Apoptosis in Spermiogenesis of Multiple Robertsonian Heterozygotes of Mus musculus domesticus
Mus musculus domesticus is a species that is characterized by a diploid number of 40 chromosomes, all telocentrics (acrocentrics). In natural populations of Mus with high frequency Robertsonian chromosome translocations (RBs), a fusion at centromere-level between two autosomal telocentric, producing metacentric Rb chromosomes and a variety of natural subspecies with diploid chromosome numbers below 40. Rb chromosomes do not affect the viability of individuals but mainly fertility of Rb heterozygotes. In their meiosis the metacentric Rb and the homologous telocentrics form trivalents that have problems in synapsis and segregation. This paper presents a comparative analysis of spermatogenesis considering the stages of the epithelium and the number of germ cells loss by apoptosis comparing heterozygote males 2n=32, carriers 8 trivalents, and homozygotes 2n=40 and 2n=24. It was found that the number of spermatocytes in first prophase was similar in the different seminiferous epithelium stages in all the chromosome constitutions. In the 2n=32 heterozygotes a significant decreased number of spermatids was reflected in the proportion of spermatocytes and spermatids that was 1:1.7. In the homozygote males rare germ cell in apoptosis (positive for caspase 3) were observed, which were mainly concentrated in the XII stage of the seminiferous epithelium. In heterozygote spermatocytes apoptotic germ cell number was significantly higher than that registered in homozygote males, and generally correspond to spermatocytes in meiotic divisions. This selective removal of cells possibly carrying anomalies, either in chromosome alignment or segregation, is consistent with the smaller number of spermatids and the relative sub-fertility observed in 2n=32 Rb heterozygotes.