Expanding the size limit of RNA viruses: Evidence of a novel divergent nidovirus in California sea hare, with a ~35.9 kb virus genome

Humberto J Debat

The availability of a public collection of NGS RNA libraries of the model organism A. californica, allowed the tentative survey of the genomic RNA landscape of AcNV. Virus RNA transcripts were detected in an outstanding diversity of target tissue and organs, suggesting e pervasive tissue tropism, showing an emergent enrichment of AcNV in the CNS. Single-cell RNAseq data allowed the surveillance of AcNV at the cellular level, including not only specific types of neuron (e.g. abdominal ganglia neurons) but also the detection of AcNV in welldefined individual neurons of A. californica (e.g. R2, L29, R15). Interestingly, AcNV RNA accumulated at high levels in R2, which is the largest known neuron in the animal kingdom, offering an outstanding and unprecedented model for the study of virus dynamics at the single cell level. Besides their accessibility, ease to identify and size, each R2 neuron may yield over 1.9ug of RNA allowing to amplification-free RNA profiling at a single-neuron level (Moroz & Kohn, 2013) . The standard of single-cell RNAseq preparation in any other organism cells, which have picogram levels of nucleic acids, is a robust amplification step, which may lead to irregular coverage, noise and imprecise quantification of sequencing data (Islam et al., 2011) .