Abstract
Transcriptome analysis of polar bear (Ursus maritimus) tissues identified sequences with similarity to Porcine Endogenous Retroviruses (PERV). Based on these sequences, four proviral copies and 15 solo long terminal repeats (LTRs) of a newly described endogenous retrovirus were characterized from the polar bear draft genome sequence. Closely related sequences were identified by PCR analysis of brown bear (Ursus arctos) and black bear (Ursus americanus) but were absent in non-Ursinae bear species. The virus was therefore designated UrsusERV. Two distinct groups of LTRs were observed including a recombinant ERV that contained one LTR belonging to each group indicating that genomic invasions by at least two UrsusERV variants have recently occurred. Age estimates based on proviral LTR divergence and conservation of integration sites among ursids suggest the viral group is only a few million years old. The youngest provirus was polar bear specific, had intact open reading frames (ORFs) and could potentially encode functional proteins. Phylogenetic analyses of UrsusERV consensus protein sequences suggest that it is part of a pig, gibbon and koala retrovirus clade. The young age estimates and lineage specificity of the virus suggests UrsusERV is a recent cross species transmission from an unknown reservoir and places the viral group among the youngest of ERVs identified in mammals.
Highlights
Retroviruses are a large and diverse family of single stranded positive sense RNA viruses [1,2]
A virome study examining the brain tissue of two polar bears (Knut of the Berlin Zoological Garden (Berlin, Germany) and Jerka of the Wuppertal Zoological Garden (Wuppertal, Germany) identified sequences ranging from 73% to 80% identity, at the DNA level, to porcine endogenous retrovirus (PERV), Mus dunni endogenous retrovirus (MDEV), Feline sarcoma virus (FeLV), and Murine leukemia virus (MLV) genes [23]
A blastn search was performed on all 72,214 scaffolds of the draft polar bear genome sequence using, as query sequences, brain derived transcriptome retroviral sequences with the highest percentage similarities to MDEV and Porcine Endogenous Retroviruses (PERV) [21]
Summary
Retroviruses are a large and diverse family of single stranded positive sense RNA viruses [1,2]. Unlike ERVs in other species, there are koalas in southern Australia that lack endogenized KoRV, they show no signs of exposure whatsoever to endogenous or exogenous KoRV. As genome data from a growing number of species becomes publicly available, a greater diversity of ERVs and their origins and evolution can be characterized. The family Ursidae is comprised of three extant genera including the (i) Ailuropoda, containing the only non-carnivorous bear species, the giant panda (Ailuropoda melanoleuka); (ii) Tremarctos, with the spectacled bear (Tremactos ornatus), the only extant species; and the (iii) Ursinae that is represented by six extant species. Genomic data has been made publicly available for several bear species providing the necessary resources for characterization of their ERV content [27,28,29]. The results suggest that along with koalas and sheep, polar bears may carry very recently endogenized retroviruses
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