The forbidden doubling: exploring rare spermatocyte polyploidy in mammals

We studied rare cases of over-ploid spermatocytes using advanced immunocytochemical methods and a cross-species approach for the first time. In subterranean rodents Ellobius tancrei, E. alaicus, E. talpinus, and Nannospalax leucodon tetraploid spermatocytes exhibited specific features during meiotic prophase I, including symmetric and asymmetric chromosome quadrivalents with partner-switching, extended asynapsis, altered recombination patterns, and variable chromatin inactivation. Notably, the quadrivalents were capable of assembling shelterin complexes at chromosome ends, which are connected to the nuclear envelope through the SUN-KASH complex as in normal spermatocytes. Anomalies suggest that meiotic checkpoints potentially triggered by failed synapsis or incomplete sex chromosome silencing may act to prevent the progression of polyploid spermatocytes.