One hundred twenty years of koala retrovirus evolution determined from museum skins
Avila-Arcoa, MC, Ho, SYW, Ishida, Y, Nikolaidis, N, Tsangaras, K, Hönig, K, Medina, R, Rasmussen, M, Fordyce, SL, Calvignac-Spencer, S, Willerslev, E, Gilbert, MTP, Helgen, KM, Roca, AL & Greenwood, AD 2013, Molecular Biology and Evolution, vol. 30, no. 2, pp. 299-304.
Historical DNA samples from museum-held koala skins displayed comparable proviral koala retrovirus (KoRV) envelope sequences to extant samples, suggesting the spread and subsequent infection of KoRV occurred over a greater period of time than previously thought. Being the only known retrovirus to be currently infecting its host’s germ line, KoRV has remained an issue for koalas since the 19th century and may have actually increased the susceptibility of koalas to chlamydial infection as a result of its evolving and expeditious nature.
Sequences of the proviral envelope region of modern koalas positive for KoRV were compared against sequences of museum koala skins to examine the hypothesis of a recent cross-species transmission of a retrovirus to northern koalas. For all specimens examined, the character states at all except for one position along the envelope gene were the same as those previously described for KoRV. Although some polymorphisms were observed, in the museum specimens one of the two character states observed at each env polymorphism was identical to that described in modern KoRV, suggesting the conservation of the retrovirus for over 100 years. Of the 16 museum-held northern koalas, 15 were positive for KoRV, inferring that KoRV has been ubiquitous and slowly spreading since the 18th century. The sedentary behaviour of koalas coupled with their infrequent interactions and geographical segregation due of human activities may account for the relatively slow spread of KoRV over two centuries.
Previous attempts to reconstruct endogenisation events in vertebrate genomes have proven difficult due to remnants of retrovirus being dated. Currently KoRV inhabits the koala’s germ line allowing for extant endogenisation studies to occur to better understand the processes of transmission that may eventuate secondary pathologies like Chlamydia over time.
By investigating the nature of KoRV throughout history, these findings enhance our modern understanding of the prevalence and spread of KoRV over time and can assist in understanding its impacts on koala populations today.
Summarised by Lauren Mousley
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