Sexual dimorphism in prophase I of meiosis in the Northern mole vole (Ellobius talpinus Pallas, 1770) with isomorphic (XX) chromosomes in males and females

The synaptonemal complex (SC) surface-spreading technique was used to visualize the process of chromosome synapsis in spermatocytes and oocytes of E. talpinus Pallas, 1770, a species with the XX sex chromosome system in both males and females. We used electron microscopy and immunofluorescent localization of synaptonemal complex protein (SCP3) and centromeric proteins to analyze the structure and behaviour of synaptonemal complexes in prophase I of meiosis, aiming to reveal signs of meiotic sexual dimorphism in this species. We present evidence of considerable differences in the structure and behaviour of the axial structures of sex bivalents in male and female meiosis, despite the isomorphic G- and C-banding patterns of mitotic sex chromosomes. During meiotic prophase I, the sex bivalent in females behaved as autosomal bivalents, but it was not involved in the formation of the bouquet configuration or it was the first to leave it. The XX chromosomes of males formed closed sex bivalents. Only short tracts of SC were formed at both ends of the sex bivalent, while large middle segments of the lateral elements remained unpaired. The male sex chromosomes also formed characteristic “sex bodies”. In fact, electron microscopy revealed dense nucleolus-like bodies associated with unpaired parts of the axial elements. These regions of the sex chromosomes were poorly immunostained, because the distribution of SCP3 had a peculiar powder-like pattern, but SCP3 was not associated with the nucleolus-like bodies. We also revealed signs of sexual dimorphism in the dynamics of formation and destruction of autosomal SCs. In males, the total SC length was shorter than in females. The chromosome bouquet configuration was preserved up to the stage of early pachytene in females. The bouquet configuration in males was not expressed. At late pachytene, gaps were revealed in the structure of autosomal SCs in spermatocytes immunostained with antibodies to SCP3. The pattern of distribution of these gaps was comparable with the G-banding patterns of mitotic chromosomes