Virolution by Frank Ryan

Author:Frank Ryan
Language: eng
Format: epub
Publisher: HarperCollins Publishers
Published: 2013-04-08T04:00:00+00:00

The virus goes by the name of the Jaagsietke sheep retrovirus, or JSRV, a translation from the Afrikaans “Jaagsietke” which means the “chasing illness”, because it resembles the breathlessness seen in sheep after they have been chased by a dog. The virus exists as two almost identical variants – the terms “ego” and “alter ego” spring to mind – with the ego taking the form of an endogenised, and thus “domesticated”, virus, in the jargon the enJSRV, and the alter ego, the JSRV, a feral exogenous variant that causes a contagious lung cancer in sheep, which is a common problem in many sheep-rearing countries, and a particular problem in the UK.3 Ego and alter ego really do appear to reflect twin origins – since more than 90% of their genetic sequences are shared by the two viruses. Long ago, the exogenous form of the virus infected the common ancestor of sheep and goats. It seems likely that the route of infection was genital, as is usual with retroviruses, after which it endogenised into the genome. From then on, the story became increasingly complicated, and interesting.

The endogenous virus took on the role of enabling placentation in sheep. This was the focus of an earlier study by Palmarini and others.4 The placenta in sheep is very different to that in humans, with no confluent syncytium, and what the virus appears to do is to alter the behaviour of the junctional cells, or trophoblasts, transforming them into placental capability, and thus enabling them to create the sheep version of the maternal-foetal barrier. At the time I visited, in March 2009, there was a heady buzz of excitement in the department that heralded another pioneering study, which had just been accepted for publication by the prestigious American journal, Science.5 This had involved a cooperative effort embracing many different countries and aimed at extrapolating patterns of chromosomal insertions of the endogenous virus as genetic markers to track the historic movement of domesticated sheep around the world. Sheep and goats were the first livestock to be domesticated, initially for their meat, and perhaps by the fifth millennium BP for secondary products such as wool, and this study was the first in which an endogenous retrovirus had been used to provide valuable insights into the history of pastoralist societies.

The outcome was fascinating in many respects. But there was one aspect that puzzled me, a facet of timing that was of general relevance to retroviruses. I asked Palmarini just how long ago the Jaagsietke virus had first infected sheep.

‘At some time before the divergence of sheep and goats – the ovine and capra lineages – which is estimated at 5 to 7 million years ago.’

‘So you find it in goats too?’

‘Only the very oldest ones.’

‘You mean the oldest – the most primitive – goats?’

‘No – the oldest proviruses. The ones that are found in both sheep and goats. Some viruses are only found in goats and some are only found in sheep.’

‘But these are similar viruses?’

‘Yes, they are.’

‘So you’re looking at different families of viruses?’

‘No – just different integration groups.

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