** What are yeast? **

[Richard Winder]

In article <thanh.9.2EACE999 at wwg3.uovs.ac.za>, thanh at wwg3.uovs.ac.za writes:
>
>Hi!
>For most of people yeasts are Sac. cerevisiae.
>For most of yeastists yeasts are the fungi with single cells stage of 
>development. But Mucor spp. can give us such kind of cells in condition 
>without O2 as well.  
>So, how do you think about "what are yeast" and "their origin"?
>
>    origin of yeasts:
>          / ...                    / fungi1             / yeasts
>    Fungi - ...        or    yeast - fungi2     or    X - fungi
>          \ yeasts                 \ fungi...           \ ...
>
>    or ...

I don't know if you can equate yeast cells with a hypothetical unicellular
protist/alagal ancestor- perhaps you can in the `lower' taxons.  Many higher 
filamentous fungi also produce yeast-like growth in liquid culture- whether 
this common adaptation is homologous or analagous to a reversion back to an 
ancestral form or not is an interesting question (essentially, is there 
recapitulation of evolution in fungi, as there seems to be in mammalian 
embryogenesis?), but I don't know if we have all the information we need to 
answer it.  Some claim that mycelia have developed analogously in the chytrid
line and the oomycete line from different ancestral precursors- why not the 
analagous development of yeasty propagules as well (particularly in the 
deuteromycetes)?  On the other hand, unicellular vegetative structures seem
to be important enough to be conserved throughout fungal evolution (I'm
thinking of germinated spores, and the ability of single-celled hyphal
fragments to propagate).  I'm not an expert in the field of fungal 
phylogeny- it would be interesting to hear what the lasted story on all of 
this is.  -RSW


  RICHARD WINDER                    Title: Research Scientist
  Canadian Forest Service           Phone: (604) 363-0773
  Victoria, B.C.                    Internet: RWINDER at A1.PFC.Forestry.CA