Abstract
Gibbon ape leukaemia virus (GALV) and koala retrovirus (KoRV) share a remarkably close sequence identity despite the fact that they occur in distantly related mammals on different continents. It has previously been suggested that infection of their respective hosts may have occurred as a result of a species jump from another, as yet unidentified vertebrate host. To investigate possible sources of these retroviruses in the Australian context, DNA samples were obtained from 42 vertebrate species and screened using PCR in order to detect proviral sequences closely related to KoRV and GALV. Four proviral partial sequences totalling 2880 bases which share a strong similarity with KoRV and GALV were detected in DNA from a native Australian rodent, the grassland melomys, Melomys burtoni. We have designated this novel gammaretrovirus Melomys burtoni retrovirus (MbRV). The concatenated nucleotide sequence of MbRV shares 93% identity with the corresponding sequence from GALV-SEATO and 83% identity with KoRV. The geographic ranges of the grassland melomys and of the koala partially overlap. Thus a species jump by MbRV from melomys to koalas is conceivable. However the genus Melomys does not occur in mainland South East Asia and so it appears most likely that another as yet unidentified host was the source of GALV.
Highlights
Koala populations in northern and central eastern Australia are currently undergoing significant declines
We screened a total of 42 native or introduced vertebrate species in Australia for the presence of related viruses, including 19 rodent and seven bat species. We considered these taxa to be the most likely candidate species, either because they transit between Australia and South East Asia or their distributions overlap the ranges of both gibbons and koalas and/or their feeding ecology could result in close contact with gibbons and koalas
The nucleotide sequences of the four Melomys burtoni retrovirus (MbRV) fragments derived from DNA samples from Melomys burtoni are remarkably similar to sequences of Gibbon ape leukaemia virus (GALV) and, to a lesser extent, of koala retrovirus (KoRV)
Summary
Koala populations in northern and central eastern Australia are currently undergoing significant declines. Retroviral particles were first observed in koala lymphosarcoma tissue [9] and partial retroviral sequences were detected in koalas with opportunistic infections [10] and in apparently healthy animals [11]. KoRV was subsequently identified in koalas in Queensland, Australia, and fully sequenced and characterised as an intact gammaretrovirus [12]. The virus has since been detected in all captive colonies tested in Australia, and the majority of freeliving koala populations [6,12,13], as well as captive animals in the USA, Japan and Germany [10,14,15]
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