PROGRESS IN PROTOZOOLOGY

248 D. L. NANNEY
among the earliest of the major eukaryotic protists, at least the earliest
represented by extant forms. Perhaps the discovery of nuclear compounding
provided the first effective means of increasing greatly the
size and motility of the eukaryotic organisms. And perhaps these new
capabilities permitted an important early radiation. That radiation could
conceivably have occurred as early as 1.5 to 1.8 billion years ago, but
the time cannot be fixed until more satisfactory data are available for
several well-calibrated presumably chronometrie molecules.
(3) Tetrahymena pyrijormis is a Complex of Species Remarkably
Similar in Certain Features
Following Sonneborn's (1937) discovery that Paramecium aurelia
is a cluster of sibling species, many named ciliate species have
been shown to be species complexes. The first systematic study of the
population structure of Tetrahymena pyrijormis was carried out by
Gruchy (1955), who found eight genetically isolated species in collections
from North America. Elliott and his collaborators continued
Species Name
Table 1
Some mating groups in the T. pyriformis complex
T. thermophila
I II III IV V VI VII
T. pigmentosa
I II III
T. canadensis
I II III IV V
T. thermophila I - + + + + + + _ _ _ ______
(mating group 1) II h + + + + - - -
m - + + + + — — —
iv - + + + — - —
V - + + — - —
VI - + — - —
VII
T. pigmentosa i - + +
(mating groups 6, 8) II - +
HI —
T. canadensis i - + + + +
(mating group 7) u - + + +
in - + +
TV - +
v —
these explorations, making collections in Central America, the Pacific
Islands, Australia and Europe (see Elliott 1973). The general situation
is illustrated in Table 1. Each species, once referred to as a "variety"
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