PROGRESS IN PROTOZOOLOGY

264 D. L. NANNEY
considerably among the species? Who would guess in advance that the
atypical linearity of the mitochondrial DNA would be found in all the
Tetrahymena species, even when their sequences are scrambled beyond
recognition? Why is the molecular weight of tubulin critical to the
structure of the cilium? How can one account for the invariance of the
molecular weight of the large cortical protein? What is so special about
the karyotype of Tetrahymena, that its five pairs are preserved even
when the base ratios of DNA shift by 8-10°/o? What is so significant
about the size of T. pyriformis? Why can't tetrahymenas grow as large
as paramecia? Why does the contractile vacuole pore open asymmetrically
on the side of the cell? on the right side? at 25% of the distance
around the cell? How does one explain the constancy of nutritional
requirements in all the species?
Our claim is, in essence, that Tetrahymena is a kind of living fossil,
an organism frozen in its organismic characteristics for a long, long time.
Organisms fixed in their attributes have a peculiar fascination, and the
longer they have persisted in their invariant adaptations the more
fascinating and useful they become. The opossum, Didelphus, seems
organismically unchanged for 75 million years, the coelocanth, Latimeria,
for 100 million, the horseshoe crab Limulus is 200 million years old.
Conceivably, Tetrahymena is 5 or even 10 times as old as that. And
we have available not one surviving species, but many, natural replicates
of evolutionary experiments in stabilizing selection.
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