A virus with an unusual route
Retroviruses take a detour in their life cycle: their genetic material is RNA, but they make a DNA copy inside an infected cell.
From RNA to DNA
Retroviruses carry an enzyme called reverse transcriptase, which copies viral RNA into DNA. Another viral enzyme, integrase, helps insert that DNA into the host cell’s chromosomes.
Once integrated, the viral DNA is called a provirus. The host cell can transcribe proviral DNA into RNA. Some RNA can become genetic material for new particles; other RNA is used to make viral proteins.
When an infection becomes inheritance
A provirus in an ordinary body cell may be copied as that cell divides, but it is not thereby passed to children. For viral DNA to become an inherited part of a species’ genome, an insertion must occur in a germ cell or its precursor, and be transmitted to offspring.
When an ancient retroviral insertion becomes fixed or persists in a population, it is called an endogenous retrovirus (ERV). Many inherited copies have accumulated mutations or deletions and can no longer produce infectious virus.
Scale matters: roughly 8% of the human genome consists of sequences derived from endogenous retroviruses. This does not mean 8% of our DNA is active virus; most of these sequences are remnants.
Can an old viral sequence become useful?
Yes. Evolution can recruit existing genetic material for a new role, a process called exaptation. Human syncytin-1 and syncytin-2 are proteins derived from retroviral envelope genes. They contribute to placental development, including the fusion of certain placental cells.
Related viral-derived genes have been independently recruited in different mammalian lineages. This is not evidence that viruses “intended” to help their hosts; it is an example of evolution acting on inherited variation.
From infection to inheritance
The route from infection to inherited sequence is uncommon and requires several steps. But it leaves a real evolutionary record: some DNA in our cells descends from ancient retroviral integrations, and a small portion has been retained for host functions.
Further reading
Coffin, Hughes and Varmus (eds.), Retroviruses, Cold Spring Harbor Laboratory Press. NCBI Bookshelf edition.
Mi et al., “Syncytin is a captive retroviral envelope protein involved in human placental morphogenesis,” Nature (2000).
For a review of endogenous viruses, see Feschotte and Gilbert, “Endogenous viruses: insights into viral evolution and impact on host biology,” Nature Reviews Genetics (2012).