Spumaretrovirinae is a subfamily of Retroviridae that includes the foamy viruses. The name refers to a characteristic foamy appearance that many isolates produce in cell culture. That visible effect is one observation within a classification also supported by molecular features and phylogenetic analysis.[ref]
Retroviridae report by the International Committee on Taxonomy of Viruses. Derivation of names, Spumaretrovirinae; Relationships within the family
A notable feature concerns the timing of reverse transcription. Released virions can already contain substantial reverse-transcribed DNA. In Orthoretrovirinae, the usual packaged genome is instead two RNA copies and reverse transcription follows entry into a new host cell. Both groups use a DNA stage in their replication cycles.[ref]
Spumaretrovirinae report by the International Committee on Taxonomy of Viruses. Distinguishing features, reverse-transcribed DNA in virions
[ref]Orthoretrovirinae report by the International Committee on Taxonomy of Viruses. Distinguishing features, RNA packaging and post-entry reverse transcription
The ICTV account also describes cytoplasmic capsid assembly and a genome of roughly 11.6 kilobases for a typical spumavirus. Its viral enzyme precursor is expressed from a spliced transcript, rather than the same fused precursor arrangement emphasized for Orthoretrovirinae.[ref]
Spumaretrovirinae report by the International Committee on Taxonomy of Viruses. Distinguishing features, assembly, typical genome and Pro/Pol expression
The timing distinction gives a useful experimental question: at which point in the cycle is a particular nucleic-acid form present? Detecting DNA in released particles and detecting newly made DNA after entry describe different stages. A single positive measurement, without identifying the sampled stage and material, cannot reconstruct when conversion occurred. Molecular form, location and sequence in time are complementary evidence.
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