The Tetrahymena meiotic chromosome bouquet is organized by centromeres and promotes interhomolog recombination

Abstract

In order to form crossovers and to undergo reductional segregation during meiosis, homologous chromosomes must pair. In Tetrahymena, meiotic prophase nuclei elongate immensely, and, within the elongated nucleus, chromosomes are arranged with telomeres assembled at one pole and centromeres at the opposite pole. This organisation is an exaggerated form of the bouquet, a meiotic chromosome arrangement that is widely conserved among eukaryotes. We show that centromere function is crucial for the formation of Tetrahymena's stretched bouquet and, thereby, for homologue pairing. This finding adds to previous reports of the importance of centromeres in chromosome pairing in budding yeast and in Drosophila. Tetrahymena's bouquet is an ataxia telangiectasia- and RAD3-related (ATR)-dependent meiotic DNA damage response that is triggered by meiotic DNA double-strand breaks (DSBs), suggesting that the bouquet is needed for DSB repair. However, in the present study we show that although homologous pairing is impeded in the absence of the bouquet, DSB repair takes place nevertheless. Moreover, recombinational DSB repair, as monitored by bromodeoxyuridine incorporation, takes place only after exit from the bouquet stage. Therefore, we conclude that the bouquet is not required for DSB repair per se, but may be necessary for the alignment of homologous loci in order to promote homologous crossovers over alternative repair pathways.