Host resistance evolution to natural enemies depends on genetic variation, symbiont associations, and trade-offs with host´s fitness traits. We selected isofemale lineages of Myzus persicae to investigate biological traits and molecular markers associated with aphid defensive responses to the parasitic wasp Diaeretiella rapae. The parasitization of M. persicae lineages by D. rapae ranged from 43 to 76%. Rickettsia was the predominant secondary symbiont. Six lineages were selected according to D. rapae parasitization rate (LP = low parasitization; HP = high parasitization) and Rickettsia association. Life history traits differed among M. persicae lineages. However, the association of adaptive costs with reduced susceptibility to D. rapae parasitization was not consistently observed. Rickettsia infection did not explain the enhanced aphid defense, but it did increase aphid fecundity. Comparative small structural variants analyses of M. persicae lineages indicated that approximately 6% of SNPs and 49% of InDels were unique to each aphid lineage or shared among LP or HP lineages. Only 1.7% of SNPs and 0.91% of InDels influenced genes involved with aphid immune response. Further exploration of non-coding variants and additional omics-related approaches are needed to fully characterize the mechanisms behind the differential parasitization success of M. persicae lineages by D. rapae.
Parasitoids use completely different life strategies when compared to true parasites as they kill their hosts upon completion of their development, putting a much stronger selection pressure for the evolution of defensive traits on their hosts. The successful exploitation of a wide range of hosts by parasitoids is partially due to the diverse strategies parasitoids evolved for host exploitation. But host defensive strategies also lead to parasitoid avoidance and parasitization escape. Host populations that are maintained under strong selective pressure from parasitoids can face a bottleneck in their genetic composition due to the elimination of susceptible genotypes that are successfully parasitized from the total genetic pool available in the population, including those represented by genotypes associated with microbial symbionts. Host-parasitoid population densities are known to fluctuate cyclically, keeping both populations in equilibrium and rarely leading to extinction events. Thus, the survival of host genotypes that escape parasitization may drive selection for hosts that are resistant to parasitization.
Diaeretiella rapae (MacIntosh) (Hymenoptera: Braconidae), a parasitoid of more than 60 species of aphids, including Myzus persicae (Sulzer) (Hemiptera: Aphididae) is an example of a parasitoid exerting strong selective pressure on its hosts. Myzus persicae is a polyphagous pest of great economic importance, with excellent developmental capabilities on hundreds of plant species in more than 50 families. Its economic importance is based on the ability to cause direct damage to plants in conjunction with its feeding habits, which result in host plant dehydration, loss of shoots and retardation of vegetative growth. In addition, aphids can cause indirect damage by transmitting viruses and promoting black mold growth by excreting honeydew. Aphid infestations of agricultural crops often require the implementation of pest control measures, and a set of new management strategies has been addressed to sum up to biocontrol methods to reduce the often-used synthetic chemical insecticides and their undesired side-effects.
D. rapae is a key parasitoid in the biological control of aphids, including M. persicae, and exerts a strong selective pressure on aphid populations, driving the evolution of host defense mechanisms and attack strategies. Previous studies on M. persicae -- D. rapae interactions have mainly focused on verifying the influence of the host plant on aphid suitability and on parasitoid attraction, the effects of the secondary symbiont Rickettsiella viridis in the rate of parasitization, and the side-effects of insecticides in this host-parasitoid interaction. There have been also studies dedicated to compare D. rapae efficacy on aphid host species with different host plant ranges. However, the genetic basis of the defensive mechanisms, the adaptive costs of resistance to parasitization and the role of secondary symbionts as defensive symbionts in this host-parasitoid interaction are seldom reported.
These gaps in knowledge highlight the complexity of the evolutionary arms race between aphids and their parasitoids. Co-evolution between hosts and parasitoids arises because of interactions throughout their evolutionary history, as hosts face selection pressure due to exposure to parasitoid-derived virulence factors whilst, at the same time, parasitoids face selection pressure from host defenses. As predicted by the red queen theory, each adaptation developed by one species faces a counteradaptation of the interacting species, such that the survival of both species depends on the continuous development of defense and attack strategies (arms race). Microorganisms can modulate their host phenotype, and secondary symbionts have shown to modify their host response to parasitoid attack, with possible implications in the evolution of parasitoid virulence. Defensive symbionts can affect host successful parasitization by resource limitation, such as lipids, and by the production and release of metabolites toxic to natural enemies.
Despite advances in the understanding of defense mechanisms against natural enemies, the physiological and behavioral strategies of hosts and their interactions with secondary symbionts are still poorly understood. Hamiltonella defensa and its associated bacteriophage are among the best-known defensive secondary symbionts. Phage-infected Hamiltonella defensa can produce toxins such as Shiga-like, CdtB and YD-repeat, which increase immature parasitoid mortality and strengthen host resistance. Serratia symbiotica is another relevant secondary symbiont that affects successful parasitization by interfering with the production of plant volatiles involved in parasitoid attraction. In aphids, other secondary symbionts (e.g., Rickettsia, Spiroplasma, Wolbachia) are also present in natural environments, but their role as defensive symbionts against natural enemies is still poorly understood, and an increase in the knowledge of parasitoid-host-symbiont interactions can open new perspectives for the development of more efficient biological control programs.
Host-parasitoid interactions are frequency-dependent, which can be observed more strongly in laboratory than in field studies. The frequency-dependent interaction combined with genetic variation allows interacting species to co-evolve. Parasitoids are selected to evade defense mechanisms of common host genotypes, contributing to the selection of hosts with rare resistant genotypes. This process is difficult to observe in field populations since the detection of resistant individuals depends on the host encounter and on the genotype of the natural enemy. Host genotypes with a lower ability to evade natural enemies' encounters are more frequently attacked. However, parasitoid genotypes are subject to similar selection pressures, resulting in a selection process that occurs without the fixation of extreme host or parasitoid genotypes. In laboratory experiments, genotypes with greater and/or lesser ability to avoid parasitoid attack face similar rates of parasitoid encounter under controlled abiotic and biotic conditions and similar patch structures, allowing the study of host/parasitoid genotypes with selected responses.
The genetic variability that results in different host responses to parasitoid attack and parasitoid phenotypes with different levels of success in host parasitization can arise from mutation, increased gene flow, genetic drift, inbreeding depression, and selection. The main and fastest way to generate genetic variability is through sexual reproduction, which allows for the perpetuation of mutations in the population, as well as genetic recombination between chromosomes. Aphids developing in temperate regions exhibit seasonal reproductive polyphenism, alternating between asexual and sexual reproduction to produce eggs that will diapause in winter. However, under tropical conditions, aphid species such as Myzus persicae reproduce exclusively by ameiotic (apomictic) parthenogenesis, in which chromosome division occurs by mitosis. In apomixis reproduction, the resulting offspring are genetically identical to their mothers, and the sources of genetic variation to promote phenotypic expression are now limited to rare genomic events, such as mutations, chromosomal rearrangements and mitotic recombination, and/or interaction with endosymbionts.
Chromosomal rearrangements and interactions with symbionts have been shown to serve as sources of genetic variation leading to the manifestation of insecticide resistant, parasitoid defense and host plant adapted phenotypes. Thus, both chromosomal and extra-chromosomal variation can influence the evolution of host defense mechanisms against natural enemies, which may invariably have associated adaptive costs. Chromosomal rearrangements often result from the emergence of structural variants that can arise through various cellular mechanisms, including DNA replication and repair. These variants can be broadly categorized into five classes: deletions, duplications, insertions, inversions, and translocations. Structural variants introduce variability in gene copy number, position, orientation, and occasionally a combination of these effects. As a result, structural variants drive genotypic change and facilitate the manifestation of diverse phenotypes.
In this context, we investigated the interaction between M. persicae and its parasitoid D. rapae, considering genetic and symbiotic factors that may influence the host response to parasitization. We sought to answer the following questions: (1) is there variation between isofemale lineages of M. persicae in their resistance to parasitism by D. rapae?; (2) does the presence of secondary symbionts influence the success of parasitism?; (3) are there biological differences between isofemale strains of M. persicae associated with the presence of secondary symbionts and their resistance to parasitism?; and (4) do small structural variants (SVs) in the M. persicae genome play a role in host adaptation and response to parasitism? Our central hypothesis is that the differential resilience to parasitism by D. rapae among M. persicae lineages is associated both with the presence of secondary symbionts and with structural variations in the genome that may modulate the host response to parasitoid attack.