beneficial virus question

[Adrian.Philbey at SMTPGWY.AGRIC.NSW.GOV.AU]

          In <4krqsb$47e at nms.telepost.no>, gronvold at telepost.no (Vidar
          Grønvold) some time ago wrote:

          >Bacteria can be either deletrious or helpful in an
          >organism.  But what about virus. Is there known any helpful
          >function of virus in nature? I find it hard to believe they
          >exist as only a deletrious thing.

          Some endogenous retroviruses of mice are thought to be
          protective against exogenous retroviruses of the same viral
          biotype. The theory is that endogenous retroviral proteins
          produced by the cell bind to surface molecules that are
          used as cell receptors, resulting in competitive exclusion
          of the exogenous retroviruses. Since exogenous retroviruses
          are capable of inducing neoplasia in mice, there is likely
          to be a selective advantage in maintaining endogenous
          retroviral sequences in the genome that produce proteins
          that interfere with infection.

          During selection programmes for beef cattle, there have
          sometimes been demands for short, stocky animals according
          to market trends; examples in Australia in the past have
          been in commercial Hereford and Poll Hereford herds.
          Persistent infection with bovine pestivirus (bovine virus
          diarrhoea virus) can contribute to short stature in cattle
          without other apparent adverse effects. There have been
          cases where the desire for short, stocky cattle has resulted
          in the selection of bulls persistently infected with
          pestivirus. Therefore, it could be said that, in this
          artificial situation, bovine pestivirus has contributed to
          perpetuation of the genetic material of individuals that are
          persistently infected. Another situation where this effect
          could occur is in animal research. One of the classical
          studies of the dynamics of pestivirus infection in cattle
          was done at a research station at Trangie in New South Wales
          where Angus cattle had been divided into two herds, one
          selected for rapid growth and the other selected for slow
          growth in order to study feed conversion efficiencies,
          amongst other parameters. The herd selected for slow growth
          was found to have a high proportion of persistently
          pestivirus-infected animals. There is also a demand in
          research for mini-breeds of several species, since they
          usually cost less to house and feed. There is the potential
          for persistently pestivirus-infected cattle to be selected
          for in such breeding programmes. However, more usually
          persistently infected animals are unthrifty, with poor
          growth rates and rough coats, as well as being liable to
          develop fatal mucosal disease, so the situations I have
          described are contrived.

          Adrian W Philbey
          Veterinary Research Officer
          Elizabeth Macarthur Agricultural Institute
          Private Mail Bag 8
          Camden  NSW  2570
          Australia
          
          Telephone: 61-46-293332
          Facsimile: 61-46-293429
          email: philbea at agric.nsw.gov.au