PROGRESS IN PROTOZOOLOGY

246 D. L. NANNEY
background against which to examine the molecular variety of the
Tetrahymena pyriformis complex.
(2) Tetrahymena Arose Very Early after the Eukaryotic Saltation
In principle the techniques of molecular biology are capable of unrevelling
the tangled history of life in whatever detail may be desired.
In practice their application is constrained. Some of the techniques
require high technology, equipment and expertise not available to everyone.
An even more severe restriction is the need, in most cases, to
have at least semi-domesticated organisms for study. The organisms need
to be capable of growth under controlled conditions which permit the
introduction of appropriate labels into their molecules. The happenstance
of Tetrahymena's domesticity, first demonstrated by Lwoff (1923)
over half a century ago, is responsible for the abundance of information
about its molecular characteristics. Tetrahymena has been a favorite
object of biochemical analysis in dozens of laboratories for dozens
of years. Unfortunately, this work has not been systematic; it has not
always been applied to properly identified strains, and it has not always
been viewed in appropriate evolutionary perspective. The useful information
is fragmentary.
I will briefly summarize here some of the studies on Tetrahymena
that place it in the context of the broad evolutionary tapestry I displayed
earlier. I wish I could show you the 16S ribosomal RNA from
Tetrahymena, but this is not yet available. Perhaps the next best thing
is the 5S ribosomal RNA recently reported by L u e h r s e n et al.
(1980). The Tetrahymena 5S RNA is without question a true eukaryotic
5S RNA, but it has some features which are distinctive. It contains, for
example, a sequence CGAAC beginning at position 40 that had previously
been found in all eubacterial 5S molecules but in no eukaryotic 5S
molecules. This molecule from Tetrahymena is, with the exception of
that from yeast, the most atypical eukaryotic molecule that has been
examined. The force of this statement is weakened by the realization
that relatively few eukaryotic 5S RNA molecules have been sequenced.
Tetrahymena thermophila is the only Tetrahymena, the only ciliate,
the only protozoan according to a restrictive definition, for which we
have 5S RNA data. Nevertheless, the data indicate that Tetrahymena
branched off the eukaryotic stem early; precisely how early is still in
question.
A second reputedly conservative molecule for which we have some
Tetrahymena data is histone H4. On superficial analysis, this chromosomal
constituent seems to be very mudh like all other H4 molecules
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