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

Abstract

Reduction of genome ploidy from diploid to haploid necessitates stable pairing of homologous chromosomes into bivalents before the start of the first meiotic division. Importantly, this chromosome pairing must avoid interlocking of non-homologous chromosomes. In spermatocytes of Drosophila melanogaster, where homolog pairing does not involve synaptonemal complex formation and crossovers, associations between non-homologous chromosomes are broken up by chromosome territory formation in early spermatocytes. Extensive non-homologous associations arise from the coalescence of the large blocks of pericentromeric heterochromatin into a chromocenter and from centromere clustering. Nevertheless, during territory formation, bivalents are moved apart into spatially separate subnuclear regions. The condensin II subunits, Cap-D3 and Cap-H2, have been implicated, but the remarkable separation of bivalents during interphase might require more than just condensin II. For further characterization of this process, we have applied time-lapse imaging using fluorescent markers of centromeres, telomeres and DNA satellites in pericentromeric heterochromatin. We describe the dynamics of the disruption of centromere clusters and the chromocenter in normal spermatocytes. Mutations in Cap-D3 and Cap-H2 abolish chromocenter disruption, resulting in excessive chromosome missegregation during M I. Chromocenter persistence in the mutants is not mediated by the special system, which conjoins homologs in compensation for the absence of crossovers in Drosophila spermatocytes. However, overexpression of Cap-H2 precluded conjunction between autosomal homologs, resulting in random segregation of univalents. Interestingly, Cap-D3 and Cap-H2 mutant spermatocytes displayed conspicuous stretching of the chromocenter, as well as occasional chromocenter disruption, suggesting that territory formation might involve forces unrelated to condensin II. While the molecular basis of these forces remains to be clarified, they are not destroyed by inhibitors of F actin and microtubules. Our results indicate that condensin II activity promotes chromosome territory formation in co-operation with additional force generators and that careful co-ordination with alternative homolog conjunction is crucial.

Condensin II is required for efficient Spindle Assembly Checkpoint activation in Drosophila male meiosis

“…In summary, our results point that Condensin II is a major regulator Drosophila spermatogenesis, acting at multiple stages to ensure meiotic fidelity. In addition to the previously reported role in territories formation and chromatin resolution [18, 34], we provide evidence that Condensin II also contributes to the fidelity of segregation through the regulation of SAC response.…”

“…In this organism, homologous chromosome pairing takes place during prophase I, when bivalents “segregate” to distinct nuclear regions, forming the so-called Chromosome Territories (CT) [33]. The mechanisms that drive this process are not fully understood but CT formation depends on condensin II activity [18, 34]. Mutants for this complex fail to display distinct CTs, possibly through the inability of resolving intra-chromosome intertwines, resulting in severe segregation defects [18].…”

Dispersive forces and resisting spot welds by alternative homolog conjunction govern chromosome shape in Drosophila spermatocytes during prophase I

“…While this long-known characteristic positioning of the major chromosome territories might be directed by a mechanism that simply maximizes the spatial separation between chromosome entities, the bivalent of the small dot-like chr4 complies with this rule only in a minority of the spermatocytes (S1 Movie). In the majority of the S5/6 spermatocytes analyzed after live imaging (72.1%, n = 48 spermatocytes from six independent cysts), the chr4 bivalent was not separated apart but closely associated with the chrXY territory, as previously described [11,13].…”

“…The lines with the following mutations or transgenes have been described before: UAS-Cas9 2;3)Eip74EF 1 , Eip74EF e/ TM6B, Tb (BDSC # 29973), His2Av-mRFP and g-cid-EGFP-cid [53], UASt-mnm-EGFP II.2 [16], UASt-EGFP-Cap-H2 [11], bamP-GAL4-VP16 [54].…”

“…Note that "M I" as used here does not include the spermatocyte growth period. Very early in spermatocytes, in the S1 stage, homolog pairing is completed [10], followed by chromosome territory formation during a few hours at the S2a/b stage transition [9][10][11]. Before territory formation, the distribution of chromatin in spermatocyte nuclei is relatively uniform.…”