retroviruses; mammals

[Don McAllister]

Eric Zurcher wrote:

> At 14:06 29/09/99 PDT, Ken Kinman wrote:
> >     Although many viruses are probably "escaped genes", I think the
> >statement by Margulis and Schwartz is perhaps a little simplistic.

Lest readers should think that viruses are all shaped to the same mold..., they
do have their own morphological as well as genetic diversity and this may
suggest natural groupings amongst them.

Helical viruses resembe long rods that may be rigid or flexible.  The nucleic
acid is found within a hollow, cylindrical capsid tha has a helical structure.

Polyhedral viruses.  Many animal, plant and bacterial viruses are polyhedral -
that is have many-sided enclosing protein coats.  Most are in the shape of a
icosahedron, an regular polyhedron with 20 triangular faces and12 corners.

Enveloped viruses.  The capsid of some viruses is covred by an envelope. These
are roughly spherical.

Complex viruses.  Some viruses, especially bacterial viruses have very
complicated structures and are called complex viruses. One looks vaguely like
the lunar landing module with a facetted 'cabin', a cylindrical base, and a
base plate with six angled 'legs.'

If one assumes that polyhedral virus structures are too complex to have arise
more than once, then this would seem to reduce the number of independent
origins of viruses, as the polyhedrals infect animal, plant and bacteria.

For myself, speaking from vast ignorance, the facts that I am aware of suggest
to me that viruses do fall into the category of living organisms - they
reproduce with DNA or RNA, admittedly with the help of non-viral species; that
they might well have had a single origin or possibly a limited number of
independent origins. Their common protein envelope and mode of reproduction is
one set of shared characterstics.  Should not Occham's razor be applied, a
single origin until disproved?  In additon there is the size factor. Most are
smaller than bacteria (hence they pass the classic filter test), but a few are
about the same size as some very small bacteria.  Lengths range from 20 to
14,000 nm in length

The total amount of nucleic acid varies from a few thousand nucleotides to as
many as 250,000.  Compare that with the bacterium, E. coli's roster of about 4
million nucleotide pairs.

Viruses are grouped by nucleic acid type, morphology, and the presence or
absence of an envelope. The International Committee on Taxonomy of Viruses has
organized the 2000 known species into 73 families each of which share
morphological characteristics and reproductive strategies.

ICTV has not yet established order through kingdom categories, except the
complex viruses will probably be placed in their own order and enveloped
viruses with one negative strand of RNA have been classed in the newly
established order Mononegavirales.

Family names end in -viridae, genus names have the suffix -virus.

don
Don McAllister