Connecting neuromodulation, brain development, and life history timing associated with ecological speciation in Rhagoletis pomonella

Host shifts are considered a key generator of insect biodiversity, yet how insects synchronize life history timing and preference for new hosts is not well understood. Neurochemicals such as biogenic amines play key roles in both development and behavior, and therefore provide a potential source for such synchronization. Here, we correlated life history timing, brain development, and corresponding levels of 14 neurochemicals in Rhagoletis pomonella (Diptera: Tephritidae), a species undergoing ecological speciation through an ongoing host shift from hawthorn to apple fruit. We found that adult brain differentiation in both host races was initiated only after pupal diapause ended, and at this stage the titers of precursors for several neurotransmitter pathways also increased significantly. Conversely, neurotransmitter product titres generally increased at later stages up to adult fly sexual maturation. Further analysis showed that the apple host race exhibited adult brain differentiation three weeks earlier than the hawthorn host race, which correlated with significantly lower titers of several neurochemicals, particularly biogenic amines, in the earlier developing apple race as compared to the hawthorn race. We thus propose that these neurotransmitter levels could influence the differences in life history timing and host preference observed in these two races, promoting their ongoing speciation.

Genetic evidence for the involvement of Dicer-like 2 and 4 as well as Argonaute 2 in the Nicotiana benthamiana response against Pelargonium line pattern virus

In plants, RNA silencing functions as a potent antiviral mechanism. Virus-derived double-stranded RNAs (dsRNAs) trigger this mechanism, being cleaved by Dicer-like (DCL) enzymes into virus small RNAs (vsRNAs). These vsRNAs guide sequence-specific RNA degradation upon their incorporation into an RNA-induced silencing complex (RISC) that contains a slicer of the Argonaute (AGO) family. Host RNA dependent-RNA polymerases, particularly RDR6, strengthen antiviral silencing by generating more dsRNA templates from RISC-cleavage products that, in turn, are converted into secondary vsRNAs by DCLs. Previous work showed that Pelargonium line pattern virus (PLPV) is a very efficient inducer and target of RNA silencing as PLPV-infected Nicotiana benthamiana plants accumulate extraordinarily high amounts of vsRNAs that, strikingly, are independent of RDR6 activity. Several scenarios may explain these observations including a major contribution of dicing versus slicing for defence against PLPV, as the dicing step would not be affected by the RNA silencing suppressor encoded by the virus, a protein that acts via vsRNA sequestration. Taking advantage of the availability of lines of N. benthamiana with DCL or AGO2 functions impaired, here we have tried to get further insights into the components of the silencing machinery that are involved in anti-PLPV-silencing. Results have shown that DCL4 and, to lesser extent, DCL2 contribute to restrict viral infection. Interestingly, AGO2 apparently makes even a higher contribution in the defence against PLPV, extending the number of viruses that are affected by this particular slicer. The data support that both dicing and slicing activities participate in the host race against PLPV.

Adaptive divergence and post-zygotic barriers to gene flow between sympatric populations of a herbivorous mite

Plant-herbivore interactions promote the generation and maintenance of both plant and herbivore biodiversity. The antagonistic interactions between plants and herbivores lead to host race formation: the evolution of herbivore types specializing on different plant species, with restricted gene flow between them. Understanding how ecological specialization promotes host race formation usually depends on artificial approaches, using laboratory experiments on populations associated with agricultural crops. However, evidence on how host races are formed and maintained in a natural setting remains scarce. Here, we take a multidisciplinary approach to understand whether populations of the generalist spider mite Tetranychus urticae form host races in nature. We demonstrate that a host race co-occurs among generalist conspecifics in the dune ecosystem of The Netherlands. Extensive field sampling and genotyping of individuals over three consecutive years showed a clear pattern of host associations. Genome-wide differences between the host race and generalist conspecifics were found using a dense set of SNPs on field-derived iso-female lines and previously sequenced genomes of T. urticae. Hybridization between lines of the host race and sympatric generalist lines is restricted by post-zygotic breakdown, and selection negatively impacts the survival of generalists on the native host of the host race. Our description of a host race among conspecifics with a larger diet breadth shows how ecological and reproductive isolation aid in maintaining intra-specific variation in sympatry, despite the opportunity for homogenization through gene flow. Our findings highlight the importance of explicitly considering the spatial and temporal scale on which plant-herbivore interactions occur in order to identify herbivore populations associated with different plant species in nature. This system can be used to study the underlying genetic architecture and mechanisms that facilitate the use of a large range of host plant taxa by extreme generalist herbivores. In addition, it offers the chance to investigate the prevalence and mechanisms of ecological specialization in nature.

Non-parallel morphological divergence following colonization of a new host plant

Divergent ecological selection may diversify populations of the same species evolving in different niches. However, for adaptation to result in speciation, the ecologically divergent populations have to experience at least some degree of reproductive isolation. While ecological selection pressures in similar environments are expected to result in convergent adaptation, sexually selected traits may diverge in different directions in different locations. Here, we use a host shift in the phytophagous peacock fly Tephritis conura, with both host races represented in two geographically separate areas, East and West of the Baltic Sea, to investigate convergence in morphological adaptations. We asked i) if there are consistent morphological adaptations to a host plant shift and ii) if the adaptations to secondary sympatry with the alternate host race are consistent across contact zones. We found low, albeit significant, divergence between host races, but only a few traits, including the female ovipositor, were consistently different. Interestingly, co-existence with the other host race significantly increased the degree of morphological divergence, but the patterns of divergence were not consistent across the two sympatric contact zones. Thus, local stochastic fixation or reinforcement could generate trait divergence, and evidence from additional sources is hence needed to conclude whether divergence is adaptive.

A Symbiotic Virus Facilitates Aphid Adaptation to Host Plants by Suppressing Jasmonic Acid Responses

Symbiotic viruses exist in many insects; however, their functions in host insects are not well understood. In this study, we explored the role of acyrthosiphon pisum virus (APV) in the interaction of its host aphid Acyrthosiphon pisum with plants. APV is primarily located in aphid salivary glands and gut and propagated in the insect. APV is horizontally transmitted to host plants during aphid feeding, but the virus does not replicate in the host plant. When the pea host race of aphids colonized two low-fitness plants, Medicago truncatula and Vicia villosa, the virus titers in both the aphids and plants significantly increased. Furthermore, APV infection strongly promoted the survival rate of the pea host race on V. villosa. Transcriptomic analysis showed that only 0.85% of aphid genes responded to APV infection when aphids fed on V. villosa, with a fold change in transcript levels of no more than fourfold. The improved survival due to APV infection was apparently related to the inhibitory effect of the virus on levels of phytohormone jasmonic acid (JA) and JA-isoleucine. Our data suggest a benefit of the symbiotic virus to its aphid host and demonstrate a novel case of symbiotic virus-mediated three-species interaction.

Increases in Genistein in Medicago sativa Confer Resistance against the Pisum Host Race of Acyrthosiphon pisum

Interspecific interaction with host plants have important consequences for the host race formation of herbivorous insects. Plant secondary metabolites, particularly those that are involved in host races specializing on plants, warrant the theory of host specialization. Acyrthosiphon pisum comprises various host races that adapt to different Fabaceae plants, which provides an ideal system for determining the behavioral and physiological mechanisms underlying host-adaptive diversification. The current study evaluated the effects of host transfer on population fitness, feeding behavior and the transcriptome-wide gene expression of the two host races of A. pisum, one of which was originally from Medicago sativa and the other from Pisum sativum. The results showed that the Pisum host race of A. pisum had a lower population abundance and feeding efficiency than the Medicago host race in terms of a longer penetration time and shorter duration times of phloem ingestion when fed on M. sativa. In contrast, few differences were found in the population abundance and feeding behavior of A. pisum between the two host races when fed on P. sativum. Meanwhile, of the nine candidate phenolic compounds, only genistein was significantly affected by aphid infestation; higher levels of genistein were detected in M. sativa after feeding by the Pisum host race, but these levels were reduced relative to uninfested controls after feeding by the Medicago host race, which suggested that genistein may be involved in the specialization of the aphid host race on M. sativa. Further exogenous application of genistein in artificial diets showed that the increase in genistein reduced the survival rate of the Pisum host race but had little effect on that of the Medicago host race. The transcriptomic profiles indicated that the transcripts of six genes with functions related to detoxification were up-regulated in the Pisum host race relative to the Medicago host race of A. pisum. These results suggested that the inducible plant phenolics and associated metabolic process in aphids resulted in their differential adaptations to their Fabaceae host.