The genome and phylogenetic analyses of tit siadenoviruses reveal both a novel avian host and viral species.

In both a Eurasian blue tit (Cyanistes caeruleus) and a great tit (Parus major), found dead in North Rhine-Westphalia, Germany, intranuclear inclusion bodies were observed in the kidneys during the histologic examination. Siadenoviruses were detected in both samples, and the nucleotide sequence of the partial DNA polymerase, obtained from the blue tit, was almost identical with that of great tit adenovirus type 1, reported from Hungary previously. The sequence, derived from the German great tit sample was more similar to great tit adenovirus 2, yet divergent enough to forecast the possible establishment of a novel viral type and species. Therefore, the complete genome was subjected to next generation sequencing. The annotation revealed a typical siadenoviral genome layout, and phylogenetic analyses proved the distinctness of the novel virus type: great tit adenovirus 3. We propose the establishment of a new species (Siadenovirus carbocapituli) within the genus Siadenovirus to contain great tit adenovirus types 2 and 3. As both of the newly-detected viruses originated from histologically confirmed cases, and several siadenoviruses have been associated with avian nephritis earlier, we assume that the renal pathology might have been also of adenoviral origin. Additionally, we performed structural studies on two virus-coded proteins. The viral sialidase and the fiber knob were modeled using the AlphaFold2 program. According to the results of the sialic acid docking studies, the fiber trimer of the new great tit adenovirus 3 binds this acid with good affinity. As sialic acid is expressed in the kidney, it can be hypothesized that it is used during the receptor binding and entry of the virus. Strong binding of sialic acid was also predictable for the viral sialidase albeit its enzymatic activity remains disputable since, within its catalytic site, an asparagine residue was revealed instead of the conserved aspartic acid.