PROGRESS IN PROTOZOOLOGY

THE MOLECULAR DIVERSITY OF TETRAHYMENA PYRIFORMIS 265
investigation of the molecular or genetic diversity of the T. pyriformis
complex. Once again I must select examples rather than catalog exhaustively,
and will choose preferentially studies of supposedly conservative
molecules.
The first molecular characteristic is chosen in part for historical
reasons. S u e o k a's (1961) study of the base composition of several
strains of Tetrahymena provided the first indication of vast genetic
distances among these species. All organismic DNA's contain of course
the same nitrogenous bases: guanine and cytosine, thymine and adenine.
And they contain these in certain fixed proportions, e.g., the molar
amounts of thymine and adenine are equal. The relative amounts of
G and C relative to the amounts of A + T are, however, permitted to
vary over a considerable range in different organisms. Within a species
the average base composition is a highly conservative characteristic.
Closely related species usually have identical base compositions. Among
all the vertebrates, the base composition varies only about 4°/o, from
40°/o G + C to 44°/o.
Yet, all the workers who have studied several Tetrahymena strains
have reported readily detected differences in base composition. The 12
strains studied by S u e o k a (Table 7) had G + C values ranging from
Table 7
Percent C+G/total bases in total DNA from different Tetrahymena strains
Species
Sueoka Suyama
Investigators
Allen,
Gibson
Flavell,
Jones
T. pyriformis complex
Amicronucleate species
GL (species unknown) 30 28 26
E (species unknown) 31
ST (species unknown) 33
W (species unknown)
Micronucleate species
33
T. thermophila (1) 25 25 24, 25, 26 25
T. americanis (2) 25
T. borealis (3) 27
T. hegewischi (5) 25
T. canadensis (7) 27 25, 26
T. pigmentosa ^
25
25
25
T. tropicalis (9) 28 30 28
T. hyperangularis (10) 31
T. australis (11) 30
T. capricornis (12) 25
T. rostrata 23
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