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First published online 3 June 2008
doi: 10.1242/jcs.031799

Journal of Cell Science 121, 2148-2158 (2008)
Published by The Company of Biologists 2008
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Research Article

DNA double-strand breaks, but not crossovers, are required for the reorganization of meiotic nuclei in Tetrahymena

Kazufumi Mochizuki1, Maria Novatchkova2 and Josef Loidl3,*

1 Institute of Molecular Biotechnology of the Austrian Academy of Sciences (IMBA), A-1030 Vienna, Austria
2 Research Institute of Molecular Pathology (IMP), A-1030 Vienna, Austria
3 Department of Chromosome Biology, Max F. Perutz Laboratories, University of Vienna, Dr Bohr Gasse 1, A-1030 Vienna, Austria

* Author for correspondence (e-mail: josef.loidl{at}univie.ac.at)

Accepted 14 April 2008

During meiosis, the micronuclei of the ciliated protist Tetrahymena thermophila elongate dramatically. Within these elongated nuclei, chromosomes are arranged in a bouquet-like fashion and homologous pairing and recombination takes place. We studied meiotic chromosome behavior in Tetrahymena in the absence of two genes, SPO11 and a homolog of HOP2 (HOP2A), which have conserved roles in the formation of meiotic DNA double-strand breaks (DSBs) and their repair, respectively. Single-knockout mutants for each gene display only a moderate reduction in chromosome pairing, but show a complete failure to form chiasmata and exhibit chromosome missegregation. The lack of SPO11 prevents the elongation of meiotic nuclei, but it is restored by the artificial induction of DSBs. In the hop2A{Delta} mutant, the transient appearance of {gamma}-H2A.X and Rad51p signals indicates the formation and efficient repair of DSBs; but this repair does not occur by interhomolog crossing over. In the absence of HOP2A, the nuclei are elongated, meaning that DSBs but not their conversion to crossovers are required for the development of this meiosis-specific morphology. In addition, by in silico homology searches, we compiled a list of likely Tetrahymena meiotic proteins as the basis for further studies of the unusual synaptonemal complex-less meiosis in this phylogenetically remote model organism.

Key words: Meiosis, Bouquet, Recombination, Chromosome pairing






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