Magnetic purification of mating Tetrahymena

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.

Alternative processing of sequences during macronuclear development in Tetrahymena thermophila

DNA is eliminated during development of the somatic MACronucleus from the germinal MICronucleus in the ciliated protozoan, Tetrahymena thermophila. Facultatively persistent sequences are a class of sequences that persist in the MAC DNA of some cell lines but are eliminated from the MAC DNA of other cell lines. One cloned MAC fragment contains a persistent sequence as well as sequences normally retained in the MAC. When this cloned fragment was used to construct MAC restriction maps of this region in cell lines whose MAC DNAs do, or do not, contain the persistent sequence, extensive variation in the map flanking this region was observed. The different DNA rearrangements of this MIC segment are epigenetically determined during or soon after MAC development. Moreover, different rearrangements may occur among the 45 copies of this MIC segment as a MAC is formed, resulting in polymorphisms that are later resolved by phenotypic assortment.

Eliminated sequences with different copy numbers clustered in the micronuclear genome of Tetrahymena thermophila

As the ciliated protozoan Tetrahymena thermophila develops a new macronucleus (MAC) from products of its micronucleus (MIC), several repetitive sequences are eliminated from the MAC genome. Four MIC DNA clones containing repetitive sequences that are eliminated from the MAC were obtained. One clone contains a representative from each of three families of eliminated sequences. One, present in 200-300 copies in the MIC, is almost completely eliminated from the MAC. A second, present in approximately 50 copies in the MIC, is scattered throughout the genome, although up to half of the family members examined could be localized to chromosome 2. Approximately one tenth of the members of this less repetitive family persist in the MAC while the rest are eliminated. The third type of eliminated sequence has three to four members, all of which are eliminated from the MAC. Three of the members are located on three of the five MIC chromosomes, and one could not be mapped. This sequence is clustered with the other two families of sequences in at least three of the four sites. All three types of eliminated sequences are found in similar arrangements in the MIC of several different inbred strains of T. thermophila.

Proteolytic processing of micronuclear H3 and histone phosphorylation during conjugation in Tetrahymena thermophila

During vegetative growth, micronuclei of the ciliated protozoan Tetrahymena thermophila contain two electrophoretically distinct forms of H3, H3S and H3F [4, 5]. Of these two forms, H3F is unique to micronuclear chromatin and is derived from H3S by a physiologically regulated proteolytic processing event [5]. While the function of this processing event is not clear, several lines of evidence [2, 5] suggest that it may be related to chromatin condensation during mitosis. In this report pulse-chase experiments have been used to study the processing of H3S into H3F during the sexual phase of the life cycle, conjugation. Our results demonstrate that even though micronuclei divide mitotically (and meiotically) several times during the mating process, processing of H3S into H3F does not occur. Failure of H3S to be converted into H3F during these divisions causes a significant increase in the amount of H3S (relative to H3F) as conjugation proceeds. By 10 h of conjugation, essentially all of the micronuclear H3 is in the form of H3S (also see [3]). As long as mating cells are maintained under starvation conditions, processing of H3S into H3F does not occur. However, if exconjugants are returned to food and allowed to proceed through the first true cell division following exconjugation, processing of H3S into H3F occurs. Thus, the return of the processing of H3(3) into H3F following conjugation seems to be tightly coupled to a division which is part of a cell division cycle (as appears to be the case with vegetatively growing cells). The relevancy of these results to the differentiation of new macro- and micronuclei is discussed. H3F is specifically phosphorylated in growing cells, and it has been suggested that this phosphorylation event may be related to chromatin condensation during mitosis [2]. Since in mating cells H3S becomes the more predominant form of H3, the pattern of histone phosphorylation was examined during stages of conjugation where micronuclei are active in mitotic division (6-7 h). While a low level of phosphate label is observed over H3S in mating cells, more phosphate label is associated with the small amount of H3F which remains in micronuclei at this stage of conjugation. We also observe significant amounts of phosphate label associated with micronuclear H2A, H2B, and H4 and each of the micronuclear H1-like molecules, alpha, beta and gamma.