Abstract
Cohesins are vital for chromosome organisation during meiosis and mitosis. In addition to the important function in sister chromatid cohesion, these complexes play key roles in meiotic recombination, DSB repair, homologous chromosome pairing and segregation. Egg-laying mammals (monotremes) feature an unusually complex sex chromosome system, which raises fundamental questions about organisation and segregation during meiosis. We discovered a dynamic and differential accumulation of cohesins on sex chromosomes during platypus prophase I and specific reorganisation of the sex chromosome complex around a large nucleolar body. Detailed analysis revealed a differential loading of SMC3 on the chromatin and chromosomal axis of XY shared regions compared with the chromatin and chromosomal axes of asynapsed X and Y regions during prophase I. At late prophase I, SMC3 accumulation is lost from both the chromatin and chromosome axes of the asynaptic regions of the chain and resolves into subnuclear compartments. This is the first report detailing unpaired DNA specific SMC3 accumulation during meiosis in any species and allows speculation on roles for cohesin in monotreme sex chromosome organisation and segregation.
Highlights
Cohesins are non-histone proteins of the structural maintenance of chromosomes (SMC) group
In order to investigate the dynamics of synaptonemal complex (SC) formation in monotreme prophase I, we used antibodies raised against the central element protein Synaptonemal Complex Protein 1 (SYCP1) and the cohesin subunit SMC3 to visualise chromosome axial cores
SYCP1 immunostaining showed an expected SC staining pattern in platypus prophase I cells, the SMC3 antibody detected remarkably strong localised accumulations unlike mouse cells at the same stage (Fig. 1). To confirm this pattern as representing cohesin complex distributions we immunostained for the meiosis specific cohesin STAG3 using a custom antibody which showed identical staining patterns described for SMC3 (Figure S3)
Summary
Cohesins are non-histone proteins of the structural maintenance of chromosomes (SMC) group They are described as the molecular glue that keeps sister chromatids physically attached after DNA replication during both mitosis and meiosis. RAD21L is recruited to the chromatin loops of the XY sex body, a pattern similar to the ɣH2AX repressive histone variant which marks silenced sex chromatin[11, 17]. Another aspect of meiotic sex chromosome regulation involves an association with nucleoli which has been implicated in their organisation and transcriptional silencing due to the close association at pachytene[18]. There is currently no information about the role of cohesin proteins in platypus meiosis
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