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.

Author Summary

Meiosis is a specialized cell division by which sexually reproducing organisms prompt the formation of specialized cells presenting a half of the species chromosomal number. These cells, the so-called gametes, are able to fertilize or be fertilized, depending on the sex in which they are produced and thus restore the species chromosomal number after fertilization. The reduction in the chromosome number is achieved by two successive rounds of chromosome segregations preceded by a single replication of the genetic material. Different proteins, mainly referred to as cohesins, are implied in the correct establishment and maintenance of an intimate association between homologous chromosomes by ensuring their close association until their separation in the first meiotic division. Grasshoppers have been considered as a gorgeous model for meiotic studies for decades due to their low chromosomal number, the large size of their chromosomes, and the well-defined meiotic stages at cytological level. On these grounds, we have combined classical grasshopper chromosome knowledge with protein immunolocalization tools in order to precisely analyze the presence of cohesins throughout the prophase of the first meiotic division. The results not only describe the dynamic loading pattern of several cohesin subunits in two grasshopper species, but they also surprisingly bring into light that different cohesins are sequentially loaded onto meiotic chromosomes throughout the first meiotic prophase. Finally, we discuss the possible roles for this sequential protein loading in relation to the processes that operate during meiosis, proposing a model for meiotic chromosome structure. Besides the novel scientific contributions for a better understanding of the meiotic process, this study clearly points out that classical cytogenetic models can be used to solve modern biological problems.