In ciliate protists, sex involves the temporary joining of two cells of compatible mating type, followed by meiosis and exchange of gametic nuclei between conjugants. Reproduction is by asexual binary fission following conjugation. For the many ciliates with fixed multiple mating types, frequency-dependent sex-ratio theory predicts equal frequencies of mating types, if sex is common in nature. Here, we report that in natural populations of Tetrahymena thermophila sexually immature cells, indicative of recent conjugation, are found from spring through fall. In addition, the seven mating types occur in approximately equal frequencies, and these frequencies appear to be maintained by interaction between complex, multiple mat alleles and environmental conditions during conjugation. Such genotype-environment interaction determining mating type frequency is rare among ciliates.Sex is an extraordinarily successful eukaryotic invention usually associated with reproduction. However, in ciliates, reproduction is by binary fission, and sex is limited to the temporary conjugal union of two cells for purposes of recombination and rejuvenation-i.e., micronuclear exchange and macronuclear replacement. To recognize suitable partners, ciliates are differentiated into mating types, a kind of self-not-self discrimination system, as opposed to true sexes. Hurst and coworkers (1, 2) have suggested that by exchanging only gametic nuclei at conjugation, ciliates minimize conflict between cytoplasmic genomes and, therefore, are free to evolve multiple mating types to maximize the choice of sexual partners. Simple extension of frequency-dependent sex-ratio theory suggests that multiple but fixed mating types should be equally frequent in breeding populations, a suggestion supported by two theoretical studies. Orias and Rohlf (3) constructed a deterministic model for three alleles at a single locus and found that there is a stable equilibrium in which mating types are equally frequent. In a more general study, Iwasa and Sasaki (4) found that evolutionary consequences greatly depend upon mating kinetics and sex-determining mechanisms. When opportunities for finding suitable conjugal partners are temporally limited, the number of sexes-i.e., mating types-increases. There is, however, limited information on ciliate sex and mating types in natural populations (5, 6) with which to test these models.Ciliates typically have two types of nuclei: a germinal, diploid micronucleus capable of meiosis and mitosis and a somatic, compound macronucleus controlling the phenotype of the cell. At conjugation, haploid gametic nuclei meiotically derived from the micronuclei are reciprocally exchanged, and, following fertilization, new macronuclei are derived from mitotic products of zygotic micronuclei, while the old macronuclei are destroyed (7). Replacement of the macronucleus not only initiates a new life cycle that may include change in mating type but also is associated with rejuvenating (8) and heterotic effects (9, 10).To determine the inciden...

Until recently, Tetrahymena thermophila has rarely been isolated from nature. With improved sampling procedures, T. thermophila has been found in ponds in many northeastern states. The availability of resident populations makes possible both population and ecological genetic studies. All seven known mating types have been recovered; no eighth mating type has been found. Crosses among whole‐genome homozygotes derived from Pennsylvania isolates reveal a spectrum genotypes with mating type alleles resembling traditional A (IV‐ and VII‐) and B(I‐) categories. The genotypes differ significantly with respect to mating type frequency, both among themselves and from previously described genotypes. One A‐category genotype appears to lack mating type II, while one A‐category and all B‐category genotypes have low frequencies of mating type III, thus accounting for the low frequency of III in the pond. The low frequency of III in all five B‐category genotypes examined suggests that the founding allele in this region was low for III. These and other differences are discussed both in terms of mating type frequencies in the pond and in terms of the possible molecular structure of mat alleles. By contrast, numerous variants of the cell surface immobilization antigen are found in addition to the previously described i‐antigens. Variants of the known SerH alleles include those with restriction fragment length polymorphisms and temperature sensitivity as well as alleles with new antigenic specificity. Multiple alleles are present in single ponds. Genes exhibiting serially dominant epistasis over SerH genes also are found. In two instances (K and C), families of antigenically similar polypeptides are expressed in place of H i‐antigen. Molecular weight differences suggest that these paralogous i‐antigen genes evolve by gene duplication and unequal crossing over within central repeats. The existence of complex patterns of epistasis together with seasonal changes in i‐ag frequencies suggest that i‐ag play an important, but as yet unknown, ecological role related to the occurrence of frequent conjugation.

In Tetrahymena thermophila mating type alleles specify temperature sensitive frequency distributions of multiple mating types. A-like alleles specify mating types I, II, III, V and VI, whereas B-like alleles specify mating types II through VII. We have characterized the mating type distributions specified by several A- and B-like genotypes segregated by genomic exclusion from cells isolated from a pond in northwestern Pennsylvania. The B-like genotypes are alike in specifying very low frequencies of mating type III, but differ with respect to the frequencies of other mating types, particularly II and VII. An A-like genotype specifies a high frequency of mating type III and is unstable in successive generations for the expression of mating type II, suggesting a possible modifier. Inter se crosses performed at 18 degrees C, 28 degrees C and 34 degrees C showed that each genotype specifies a frequency distribution that is uniquely affected by temperature. No mating type was affected the same way by temperature in all genotypes. In A/B heterozygotes, the B-like genotype exhibited partial dominance. The genotypes described here differ significantly from previously described genotypes from the same pond, indicating that there are numerous mating type alleles. For frequency-dependent selection to equalize mating type frequencies, it must act not only on complex multiple alleles but also on the response of mating type alleles to temperature.

Autophagy serves as a turnover mechanism for the recycling of redundant and/or damaged macromolecules present in eukaryotic cells to re-use them under starvation conditions via a double-membrane structure known as autophagosome. A set of eukaryotic genes called autophagy-related genes (ATGs) orchestrate this highly elaborative process. The existence of these genes and the role they play in different eukaryotes are well-characterized. However, little is known of their role in some eukaryotes such as ciliates. Here, we report the computational analyses of ATG genes in five ciliate genomes to understand their diversity. Our results show that Oxytricha trifallax is the sole ciliate which has a conserved Atg12 conjugation system (Atg5-Atg12-Atg16). Interestingly, Oxytricha Atg16 protein includes WD repeats in addition to its N-terminal Atg16 domain as is the case in multicellular organisms. Additionally, phylogenetic analyses revealed that E2-like conjugating protein Atg10 is only present in Tetrahymena thermophila. We fail to find critical autophagy components Atg5, Atg7 and Atg8 in the parasitic ciliate Ichthyophthirius multifiliis. Contrary to previous reports, we also find that ciliate genomes do not encode typical Atg1 since all the candidate sequences lack an Atg1-specific C-terminal domain which is essential for Atg1 complex formation. Consistent with the absence of Atg1, ciliates also lack other members of the Atg1 complex. However, the presence of Atg6 in all ciliates examined here may rise the possibility that autophagosome formation could be operated through Atg6 in ciliates, since Atg6 has been shown as an alternative autophagy inducer. In conclusion, our results highlight that Atg proteins are partially conserved in ciliates. This may provide a better understanding for the autophagic destruction of the parental macronucleus, a developmental process also known as programmed nuclear death in ciliates.