RE: Mice given bat-like forelimbs through gene switch

[Tim Williams <twilliams_alpha@hotmail.com>]

Jura wrote:


> Technically bacteria and viruses are not organisms, as
> they are only single celled. 


I don't know where you got this from, but it is simply not true.  Bacteria are 
certainly organisms.  They are single-celled organisms.  So are archaea.  Many 
eukaryotes are single-celled too, especially those critters historically known 
as "protists" or "protozoans".


Viruses are not "single celled".  They are "no celled".  Viruses are RNA- or 
DNA-containing entities that cannot reproduce outside a host cell.  Whether or 
not viruses constitute an "organism" is principally a matter of semantics.


David Marjanovic wrote:


> In fact, what is a bacterial 'species' or an archaeal one; Most species 
> concepts are simply not applicable, and the rest needs arbitrary cutoff 
> points; similarity in genome sequence; similarity in AT content. It 
> would make a lot more sense to just talk about clades. 


Most prokaryote (bacteria + archaea) taxonomy is quantitative, and based on the 
degree of similarity (sequence homology) of the 16S rRNA gene.  So yes, the 
erection of a new prokaryote genus or species does tend to follow a 'cut-off'.  
There are exceptions to this practice: certain highly pathogenic or infectious 
bacterial strains, which based solely on their 16S rRNA gene sequences would be 
sunk, have been upheld as valid genera or species due to their medical 
importance (e.g., _Shigella_spp., which otherwise would be referred to _E. 
coi_).


Prokaryote taxonomy does follow a clade system to some degree.  Classification 
is based on the sequence homology of the 16S rRNA genes.  Aside from that, the 
concept of 'clade' doesn't really hold for bacteria and archaea, given the 
propensity of interspecific gene transfer.  It's not uncommon to find genes 
sharing high sequence homology shared between distantly related organisms (like 
David's example of thermophilic archaea and bacteria).  As Dann mentioned, this 
famously applies to genes responsible for antibiotic resistance - much to the 
detriment of the human species.  


Cheers

Tim
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