dd@a.UUCP (Dan Davison) (08/21/87)
A while back (two months or so) I mentioned in a posting on another matter that the urkaryotes are still with us, that is, we is them. That is incorrect and comes from remembering the last half of a sentence in a paper rather than the first half. For those who came in late, it has been proposed (Woese and Fox, PNAS 74(11):5088-5090 1977, and J. Molecular Evolution 10(1):1-6, 1977) that the eucaryotic cytoplasm represents a line of descent that is separate from the typical bacterial line. This is now known to workers in the ribosomal RNA field as the "Three kingdoms"- archebacteria (salt and high temperature- loving anuclear cells), eubacteria (the cells meant by the word "bacteria") and eucaryotes. This division is based on analysis of small ribosomal subunit RNAs (the 16S-like rRNAs) that clearly shows three distinct branches. [Oops, the archaebacteria also include methanogens.] When the subject came up last time around I made a comment that the urkaryote was still around-it's our eucaryotic cell. As you can guess, that's half right. The urkaryote is the source of the eucaryotic cytoplasm, "an ameboid anaerobic entity free of organelles and the like" (JME reference, pg. 5). If you've been reading this newsgroup, organisms mentioned by "baby doc" werner and myself in recent postings fit some of these requirements. The point left open by the papers is...did the urkaryote have an enveloped genetic material (nucleus)? If you've read Margulis and Sagan's _The Origins of Sex: Three Billion Years of Genetic Recombination_ you'll have your own ideas about this one. Anyone care to speculate, based on knowledge of appropriately weird organisms? dan davison/theoretical biology/t-10 ms k710/los alamos national lab/los alamos NM 87545/ dd@lanl.gov (arpa) / uucp: ...cmcl2!lanl!dd (maybe)