Sex chromosomes, synapsis, and cohesins: a complex affair

Abstract

During first meiotic prophase, homologous chromosomes are held together by the synaptonemal complex, a tripartite proteinaceous structure that extends along the entire length of meiotic bivalents. While this feature is applicable for autosomes, sex chromosomes often escape from this rule. Many species present sex chromosomes that differ between them in their morphology, length, and gene content. Moreover, in some species, sex chromosomes appear in a single dose in one of the sexes. In all of these cases, the behavior of sex chromosomes during meiosis is conspicuously affected, and this includes the assembly and dynamics of the synaptonemal complex. We review in this study the structure of the synaptonemal complex in the sex chromosomes of three groups of organisms, namely: mammals, orthopterans, and hemipterans, which present different patterns of sex chromosome structure and behavior. Of special interest is the analysis of the organization of the axial/lateral elements of the synaptonemal complex in relation to other axial structures organized along meiotic chromosomes, mainly the cohesin axis. The differences found in the behavior of both axial structures reveal that while the organization of a cohesin axis along sex chromosomes is a conserved feature in most organisms and it shows very little morphological variations, the axial/lateral elements of the synaptonemal complex present a wide range of structural modifications on these chromosomes.

Sex chromosome silencing in the marsupial male germ line

“…1 D and DЈ). The DP appeared to be attached to the nuclear envelope, consistent with previous description (48). At diplotene, SCP3 staining of the DP remained intense but became extremely weak on autosomes ( 1 E and EЈ).…”

“…Unlike observations in eutherians, X-Y association occurred late during pachytene and only after the autosomes had synapsed ( Fig. 1 C and CЈ), confirming delayed sex chromosome association described previously in M. domestica (47) and other marsupials (48). This contrasts with eutherian pachytene, during which partial homology between the X and Y enables synaptonemal complex formation and true homologous pairing, akin to what is observed among autosomal pairs.…”

“…Furthermore, we noted that, although it resides at a peripheral nuclear location, the opossum XY body does not protrude out of the nucleus at mid-late pachytene as is characteristic of the mouse XY body. Overall, the behavior of the M. domestica sex chromosomes during male meiosis is similar to what has been described for other marsupials (48).…”

“…1 C and CЈ). However, the absence of any obvious homology between the sex chromosomes appeared to preclude the formation of synaptonemal complex (48). Instead, the X and Y associate through a dense plate (DP) (49) between the X and Y arcs ( Fig.…”

Cohesins and Cohesin-Regulators in Meiosis

“…Thus, structural modifications in the unsynapsed AEs of XY pair have been identified. Moreover, modifications in the distribution of different cohesin subunits have been also observed, as the preferential location of REC8 in the synapsed region of the XY body at pachytene (Page et al, 2006). However, our results show that the ESCO2 labelling embrace all the chromatin of sex chromosomes, similar signals have been detected after immunostaining of -H2AX and surprisingly the cohesin subunit RAD21L has been also observed in the XY chromatin at pachytene and diplotene , Ishiguro, et al, 2011.…”