Whitefly endosymbionts: IPM opportunity or tilting at windmills?

Endosymbionts as hidden players in tripartite pathosystem of interactions and potential candidates for sustainable viral disease management

The convoluted relationships between plants, viruses, and arthropod vectors housing bacterial endosymbionts are pivotal in the spread of harmful plant viral diseases. Endosymbionts play key roles in: manipulating host responses, influencing insect resistance to pesticides, shaping insect evolution, and bolstering virus acquisition, retention, and transmission. This interplay presents an innovative approach for developing sustainable strategies to manage plant diseases. Recent progress in targeting specific endosymbionts through genetic modifications, biotechnological advancements, and RNA interference shows potential for curbing viral spread and disease progression. Additionally, employing synthetic biology techniques like CRISPR/Cas9 to engineer endosymbionts and disrupt crucial interactions necessary for viral transmission in arthropod vectors holds promise for effective control measures. In this review, these obligate and facultative bacterial cruxes have been discussed to elaborate on their mechanistic involvement in the regulation and/or inhibition of tripartite pathways of interactions. Furthermore, we provide an in-depth understanding of endosymbionts' synergistic and antagonistic effects on: insect biology, plant immunity, and virus acquisition and transmission. Finally, we point out open questions for future research and provide research directions concerning the deployment of genetically engineered symbionts to affect plant-virus-vector interactions for sustainable disease management. By addressing existing knowledge gaps and charting future research paths, a deeper comprehension of the role of endosymbionts in plant-virus-vector interactions can pave the way for innovative and successful disease management strategies. The exploration of antiviral therapies, paratransgenesis, and pathogen-blocking tactics using engineered endosymbionts introduces pioneering solutions for lessening the impact of plant viral diseases and green pest management.

Heterogeneous distribution of Cardinium in whitefly populations is associated with host nuclear genetic background

Inherited bacterial symbionts are very common in arthropods, but infection frequency can vary widely among populations. Experiments and interpopulation comparisons suggest that host genetic background might be important in explaining this variation. Our extensive field investigation showed that the infection pattern of the facultative symbiont Cardinium was heterogeneous across geographical populations of the invasive whitefly Bemisia tabaci Mediterranean (MED) in China, with genetic nuclear differences evident in 2 of the populations: 1 with a low infection rate (SD line) and 1 with a high infection rate (HaN line). However, whether the heterogeneous frequency of Cardinium is associated with the host genetic background remains poorly understood. Here, we compared the fitness of the Cardinium-infected and uninfected sublines with similar nuclear genetic backgrounds from SD and HaN lines, respectively, and further determine whether host extranuclear or nuclear genotype influenced the Cardinium-host phenotype by performing 2 new introgression series of 6 generations between SD and HaN lines (i.e., Cardinium-infected females of SD were backcrossed with uninfected males of HaN, and vice versa). The results showed that Cardinium provides marginal fitness benefits in the SD line, whereas Cardinium provides strong fitness benefits in the HaN line. Further, both Cardinium and the Cardinium-host nuclear interaction influence the fecundity and pre-adult survival rate of B. tabaci, whereas the extranuclear genotype does not. In conclusion, our results provide evidence that Cardinium-mediated fitness effects were closely associated with the host genetic background, which provides a fundamental basis for understanding the mechanism underlying the heterogeneous distribution of Cardinium in B. tabaci MED populations across China.

The Susceptibility of Bemisia tabaci Mediterranean (MED) Species to Attack by a Parasitoid Wasp Changes between Two Whitefly Strains with Different Facultative Endosymbiotic Bacteria

In this study, two strains of the mitochondrial lineage Q1 of Bemisia tabaci MED species, characterized by a different complement of facultative bacterial endosymbionts, were tested for their susceptibility to be attacked by the parasitoid wasp Eretmocerus mundus, a widespread natural enemy of B. tabaci. Notably, the BtHC strain infected with Hamiltonella and Cardinium was more resistant to parasitization than the BtHR strain infected with Hamiltonella and Rickettsia. The resistant phenotype consisted of fewer nymphs successfully parasitized (containing the parasitoid mature larva or pupa) and in a lower percentage of adult wasps emerging from parasitized nymphs. Interestingly, the resistance traits were not evident when E. mundus parasitism was compared between BtHC and BtHR using parasitoids originating from a colony maintained on BtHC. However, when we moved the parasitoid colony on BtHR and tested E. mundus after it was reared on BtHR for four and seven generations, we saw then that BtHC was less susceptible to parasitization than BtHR. On the other hand, we did not detect any difference in the parasitization of the BtHR strain between the three generations of E. mundus tested. Our findings showed that host strain is a factor affecting the ability of E. mundus to parasitize B. tabaci and lay the basis for further studies aimed at disentangling the role of the facultative endosymbiont Cardinium and of the genetic background in the resistance of B. tabaci MED to parasitoid attack. Furthermore, they highlight that counteradaptations to the variation of B. tabaci defence mechanisms may be rapidly selected in E. mundus to maximize the parasitoid fitness.

Plant-mediated rifampicin treatment of Bemisia tabaci disrupts but does not eliminate endosymbionts

Whiteflies are among the most important global insect pests in agriculture; their sustainable control has proven challenging and new methods are needed. Bacterial symbionts of whiteflies are poorly understood potential target of novel whitefly control methods. Whiteflies harbour an obligatory bacterium, Candidatus Portiera aleyrodidarum, and a diverse set of facultative bacterial endosymbionts. Function of facultative microbial community is poorly understood largely due to the difficulty in their selective elimination without removal of the primary endosymbiont. Since the discovery of secondary endosymbionts, antibiotic rifampicin has emerged as the most used tool for their manipulation. Its effectiveness is however much less clear, with contrasting reports on its effects on the endosymbiont community. The present study builds upon most recent method of rifampicin application in whiteflies and evaluates its ability to eliminate obligatory Portiera and two facultative endosymbionts (Rickettsia and Arsenophnus). Our results show that rifampicin reduces but does not eliminate any of the three endosymbionts. Additionally, rifampicin causes direct negative effect on whiteflies, likely by disrupting mitochondria. Taken together, results signify the end of a rifampicin era in whitefly endosymbiont studies. Finally, we propose refinement of current quantification and data analysis methods which yields additional insights in cellular metabolic scaling.

Wolbachia pipientis: first detection in populations of Glycaspis brimblecombei (Hemiptera: Aphalaridae) and Psyllaephagus bliteus (Hymenoptera: Encyrtidae) in Brazil

The sucking insect, Glycaspis brimblecombei Moore (Hemiptera: Aphalaridae), is originally from Australia and reduces the productivity of Eucalyptus crops. The parasitoid Psyllaephagus bliteus Riek (Hymenoptera: Encyrtidae) is the main agent used in the integrated management of G. brimblecombei. Endosymbionts, in insects, are important in the adaptation and protection of their hosts to the environment. The intracellular symbionts Wolbachia, induces reproductive changes such as cytoplasmic incompatibility, feminization, male death and parthenogenesis. The objective of this study was to report the first record of Wolbachia pipientis in populations of G. brimblecombei and of its parasitoid P. bliteus in the field in Brazil. Branches with adults of G. brimblecombei and P. bliteus were collected from eucalyptus trees in commercial farms in six Brazilian states and, after emergence, the insects obtained were frozen at -20 °C. Polymerase chain reaction (PCR) was performed to detect the Wolbachia endosymbiont. Wolbachia pipientis was identified in individuals of G. brimblecombei and its parasitoid P. bliteus from populations of the counties of Agudos and Mogi-Guaçu (São Paulo State), Itamarandiba (Minas Gerais State) and São Jerônimo da Serra (Paraná State) in Brazil.