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Outer membrane protein A gene sequencing demonstrates the polyphyletic nature of koala Chlamydia pecorum isolates

Jackson, M, Giffard, P & Timms, P 1997, Systematic and Applied Microbiology, vol. 20, no. 1, pp. 187-200.

When the phylogenetic relationships of sequences of a chlamydial major outer membrane protein gene from Chlamydia pecorum isolates of both koala and non-koala origin were examined, the genotypes sequenced were found to be genetically distinct. Rather than being represented as a single branch in the C. pecorum phylogenetic tree, the koala isolates separated into five unique genotypes. Of these five genotypes, two were specific to koalas, and three were shared with sheep, cattle or pigs.

  The purpose of this study was to describe the genetic diversity and evolutionary relationships of C. pecorum isolates from koalas to isolates from both other koalas and other animals. DNA sequences of the section of the chlamydial major outer membrane protein called ompA VD4 from C. pecorum-positive swab samples of 15 koalas and ten other animals were analysed. Other chlamydial strains from various origins documented in the literature were included in the study for phylogenetic comparison. Phylogenetic analysis revealed that the DNA sequences of C. pecorum of koala origin separated into five distinct groups, and these were named genotypes A-E. Genotypes A and E each comprised five near-identical isolates and were specific to koalas. Genotypes B, C and D were each represented by only a single koala strain, and in each case koala C. pecorum isolates were very similar to C. pecorum isolates from sheep, cattle or pigs. Each of the five koala C. pecorum genotypes was more closely related to isolates from sheep, cattle or pig origin than to other koala genotypes. This implies the unlikelihood of all koala C. pecorum strains having descended from a common ancestor that was once transmitted to a koala. Instead, the results suggest that koala C. pecorum strains are polyphyletic, meaning they are grouped into a category that does not include their most recent common ancestor because the characteristics of each are not necessarily shared.

  Chlamydia is one of the most significant threats to koalas, causing keratoconjunctivitis, rhinitis, urinary tract disease and ultimately infertility. In line with the evidence presented in this study, the most feasible explanation for the polyphyletic nature of koala C. pecorum strains is that koalas acquired the pathogen via multiple cross-species transmission events. This is conceivable given the transformation of the Australian continent post-colonisation, with a geographically isolated landscape abruptly exposed to a variety of exotic species. It is possible, for example, that the grazing of cattle adjacent to remnant koala habitat led to the transmission of bovine C. pecorum to the vulnerable koala via contamination of its habitat with parasites in cattle faeces.

  The C. pecorum isolates examined in this study were from a small and possibly biased sample of koalas, and therefore broad conclusions about their genetic origins and relations to clinical disease cannot be made. As a result of the findings presented here, the authors suggest that future research utilise more extensive methods to examine whether different C. pecorum genotypes are associated with different manifestations of disease.

 

Summarised by Joanna Horsfall

 

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