Meiosis in a ciliate system

Members of all major branches of the eukaryotic family tree undergo meiosis in order to produce haploid cells. These cells or their mitotic descendants can fuse into a zygote at the start of a diploid cell generation. Despite the conservation of the meiotic process over large evolutionary distances, even closely related organisms can show considerable variation. For instance, not all organisms require recombination for the pairing of homologs and not all of them require a synaptonemal complex (SC) to stabilize this pairing. This project is intended to reappraise our current view of a "consensus" meiotic process that has emerged from studies in a limited set of animal, plant and fungal model organisms. We will establish Tetrahymena thermophila as a meiotic model system and study various aspects of its meiosis, notably pairing and recombination. Tetrahymena is a ciliate protist which is vastly different in terms of evolutionary relationship to common meiotic model systems. It has several advantages (such as inducible and highly synchronous meioses, an alternative vegetative propagation cycle, and its amenability to advanced molecular, cytological and genetic tools) which can be exploited to study meiosis. Previous studies have revealed that the early steps in Tetrahymena meiosis are accompanied by the extreme elongation of nuclei. In these thread-shaped nuclei chromosomes are arranged in a bouquet-like fashion with the centromeres and telomeres clustered at opposite ends, and recombination and pairing take place. Moreover, Tetrahymena does not employ an SC. Analysis of this unusual meiosis will surely impact our current understanding of how chromosome pairing works. Moreover, in ciliates there exists a nuclear dualism, with an amitotically dividing somatic macronucleus and a generative micronucleus, which undergoes mitotic cycles and meiosis. This provides the opportunity to compare the role of cohesin in chromosome axis organization between fundamentally different types of nuclei, and thereby to clarify its specific requirement for meiotic pairing. To this end, we will identify and knock out meiotic genes and study the consequences of their loss on meiotic chromosome behavior. In the long term we will extend our studies to meioses of other ciliates to fully exploit the wealth of variability displayed by the meiotic processes in some of the more "exotic" organisms. This will serve the final goal to learn about the origin of conserved meiotic features such as the SC, and finally, the evolution of meiosis itself.

Members of all major branches of the eukaryotic family tree undergo meiosis in order to produce haploid cells. These cells or their mitotic descendants can fuse into a zygote at the start of a diploid cell generation. Despite the conservation of the meiotic process over large evolutionary distances, even closely related organisms can show considerable variation. We have established Tetrahymena thermophila as a meiotic model system and studied various aspects of its meiosis, notably pairing and recombination. The main question that motivates our work, is whether our picture of the meiotic process, as has been conveyed by the study of yeasts and a selected choice of animals and plants, is universal and complete. Tetrahymena is a ciliated protist, which is vastly different from common meiotic model systems in terms of evolutionary relationship. Most notably, Tetrahymena does not employ a synaptonemal complex, the conserved meiotic pairing structure. Our previous studies revealed that the early steps in Tetrahymena meiosis are accompanied by the extreme elongation of nuclei. Here, we found that in these thread-shaped nuclei chromosomes are arranged in a bouquet-like fashion with the centromeres and telomeres clustered at opposite ends. It is believed that in meiosis two paralogous proteins, Rad51 and Dmc1, both form nucleofilaments with single-stranded DNA to support strand exchange between homologous DNA sequence tracts. Tetrahymena is exceptional in that we could observe notable amounts of only Dmc1. We concluded that Dmc1 plays a predominant role in pairing and recombination, and Rad51 only assists Dmc1 in this task. We also studied how DNA double-strand breaks, the initial events in meiotic recombination, are transformed to crossovers, the reciprocal exchange of chromosome parts. It is generally believed that there exist two pathways towards crossing over. In most organisms they are partially redundant, whereas in the fission yeast only one, and in C. elegans only the other exists. In Tetrahymena, the fission yeast pathway seems to prevail. It is a more simple process that largely relies on factors, which are also active in mitotic DNA damage repair. Thus, fission yeast and Tetrahymena, despite their large evolutionary distance, have introduced similar measures to make their meiosis simple. Altogether, the picture that has emerged from this study is that core functions of meiosis are largely conserved among eukaryotic kingdoms, but that details of the meiotic process are more divergent than expected.