the seven S. avenae populations studied can be divided into four distinct clusters; (i) Hubei, (ii) Shanxi, (iii) Beijing and Hebei, and (iv) Shandong, Henan, and Jiangsu. The present results provide a basis for potentially optimizing integrated pest management (IPM) programs in China, through adapting control methods that target biological traits shared by various populations of the same genotype.

Presence of a functional (TTAGG)(n) telomere-telomerase system in aphids

The structure of the telomeres of four aphid species (Acyrthosiphon pisum, Megoura viciae, Myzus persicae and Rhopalosiphum padi) was evaluated by Southern blotting and fluorescent in situ hybridization, revealing that each chromosomal end consists of a (TTAGG)(n) repeat. The presence of a telomerase coding gene has been verified successively in the A. pisum genome, revealing that aphid telomerase shares sequence identity ranging from 12% to 18% with invertebrate and vertebrate homologues, and possesses the two main domains involved in telomerase activity. Interestingly, telomerase expression has been verified in different somatic tissues suggesting that, in aphids, telomerase activity is not as restricted as in human cells. The study of telomeres in a M. persicae strain with a variable chromosome number showed that aphid telomerase can initiate the de novo synthesis of telomere sequences at internal breakpoints, resulting in the stabilization of chromosomal fragments.