Proteinaceous structure of the meiotic sex chromosomes in the domestic cats

M. I. RAHN; A J SOLARI; R. B. SCIURANO; DE LUCA GERALDINE

Cavtat

Congreso; EMBO Conference for Meiosis; 2017

Institute of Biochemistry II, Goethe University

The X and Y chromosomes behave in a special way during the first meiotic prophase in mammalian spermatocytes compared to autosomes (reviewed in Solari, 1994 and in Handel, 2004). The heteromorphic X and Y chromosomes, generally, synapse only partially at the pseudoautosomal region (PAR) leaving two unsynapsed (or unpaired) differential regions. Moreover, both chromosomes form a well-known subnuclear structure characterized by the special compaction of its chromatin fibers: the XY body (Solari, 1974). Testes from four European shorthair cats and one Siamese cat were surgically removed by veterinarians during a normal castration operation at the Instituto de Zoonosis ?Luis Pasteur? (C.A.B.A., Argentina) following all institutional and national guidelines for the care of animals. For each animal, two pieces of tissue were processed for semithin (0.5 μm thick) and thin (0.08 μm thick) sections to analyze the seminiferous epithelium in detail by light microscopy and the ultrastructure of the XY body in spermatocyte nuclei by electron microscopy (EM), respectively. Another piece of tissue was used to perform spermatocyte spreads for synaptonemal complexes. The aim of the present work is to analyze the fine structural characteristics of the sex chromosomes and the immunolocalization of the meiotic proteins SYCP3, SYCP1, SYCP2, SYCE3, SMC3, γ-H2AX, BRCA1, 3meH3K27 and MLH1 in the primary spermatocyte nuclei of the domestic cat. The analysis of those primary spermatocytes shows that the X and Y chromosomes undergo sequential changes along pachytene stage: first bullous expansions, then subdivision into multiple fibrils, all involving axial thickening. The number of split fibers, as well as their narrowness, is very different from the substructure of the autosomal axes. That multistranded, split appearance of the X and Y axes is due to the appearance of fibrils containing SYCP3 protein, which develop into tangles and bridges in the late substages of pachytene. Some of those fibrils make a bridge across the space between the X and Y chromosomes at a few sites. In some short stretches of the multistranded X-axes, SYCP1 is located at the inner side. Unexpectedly, one of those cross-bridges in the PAR co-localize with the late recombination protein marker, MLH1. The chromatin of the XY body shows the immunolocalization of γ-H2AX and 3meH3K27. At mid-late pachytene stages, the XY pair undergoes a premature desynapsis as a consequence of the disassembly of the synaptonemal complex at the PAR. Those results show that even if the XY body of the domestic cat shows the usual patterns of meiotic sex chromosome inactivation (MSCI) in mammals, striking variations in the structure of unpaired axes and their behavior during meiotic prophase are observed. The present work will discuss the supramolecular structure of the synaptonemal complex under the light of those observations.