Torix Rickettsia: aspects of diversity, host range and symbiont-host interaction.



Thongprem, Panupong ORCID: 0000-0001-6542-235X
(2021) Torix Rickettsia: aspects of diversity, host range and symbiont-host interaction. PhD thesis, University of Liverpool.

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Abstract

Rickettsia bacteria have traditionally been considered as the aetiologic agent of deadly arthropod-borne diseases in humans and livestock. However, more recent studies have discovered Rickettsia as non-vertebrate pathogens that are actually important to invertebrate evolution as symbionts. Recently, Rickettsia in the ‘torix’ clade were described from glossiphoniid leeches. This clade has since been observed to infect a wide range of invertebrate species and is thought to be most common in host species associated with freshwater habitats. This leads to a general hypothesis that torix Rickettsia are a common endosymbiont of freshwater taxa. However, this hypothesis is yet to be formally tested. To assess this hypothesis, I firstly investigated in-depth a freshwater-associated insect order, the Odonata (dragonflies and damselflies), in which torix Rickettsia had not been previously recorded. This study revealed the first incidence of torix Rickettsia in odonates, present in roughly 10% of the screened species. Maternal transmission of this endosymbiont was observed in a damselfly (Coenagrion puella), and this strain has likely driven mtDNA introgression between the insect and its sister species (C. pulchellum). Then, I expanded the screen to test for torix Rickettsia in other invertebrate taxa and compared the infection frequency between freshwater and terrestrial communities. Fisher’s exact test indicated that the proportions of infected species from freshwater community is significantly higher than the terrestrial group in three representative insect orders. In addition to this broad screen, torix Rickettsia in a few blood-feeding insects are recorded for the first time, including mosquitos (Anopheles plumbeus), black flies (Simulium aureum) and the common bed bug (Cimex lectularius). Bed bugs were then established as a model system to study biological impacts of torix Rickettsia carriage. Symbionts in the bed bug were transmitted via matrilines only. There were no signs of reproductive parasitism, sex ratio distortion or cytoplasmic incompatibility phenotypes. Torix Rickettsia only express mild parasitic impacts on C. lectularius biology by slowing development time and reducing fecundity. Finally, this thesis raises three questions for onward study; i) why torix Rickettsia are abundant in freshwater biomes, ii) how do torix strains transition into terrestrial species and iii) how torix Rickettsia are associated with broad spectrum of eukaryotic hosts. Possible scenarios for these three questions are discussed for future study.

Item Type: Thesis (PhD)
Divisions: Faculty of Health and Life Sciences
Depositing User: Symplectic Admin
Date Deposited: 22 Apr 2021 15:54
Last Modified: 18 Jan 2023 22:52
DOI: 10.17638/03119474
Supervisors:
URI: https://livrepository.liverpool.ac.uk/id/eprint/3119474