Almost There: Transmission Routes of Bacterial Symbionts between Trophic Levels

Many intracellular microbial symbionts of arthropods are strictly vertically transmitted and manipulate their host's reproduction in ways that enhance their own transmission. Rare horizontal transmission events are nonetheless necessary for symbiont spread to novel host lineages. Horizontal transmission has been mostly inferred from phylogenetic studies but the mechanisms of spread are still largely a mystery. Here, we investigated transmission of two distantly related bacterial symbionts – Rickettsia and Hamiltonella – from their host, the sweet potato whitefly, Bemisia tabaci, to three species of whitefly parasitoids: Eretmocerus emiratus, Eretmocerus eremicus and Encarsia pergandiella. We also examined the potential for vertical transmission of these whitefly symbionts between parasitoid generations. Using florescence in situ hybridization (FISH) and transmission electron microscopy we found that Rickettsia invades Eretmocerus larvae during development in a Rickettsia-infected host, persists in adults and in females, reaches the ovaries. However, Rickettsia does not appear to penetrate the oocytes, but instead is localized in the follicular epithelial cells only. Consequently, Rickettsia is not vertically transmitted in Eretmocerus wasps, a result supported by diagnostic polymerase chain reaction (PCR). In contrast, Rickettsia proved to be merely transient in the digestive tract of Encarsia and was excreted with the meconia before wasp pupation. Adults of all three parasitoid species frequently acquired Rickettsia via contact with infected whiteflies, most likely by feeding on the host hemolymph (host feeding), but the rate of infection declined sharply within a few days of wasps being removed from infected whiteflies. In contrast with Rickettsia, Hamiltonella did not establish in any of the parasitoids tested, and none of the parasitoids acquired Hamiltonella by host feeding. This study demonstrates potential routes and barriers to horizontal transmission of symbionts across trophic levels. The possible mechanisms that lead to the differences in transmission of species of symbionts among species of hosts are discussed

Rickettsia Phylogenomics: Unwinding the Intricacies of Obligate Intracellular Life

BACKGROUND: Completed genome sequences are rapidly increasing for Rickettsia, obligate intracellular alpha-proteobacteria responsible for various human diseases, including epidemic typhus and Rocky Mountain spotted fever. In light of phylogeny, the establishment of orthologous groups (OGs) of open reading frames (ORFs) will distinguish the core rickettsial genes and other group specific genes (class 1 OGs or C1OGs) from those distributed indiscriminately throughout the rickettsial tree (class 2 OG or C2OGs). METHODOLOGY/PRINCIPAL FINDINGS: We present 1823 representative (no gene duplications) and 259 non-representative (at least one gene duplication) rickettsial OGs. While the highly reductive (approximately 1.2 MB) Rickettsia genomes range in predicted ORFs from 872 to 1512, a core of 752 OGs was identified, depicting the essential Rickettsia genes. Unsurprisingly, this core lacks many metabolic genes, reflecting the dependence on host resources for growth and survival. Additionally, we bolster our recent reclassification of Rickettsia by identifying OGs that define the AG (ancestral group), TG (typhus group), TRG (transitional group), and SFG (spotted fever group) rickettsiae. OGs for insect-associated species, tick-associated species and species that harbor plasmids were also predicted. Through superimposition of all OGs over robust phylogeny estimation, we discern between C1OGs and C2OGs, the latter depicting genes either decaying from the conserved C1OGs or acquired laterally. Finally, scrutiny of non-representative OGs revealed high levels of split genes versus gene duplications, with both phenomena confounding gene orthology assignment. Interestingly, non-representative OGs, as well as OGs comprised of several gene families typically involved in microbial pathogenicity and/or the acquisition of virulence factors, fall predominantly within C2OG distributions. CONCLUSION/SIGNIFICANCE: Collectively, we determined the relative conservation and distribution of 14354 predicted ORFs from 10 rickettsial genomes across robust phylogeny estimation. The data, available at PATRIC (PathoSystems Resource Integration Center), provide novel information for unwinding the intricacies associated with Rickettsia pathogenesis, expanding the range of potential diagnostic, vaccine and therapeutic targets

FISH of <i>Er. emiratus</i> stained with <i>Rickettisa</i> specific probe (blue).

<p>Left panel-<i>Rickettsia</i> probe fluorescent channel; right panel- overlay of fluorescent and brightfield channels. Arrows pointing to parasitoid gut. A- parasitoid larva (dark, ovoid sphere in the center of the host). Note <i>Rickettsia</i> in the parasitoid gut, as well in the whitefly's body remnants, surrounding the parasitoid. B- parasitoid pre-pupa. C- parasitoid pupae (note the autofluorescence of the anus and mouthpart); 1C, right image- brightfield channel only. D- parasitoid adult abdomen.</p

A diagram illustrating the design of experiment 7, transmission of symbionts from <i>B. tabaci</i> to parasitoids, and 8, vertical transmission of symbionts in parasitoids.

<p>Infection status is indicated either by red “+” sign or blue “−” sign. R = <i>Rickettsia</i>, H = <i>Hamiltonella</i>. TRT = treatment. Whitefly hosts are illustrated as small yellow ovals on the (green) leaf disks. To test transmission of symbionts from <i>B. tabaci</i> to parasitoids, one female parasitoid was introduced to each leaf disk for 24 h, after which they were tested by PCR. From the emerging F₁, one or two females from each replicate were used to continue to the vertical transmission experiment, while the rest of the cohort was tested by PCR (two-five from each cohort). The emerging F₂ were all collected and two-five from each cohort were tested by PCR.</p

<i>Rickettsia</i> (white arrows) in <i>Er. eremicus</i> germarium area, between nuclei of stem/pre-follicle/nurse cells as well as outside the ovary, next to the Tunica propria (TP).

<p>Note mature oocyte on the bottom left corner area. N-nucleus; EnC- endochorion; ExC- Exochorion; VE- Vitellin envelope.</p