The hidden diversity of ancient bornaviral sequences from X and P genes in vertebrate genomes

Endogenous viral elements are not only derived from retroviruses but also from bornaviruses. Existence of endogenous bornavirus-like elements (EBLs) is attributed to reverse transcription of bornavirus mRNA followed by integration into host genome by long interspersed nuclear element-1 retro-transposon. However, detection of EBLX/Ps is limited as compared to EBLs derived from bornaviral N, P, M, G and L genes. Low detection of EBLX/Ps was hypothesized to result from limitations of sequence similarity search using tBLASTn. A 1.9-kb miEBLN-1 insertion present in genome of miniopterid bat was used for analysis. The 1.9-kb insertion consisted of 1.2-kb miEBLN-1 region and 0.7-kb unannotated region, and was similar in size to N/X/P mRNA of present orthobornaviruses. The insertion is shown to have originated from ancient N/X/P transcript which integrated into the genome of miniopterid bat. The findings illustrate existence of hidden EBLN/X/Ps or EBLX/Ps in vertebrate genomes. Despite of high evolutionary rates of X and P genes, the genes are likely to have been maintained for million years as demonstrated by intact X and P-like open reading frames of miEBLX/P which were expressed as a fusion transcript with cellular gene, ZNF451. Expression of ZNF451/miEBLP-4 chimeric transcript suggested potential co-option of EBLX/P in miniopterid bats.
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