Drivers of Mating Type Composition in Tetrahymena thermophila

Sex offers advantages even in primarily asexual species. Some ciliates appear to utilize such reproductive strategy with many mating types. However, the factors determining the composition of mating types in the unicellular ciliate Tetrahymena thermophila are poorly understood, and this is further complicated by non-Mendelian determination of mating type in the offspring. We therefore developed a novel population genetics model to predict how various factors influence the dynamics of mating type composition, including natural selection. The model predicted either the coexistence of all seven mating types or fixation of a single mating type in a population, depending on parameter combinations, irrespective of natural selection. To understand what factor(s) may be more influential and to test the validity of theoretical prediction, five replicate populations were maintained in laboratory such that several factors could be controlled or measured. Whole-genome sequencing was used to identify newly arising mutations and determine mating type composition. Strikingly, all populations were found to be driven by strong selection on newly arising beneficial mutations to fixation of their carrying mating types, and the trajectories of speed to fixation agreed well with our theoretical predictions. This study illustrates the evolutionary strategies that T. thermophila can utilize to optimize population fitness.

Selecting one of several mating types through gene segment joining and deletion in Tetrahymena thermophila

In Tetrahymena, a multi-sexed single-celled organism, the sex of the progeny is randomly determined by site-specific recombination events that assemble one complete gene pair and delete all others.

The unicellular eukaryote Tetrahymena thermophila has seven mating types. Cells can mate only when they recognize cells of a different mating type as non-self. As a ciliate, Tetrahymena separates its germline and soma into two nuclei. During growth the somatic nucleus is responsible for all gene transcription while the germline nucleus remains silent. During mating, a new somatic nucleus is differentiated from a germline nucleus and mating type is decided by a stochastic process. We report here that the somatic mating type locus contains a pair of genes arranged head-to-head. Each gene encodes a mating type-specific segment and a transmembrane domain that is shared by all mating types. Somatic gene knockouts showed both genes are required for efficient non-self recognition and successful mating, as assessed by pair formation and progeny production. The germline mating type locus consists of a tandem array of incomplete gene pairs representing each potential mating type. During mating, a complete new gene pair is assembled at the somatic mating type locus; the incomplete genes of one gene pair are completed by joining to gene segments at each end of germline array. All other germline gene pairs are deleted in the process. These programmed DNA rearrangements make this a fascinating system of mating type determination.

Tetrahymena thermophila is a single-celled organism with seven sexes. After two cells of different sexes mate, the progeny cells can be of any one of the seven sexes. In this article we show how this sex decision is made. Every cell has two genomes, each contained within a separate nucleus. The germline genome is analogous to that in our ovaries or testes, containing all the genetic information for the sexual progeny; the somatic or working genome controls the operation of the cell (including its sex). We show that the germline genome contains a tandem array of similarly organized but incomplete gene pairs, one for each sex. Sex is chosen after fertilization when a new somatic genome is generated by rearrangement of a copy of the germline genome. One complete sex gene pair is assembled when the cell joins DNA segments at opposite ends of the array to each end of one incomplete gene pair; this gene pair is thus completed and becomes fully functional, while the remaining sex gene pairs are excised and lost. The process involves programmed, site-specific genome rearrangements, and the physically independent rearrangements that occur at opposite ends of the selected gene pair happen with high reliability and precision.

Genetic and environmental factors affecting mating type frequency in natural isolates of Tetrahymena thermophila

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.

Mapping the mating type locus of Tetrahymena thermophila: meiotic linkage of mat to the ribosomal RNA gene

Tetrahymena thermophila has a multiple mating type system. While a sexually mature cell usually expresses only one mating type, its germline (micronucleus) carries the genetic potential for 5 to 7 mating types. The set of allowed mating types is specified by the mat locus. The choice of which particular mating type is expressed by a cell reflects a somatically inherited, developmentally programmed differentiation of the somatic nucleus (macronucleus). In this work we report that the mat locus maps to the left arm of chromosome 2, as determined by nullisomic deletion mapping. We also report a distance of 29 cM between the mat locus and the ribosomal RNA gene, previously mapped to chromosome 2L. This represents another (rare) case of meiotic linkage in Tetrahymena.