PROGRESS IN PROTOZOOLOGY
PROGRESS IN PROTOZOOLOGY
PROGRESS IN PROTOZOOLOGY
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THE MOLECULAR DIVERSITY OF TETRAHYMENA PYRIFORMIS 245<br />
ing eukaryotic beginnings and protistan relationships, but the means<br />
are at hand for definitive solutions and they will soon be ours.<br />
The firsl; modern eukaryote probably appeared on the evolutionary<br />
scene something like 1.8-2.0 billions of years ago, about half the time<br />
since the appearance of the progenote. It likely appeared as the result<br />
of a single improbable collusion — a genetic conspiracy involving two<br />
or more evolutionary lineages that had been separated for some 2 billions<br />
of years. The saltation (or punctuation) created in some as yet uncertain<br />
way a "community" organization with vastly improved capabilities.<br />
This organization involved at least three major improvements: a plasma<br />
membrane capable of phagocytosis, pinocytosis, excitability and enhanced<br />
sensitivity; a microtubular system capable of forming the 9 + 2 external<br />
motility organelles, and the corresponding centrioles and associated<br />
elements responsible for moving internal structures; the nucleosomic<br />
apparatus, consisting of the 5 classes of basic proteins called histones,<br />
which make chromosomes possible and through them the storage and<br />
manipulation of orders of magnitude more nucleic information than<br />
could be managed by more primitive organisms.<br />
The important thing for present considerations is that the new eukaryote<br />
inventions, like the earlier ribosome, were quickly brought to<br />
a state of efficiency incapable of improvement without radical redesign.<br />
The 9 + 2 structure of cilia and flagella does not vary significantly<br />
between Chlamydomonas and Homo. The excitable membrane of Paramecium<br />
has properties remarkably similar to those in the nervous<br />
system of the squid. The nucleosomes of Tetrahymena are scarcely distinguishable<br />
from those of Drosophila or maize.<br />
These fundamental eukaryotic structures provide, therefore, conservative<br />
molecules whose variations can be used to measure large<br />
evolutionary distances. Not all molecules are equally useful in different<br />
kinds of evolutionary enquiry. Variations between certain histone molecules<br />
are essentially useless in studying vertebrate relations, for example;<br />
the vertebrates are of far too recent origin to have accumulated<br />
a statistically useful number of histone differences. On the other hand,<br />
evolutionarily labile molecules such as esterases may be useful in discriminating<br />
among insect subspecies, but may be of no value in examining<br />
more ancient evolutionary separations; they are so variable<br />
as to supply only meaningless noise when the organisms being compared<br />
are of more distant relationships.<br />
I apologize (insincerely) for taking so much time to display my<br />
prejudices concerning the principles of molecular evolution, and also<br />
for not documenting more fully the evidences for them. Time does not<br />
permit a full explanation, however, and a summary is necessary for<br />
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