HIGH-FREQUENCY OF SEX AND EQUAL FREQUENCIES OF MATING TYPES IN NATURAL-POPULATIONS OF THE CILIATE TETRAHYMENA-THERMOPHILA

In ciliate protists, sex involves the temporary joining of two cells o f compatible mating type, followed by meiosis and exchange of gametic nuclei between conjugants. Reproduction is by asexual binary fission f ollowing conjugation. for the many ciliates with fixed multiple mating types, frequency-dependent sex-ratio theory predicts equal frequencie s of mating types, if sex is common in nature. Here, we report that in natural populations of Tetrahymena thermophila sexually immature cell s, indicative of recent conjugation, are found from spring through fal l. In addition, the seven mating types occur in approximately equal fr equencies, and these frequencies appear to be maintained by interactio n between complex, multiple mat alleles and environmental conditions d uring conjugation. Such genotype-environment interaction determining m ating type frequency is rare among ciliates.