Re: mitochondria virus?

Regarding the possibility that a virus exists which could specifically 
infect mitochondria and use mitochondrial DNA:

Because of the theory that mitochondria were once organisms, rather than 
organelles, it is indeed possible and even likely that there were viruses 
which could infect the mitochondria.  It is also possible, although less 
likely, that these viruses would still exist today.  Whether or not they 
still exist would primarily depend on how they were able to attack the 
mitochondria.

It is important to keep in mind that mitochondria have an extra advantage 
over their cells in fighting viruses: their double membrane.  In order for 
a virus to infect a cell, the viral particle must convince the cell 
membrane to let it in.  In order to do this, viruses tend to mimic other 
substances that the cell normally takes in.  For a virus to infect the 
mitochondria, however, it would have to pass through three separate 
membranes.  It would first have to convince the cell membrane to let the 
virus pass, then the outer mitochondrial membrane, then the inner 
membrane.  Very few substances normally have access to all three; 
therefore, the virus may have to mimic multiple substances.  
Unfortunately, many viruses do this all too well.

Another defense the mitochondria has is that some viruses bind to the cell 
membrane and inject the viral DNA/RNA into the cytoplasm; the viral 
protein coat does not enter the cell.  Because the viral coat is what 
fools the membrane into letting the virus in, it cannot access the 
mitochondria.

Because of the extra anti-viral defenses mitochondria have as organelles, 
it is probable that any mitochondria-targetting viruses would have died 
off for lack of hosts.  Unless the virus was able to adapt quickly enough 
to overcome the extra cell membranes when mitochondria became organelles, 
they would no longer be able to infect the mitochondria, and thus would 
not be able to replicate.  If this virus was not able to infect any other 
cell types, it would eventually die off.