PROGRESS IN PROTOZOOLOGY

250 D. L. NANNEY
or "syngen" but now given a Latin binomial (Nanney and McCoy
1976), contains two or more different mating types which mate in all
heterologous combinations within a species. The 13 currently recognized
breeding species (Table 2) include three recently named by N y b e r g
(1981) and one discovered by Simon (personal communication) in
Malaysia.
Not all strains conforming to the type T. pyriformis (see Corliss
1952, 1979) are represented in the list of breeding species. A considerable
fraction isolated from nature self in unmixed cultures and cannot
therefore be assigned to species by mating reactions. An even larger
number of isolates are amicronucleate, and generally these amicronucleate
strains cannot be induced to interact with mating straijis. Relatively
little attention has been paid to the selfing and amicronucleate
strains, but several amieronucleates have found their way into research
laboratories. Although they cannot be classified by mating reactions,
amicronucleates can be readily discriminated by molecular analysis
(Borden et al. 1973 a) and four groups have been assigned Latin
binomials (Nanney and McCoy 1976). The amicronucleate "species"
seem to have few distinctive characteristics in common. None has
been convincingly associated with any particular micronueleate species.
The genus Tetrahymena contains at least 8 "morphological" species
in addition to the T. pyriformis group (Corliss 1979). Some of these
are certainly species complexes also, but in the absence of adequate
breeding and systematic biochemical analysis, the extent of species crypticity
in them is uncertain. Our present understanding of the complexity
of the genus is summarized in Table 3.
The genetic isolation of the cryptic species of T. pyriformis has provoked
several efforts to find some useful biological correlates of their
evolutionary disjunction. Generally speaking, studies of supramolecular
characteristics have not been very helpful in this respect. I will not
discuss all these inconclusive studies, but will summarize briefly some
representative ones.
First, let us consider size. The size of a ciliate varies by a factor of
two in each replication cycle, and it is also sensitive to growth conditions.
To assess size accurately, one must control the cultural conditions
and examine cells at a fixed time in the cell cycle. One measure of
size (surface area) is provided by an assessment of the number of
ciliary units (basal bodies) on the cell surface. It can be converted to
a measure of mass and length, but can also be left as a surface measure.
In a comparative study (Nanney et al. 1978) the number of basal
bodies in the posterior cell at division was found to vary from a low of
234 in the amicronucleate T. furgasoni to a high of 481 in T. capricornis.
Assuming equivalent sizes for cortical units in the species, the surfaces
vary by a maximum factor of about two. Again making plausible but
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