PROGRESS IN PROTOZOOLOGY

258
the macronucleus, the rDNA is released from the chromosome, transformed
into a palindrome, trimmed and patched and amplified to a level
a couple of orders of magnitude greater than that of other known macronuclear
components.
The rDNA's of several Tetrahymena species have been analyzed by
Din and Engberg (1979). The technique used in this case was restriction
enzyme cleavage, which is capable of yielding molecular maps.
When several different endonucleases with different sites of action are
employed, a reasonably detailed map can be developed, and the maps
for molecules of different origin can be compared. The maps now at
hand show numerous differences in the rDNA's of the species compared,
and show some possibly significant distinctions among the species.
Particularly, T. pyriformis (sensu stricto) and T. thermophila are very
different from each other and from the other species examined. The
latter, though distinguishable, are more similar. Thus, even these very
conservative molecules provide evidence of considerable evolutionary distance
between the species of the complex even though we are not able
to calibrate those distances very precisely.
i A last DNA molecule for which we have some comparative data is
notoriously labile. The mitochondrial DNA's of the amicronucleate species
have been studied by Goldbach et al. (1977), using DNA-DNA
hybridization to test for affinities. The results, summarized in Table 9,
include comparisons in all combinations, and not just against a single
Table 9
Comparisons of the mitochondrial DNAs of four amicronucleate species of the T. pyriformis
complex
Species
Reannealing Percentage at 65°C Molecular
T. pyriformis T. elliotti T. furgasoni T. Iwoffi Weight x 10- 6
T. pyriformis 100 4 9 4 28.4
(sensu stricto)
T. elliotti 6 100 7 9 31.0
T. furgasoni 11 8 100 5 25.8
T. Iwoffi 4 8 5 100 31.0
Goldbach et al. (1977).
reference strain as in the case of the study of Allen and L i. As
might be expected for such labile molecules, the cross-annealing values
are all very low, in the range of 5-10%. The residual hybridizations
probably reflect the conservatism of a portion of the otherwise labile
structures — particularly the portion responsible for the ribosomal apparatus
of the mitochondrial synthetic system. The mitochondrial data
do not help us to order the Tetrahymena species in any phylogenetic
http://rcin.org.pl