Sequential Loading of Cohesin Subunits during the First Meiotic Prophase of Grasshoppers

Abstract

The cohesin complexes play a key role in chromosome segregation during both mitosis and meiosis. They establish sister chromatid cohesion between duplicating DNA molecules during S-phase, but they also have an important role during postreplicative double-strand break repair in mitosis, as well as during recombination between homologous chromosomes in meiosis. An additional function in meiosis is related to the sister kinetochore cohesion, so they can be pulled by microtubules to the same pole at anaphase I. Data about the dynamics of cohesin subunits during meiosis are scarce; therefore, it is of great interest to characterize how the formation of the cohesin complexes is achieved in order to understand the roles of the different subunits within them. We have investigated the spatio-temporal distribution of three different cohesin subunits in prophase I grasshopper spermatocytes. We found that structural maintenance of chromosome protein 3 (SMC3) appears as early as preleptotene, and its localization resembles the location of the unsynapsed axial elements, whereas radiation-sensitive mutant 21 (RAD21) (sister chromatid cohesion protein 1, SCC1) and stromal antigen protein 1 (SA1) (sister chromatid cohesion protein 3, SCC3) are not visualized until zygotene, since they are located in the synapsed regions of the bivalents. During pachytene, the distribution of the three cohesin subunits is very similar and all appear along the trajectories of the lateral elements of the autosomal synaptonemal complexes. However, whereas SMC3 also appears over the single and unsynapsed X chromosome, RAD21 and SA1 do not. We conclude that the loading of SMC3 and the non-SMC subunits, RAD21 and SA1, occurs in different steps throughout prophase I grasshopper meiosis. These results strongly suggest the participation of SMC3 in the initial cohesin axis formation as early as preleptotene, thus contributing to sister chromatid cohesion, with a later association of both RAD21 and SA1 subunits at zygotene to reinforce and stabilize the bivalent structure. Therefore, we speculate that more than one cohesin complex participates in the sister chromatid cohesion at prophase I.

Relationship between incomplete synapsis and chiasma localization

“…Then, only the non-centromeric chromosomal regions, those which have previously developed cohesin axes and loaded the meiotic recombination machinery, will be able to achieve pairing and synapsis. However, DSBs could occur concomitantly with the axis formation (van Heemst and Heyting 2000;Kleckner 2006;Lorenz and Whitby 2006;Sánchez-Morán et al 2007). Consequently, although synapsis is severely reduced, SC is always formed at the non-centromeric ends of each autosome.…”

“…In order to establish accurately the pattern of synapsis progression in P. meridionalis, we have used, as in previous studies of other orthopteran species (Viera et al 2004a, b;Calvente et al 2005;Valdeolmillos et al 2007), antibodies against SMC3, which is widely conserved in eukaryotes (Hirano 2002). We have also analysed the dynamics of another cohesin subunit, SMC1α.…”

Coupling meiotic chromosome axis integrity to recombination

“…Exchange is apparent from images of chiasmata in diplotene chromosomes, which further show that sister chromatid axes are locally separated at these sites (see picture in Jones and Franklin 2006). Indeed, certain axial structural components, including cohesins, are already known to be specifically absent from chiasmata sites; e.g., Rec8 in mouse (Eijpe et al 2003) and MCD1/SCC1/ RAD21 plus SMC3 in grasshoppers (Valdeolmillos et al 2007). Occurrence of axis changes being specific to sites of COs, it would be sensible for these local axis changes to begin at the time of CO designation, thus much earlier than when observed at chiasmata sites during diplotene, and then to proceed in concert with the DNA events of recombination (e.g., Blat et al 2002).…”

Cohesin axis maturation and presence of RAD51 during first meiotic prophase in a true bug

“…Here, this situation has been circumvented by using antibodies generated against the cohesin subunits, SMC3 and SMC1α. The analysis of the SMC3 cohesin axes throughout prophase I has allowed the identification of the different stages in several insect species (Calvente et al 2005;Valdeolmillos et al 2007;Viera et al 2004aViera et al , b, 2009b. In this sense, we have previously shown that, in squashed pachytene spermatocytes of G. italicum, SMC3 appears along axes of both autosomes and sex chromosomes (Page et al 2006), and it has also been reported that the cohesin axes reflect the position of the underlying axial elements/lateral elements (AEs/LEs ; Pelttari et al 2001;Viera et al 2004b).…”