Chromosome separation during Drosophila male meiosis I requires separase-mediated cleavage of the homolog conjunction protein UNO

Abstract

Regular chromosome segregation during the first meiotic division requires prior pairing of homologous chromosomes into bivalents. During canonical meiosis, linkage between homologous chromosomes is maintained until late metaphase I by chiasmata resulting from meiotic recombination in combination with distal sister chromatid cohesion. Separasemediated elimination of cohesin from chromosome arms at the end of metaphase I permits terminalization of chiasmata and homolog segregation to opposite spindle poles during anaphase I. Interestingly, separase is also required for bivalent splitting during meiosis I in Drosophila males, where homologs are conjoined by an alternative mechanism independent of meiotic recombination and cohesin. Here we report the identification of a novel alternative homolog conjunction protein encoded by the previously uncharacterized gene univalents only (uno). The univalents that are present in uno null mutants at the start of meiosis I, instead of normal bivalents, are segregated randomly. In wild type, UNO protein is detected in dots associated with bivalent chromosomes and most abundantly at the localized pairing site of the sex chromosomes. UNO is cleaved by separase. Expression of a mutant UNO version with a non-functional separase cleavage site restores homolog conjunction in a uno null background. However, separation of bivalents during meiosis I is completely abrogated by this non-cleavable UNO version. Therefore, we propose that homolog separation during Drosophila male meiosis I is triggered by separase-mediated cleavage of UNO.

Bivalent individualization during chromosome territory formation in Drosophila spermatocytes by controlled condensin II protein activity and additional force generators

“…To detect the metaphase-to-anaphase I transition more clearly in our time-lapse imaging with Cap-D3 mutant spermatocytes, they expressed not just UNO-EGFP and His2Av-mRFP but also Cenp-A/Cid-EGFP ( Fig 5B ). As shown previously [ 53 ], the strong UNO-EGFP dot on the chrXY pairing region can be distinguished readily from centromeric Cid-EGFP dots based on intensity and shape. As in normal spermatocytes, a single prominent UNO-EGFP dot was present in Cap-D3 mutant spermatocytes just prior to NEBD I ( Fig 5B ), indicating that AHC protein localization is not affected, at least on the sex chromosome bivalent.…”

“…In principle, the chromosome bridges during exit from M I in Cap-D3 and Cap-H2 mutants, might result from a failure in elimination of alternative homolog conjunction (AHC). Normally, AHC is eliminated by separase-mediated cleavage of the AHC protein UNO during the metaphase-to-anaphase I transition [ 53 ]. To address whether condensin II proteins are required to remove AHC during M I, we performed time-lapse imaging with Cap-D3 EY00456 / Df(2L)Exel7023 spermatocytes expressing UNO-EGFP.…”

“…Normally, UNO-EGFP is co-localized with other AHC proteins throughout spermatocyte maturation [ 53 ]. At the start of M I, it most strongly enriched on the chrXY pairing region, where it forms a very prominent dot that disappears within minutes at the onset of anaphase I ( Fig 5A and 5C ) [ 53 ]. UNO-EGFP signals on autosomes, which disappear concomitantly, are far lower and difficult to detect above background [ 53 ].…”

“…At the start of M I, it most strongly enriched on the chrXY pairing region, where it forms a very prominent dot that disappears within minutes at the onset of anaphase I ( Fig 5A and 5C ) [ 53 ]. UNO-EGFP signals on autosomes, which disappear concomitantly, are far lower and difficult to detect above background [ 53 ]. These weak autosomal UNO-EGFP signals are below detection limit with the chosen display settings ( Fig 5A ).…”

“…COs stabilize bivalents until anaphase I in females. An alternative homolog conjunction (AHC) system is used in spermatocytes [ 50 – 53 ]. Several genes ( teflon , mnm , snm and uno ) are known to be required specifically for AHC.…”