Novel meiotic genes in the protist Tetrahymena

Meiosis is the cell division by which sexually reproducing organisms produce gametes or their precursors. These cell types contain a single genome, and they fuse to produce zygotes and sexual progeny with a doubled genome. Meiotic defects cause sterility, stillbirth or congenital diseases. Meiosis is thought to have originated in the common ancestor of all eukaryotes, however, it has undergone diversification and adaptations in extant organisms. In particular, the protist Tetrahymena is known for some notable deviations from meioses in all major model organisms. It is one of a few organisms that do not possess a synaptonemal complex (SC) - an otherwise ubiquitous and highly conserved structure involved in meiotic chromosome pairing, and it does not possess a number of genes that are essential for meiotic recombination (crossover) in other groups. As an alternative to SC-based pairing, Tetrahymena prearranges chromosomes in dense bundles within its immensely elongated meiotic nucleus. Altogether, Tetrahymena seems to have abandoned many meiotic specializations during its evolution, and most of its meiotic functions are exerted by genes that have also mitotic roles. We want to learn how this minimal meiosis works, and thereby also to identify possible causes for why most organisms maintain a more embellished meiosis. To this end, we want to identify and characterize additional meiosis- specific genes in Tetrahymena. We build this project upon a rich published resource of developmental stage-specific gene expression data, which allows us to narrow down potential meiotic genes to a manageable number for experimental examination. We will mutate or silence these genes and study their roles in meiosis by a set of functional tests. Also, we will determine the cellular localization of gene products by protein tagging or by having antibodies raised against them. By the identification and functional testing of additional meiosis genes we hope to get some insight into the workings of Tetrahymenas exotic meiosis. In particular, we want to identify factors that drive nuclear elongation as Tetrahymenas unique strategy to pair homologous chromosomes, and factors that define its crossover pathway(s).

Meiosis is the cell division that produces germ cells (egg or sperm) from normal body cells with two copies of each chromosome. 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. We knocked out 78 hitherto uncharacterized genes that are highly expressed in early mating cells (where meiosis occurs) and studied the consequences of their loss. Only ~22% of these genes were found to be indispensable for meiotic pairing or recombination, whereas ~27% probably function in the postmeiotic development of gametes or the maturation of sexual progeny. The knockout of ~34% 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. The remaining genes appeared to have functions in general chromosomal structure or in the regulation of the meiotic cell cycle. Four novel genes were found to be required for wild-type level crossover formation. Three of them (BIME1, BIME2, ZHP3) seem to promote the interaction of homologous chromosomes by the invasion of DNA strands. One gene (PARS11) turned out to promote and control the formation of double-stranded DNA breaks by Spo11. The deletion of two genes (EMIT1, RIB1) was found to impair the transcription of noncoding RNA, the so-called scan-RNA, which is part of a mechanism for programmed somatic DNA elimination in Tetrahymena. The elimination of about 30% of the genome is important for the development of sexual progeny. Another gene, SEMI1, turned out to encode a protein with a crucial role in selecting one of the four meiotic products to become the gamete nucleus, which fertilizes the gamete nucleus of the cells conjugation partner to produce sexual progeny. 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.