Meiotic DNA double-strand breaks in Tetrahymena

Meiosis is the cell division that produces germ cells (egg or sperm) from normal body cells with two copies of each chromosome. Germ cells have only one copy, which ensures that a double set comprising paternal and maternal chromosomes is restored by fertilization. During meiosis, two corresponding (homologous) chromosomes (one from each parental set) form pairs. This pairing facilitates the faithful separation of homologous partners via a simple sorting mechanism. At the same time, parts of homologous chromosomes are exchanged, which leads to new gene combinations in the gametes and the progeny. This recombination works via deliberate DNA breakage and subsequent break repair due to the crosswise linking of homologous chromosome segments. Meiotic recombination is a risky process because DNA breaks are dangerous. Nevertheless, most higher organisms (eukaryotes) reproduce sexually in this way because genetic recombination allows them to adapt to changes in the environment. As a consequence of meiotic defects, germ cells may receive truncated chromosomes or two copies of the same chromosome (such that three copies are present after fertilization). Embryos with affected chromosomes die or develop abnormally. A common consequence of an error in meiotic chromosome segregation in humans is trisomy 21, which causes Down syndrome. Meiosis proceeds similarly in animals and plants. It is an immensely complex process (requiring about one quarter of all genes) and many of its details are still poorly understood. Insight into meiotic mechanisms is important not only for reproductive medicine but also for eveloping methods to produce favorable combinations of traits in animal and plant breeding. To understand the proteins involved in meiotic structures and processes, mutations can be generated in the responsible genes and their normal function can be deduced from the resulting defects. However, we have been pursuing a different strategy: We are studying the unconventional simplified meiosis of Tetrahymena, a single-celled organism that diverged from animals and plants early in evolution. Tetrahymenas meiosis lacks some genes and control mechanisms, indicating that these are dispensable for basal meiotic DNA repair, chromosome pairing, and recombination. However, more elaborate processes may have evolved in other lineages as adaptations and optimizations. Conversely, some structures and processes are more pronounced in Tetrahymena than in common model organisms, clarifying their roles in the meiotic process. In the present project, we will use microscopic and molecular methods to investigate the contribution of some key proteins to DNA repair and subsequent steps in meiosis. Commonalities and differences in the meiotic programs of Tetrahymena and higher multicellular organisms will show how extant meiosis acquired its complexities and extended functions during the course of evolution.

Meiosis is the cell division that produces germ cells (egg or sperm) from normal body cells with two copies of each chromosome. Germ cells have only one copy, which ensures that a double set comprising paternal and maternal chromosomes is restored by fertilization. During meiosis, two corresponding (homologous) chromosomes (one from each parental set) form pairs. This pairing facilitates the faithful separation of homologous partners via a simple sorting mechanism. At the same time, parts of homologous chromosomes are exchanged; this so-called crossing over leads to new gene combinations in the gametes and the progeny. Meiosis proceeds similarly in animals and plants. It is a complex process and many of its details are still poorly understood. Insight into meiotic mechanisms is important both for understanding the cause of congenital defects in humans and for the development of methods to produce favorable combinations of traits in animal and plant breeding. Here, we studied the meiosis of Tetrahymena thermophila, a single-celled organism that diverged from animals and plants early in evolution. We wanted to identify primordial meiotic processess that are shared by all eukaryotes and distinguish them from modified or specialized features of meiosis that appeared later in evolution. In the course of the present and previous projects we knocked out more than 100 hitherto uncharacterized genes that are highly expressed in early mating cells (where meiosis occurs) and studied the consequences of their loss. Only about one quarter of these genes were found to be indispensable for meiotic pairing or recombination, whereas another quarter probably functions in the postmeiotic development of gametes or the maturation of sexual progeny. Ca. 10% of genes appeared to have functions in general chromosomal structure or in the regulation of the meiotic cell cycle.The knockout of of the remainder of the genes did not cause a notable defect, suggesting that they are working redundantly in parallel processes or that their effect on sexual reproduction is subtle and not decisive under laboratory conditions. Altogether, Tetrahymena turned out to utilize a surprisingly small set of genes that are essential for meiotic DNA repair, chromosome pairing, and recombination processes. This suggests that fungi, animals and plants developed specialized and more complex meiotic programs in the course of evolution. Thus, Tetrahymena may serve as a manageable model organism for the study of core meiotic functions. The title of a recent review article "Tetrahymena meiosis: Simple yet ingenious" (Loidl 2021, PLoS Genetics 17, e1009627) mirrors the insight that Tetrahymenas limited meiotic toolbox enables (or forces) it to employ a sophisticated alternative mechanism for meiotic chromosome pairing, namely by juxtaposing it within an extremely reorganized tube-shaped nucleus.