title: Wolbachia pseudogenes and low prevalence infections in tropical but not temperate Australian tephritid fruit flies: manifestations of lateral gene transfer and endosymbiont spillover?
abstract: BACKGROUND
  Maternally inherited Wolbachia bacteria infect many insect species. They can also be transferred horizontally into uninfected host lineages. A Wolbachia spillover from an infected source population must occur prior to the establishment of heritable infections, but this spillover may be transient. In a previous study of tephritid fruit fly species of tropical Australia we detected a high incidence of identical Wolbachia strains in several species as well as Wolbachia pseudogenes. Here, we have investigated this further by analysing field specimens of 24 species collected along a 3,000 km climate gradient of eastern Australia.
  
  
  RESULTS
  Wolbachia sequences were detected in individuals of nine of the 24 (37 %) species. Seven (29 %) species displayed four distinct Wolbachia strains based on characterisation of full multi locus sequencing (MLST) profiles; the strains occurred as single and double infections in a small number of individuals (2-17 %). For the two remaining species all individuals had incomplete MLST profiles and Wolbachia pseudogenes that may be indicative of lateral gene transfer into host genomes. The detection of Wolbachia was restricted to northern Australia, including in five species that only occur in the tropics. Within the more widely distributed Bactrocera tryoni and Bactrocera neohumeralis, Wolbachia also only occurred in the north, and was not linked to any particular mitochondrial haplotypes.
  
  
  CONCLUSIONS
  The presence of Wolbachia pseudogenes at high prevalence in two species in absence of complete MLST profiles may represent footprints of historic infections that have been lost. The detection of identical low prevalence strains in a small number of individuals of seven species may question their role as reproductive manipulator and their vertical inheritance. Instead, the findings may be indicative of transient infections that result from spillover events from a yet unknown source. These spillover events appear to be restricted to northern Australia, without proliferation in host lineages further south. Our study highlights that tropical fruit fly communities contain Wolbachia pseudogenes and may be exposed to frequent horizontal Wolbachia transfer. It also emphasises that global estimates of Wolbachia frequencies may need to consider lateral gene transfer and Wolbachia spillover that may be regionally restricted, transient and not inherited.
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, 2751, Australia. j.morrow@westernsydney.edu.au.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Morrow
authors.affiliation: Evolution and Ecology Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, 2052, Australia. m.frommer@unsw.edu.au.
authors.forename: Marianne
authors.initials: M
authors.lastname: Frommer
authors.affiliation: Queensland Department of Agriculture and Fisheries, EcoSciences Precinct, 41 Boggo Road, Dutton Park, QLD, 4102, Australia. jane.royer@daf.qld.gov.au.
authors.forename: Jane E
authors.initials: JE
authors.lastname: Royer
authors.affiliation: Evolution and Ecology Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW, 2052, Australia. d.shearman@unsw.edu.au.
authors.forename: Deborah C A
authors.initials: DC
authors.lastname: Shearman
authors.affiliation: Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, 2751, Australia. m.riegler@westernsydney.edu.au.
authors.forename: Markus
authors.initials: M
authors.lastname: Riegler
mesh_terms: D000818:Animals; D001315:Australia; D003001:Cloning, Molecular; D022761:Gene Transfer, Horizontal; D006239:Haplotypes; D058885:Multilocus Sequence Typing; D010802:Phylogeny; D011544:Pseudogenes; D033621:Tephritidae / Q000145:classification* / Q000382:microbiology* / Q000502:physiology; D020577:Wolbachia / Q000235:genetics* / Q000302:isolation & purification* / Q000502:physiology