Comparative Male Meiosis in Seven Species of Cotton Rats, Genus Sigmodon

A study of the male meiosis in seven species of cotton rats, genus Sigmodon, was carried out. The general course of meiosis in four species, S. hispidus, S. leucotis, S. ochrognathus, and S. alleni, is similar to that in other mammals studied. In pachynema, the sex bivalent is negatively heteropycnotic and tightly coiled to form the sex vesicle. In diakinesis, the sex bivalent is elongated and stains lightly. All of these species have a diploid number of 52, and 26 bivalents are seen in diakinesis. In the remaining three species, S. arizonae, S. mascotensis, and S. fulviventer, the sex bivalent is negatively heteropycnotic in pachynema with no indication of condensation. It would appear that the functional activity of the sex chromosomes in meiosis in these species is completed before pachynema. Diakinesis stages in S. arizonae (2n=22, 24) possess 11 or 12 bivalents; in S. fulviventer (2n=28, 29, or 30) 14 or 15 bivalents; and S. mascotensis (2n=28) 14 bivalents. Differences found in activity of the sex bivalent can be correlated with the two groups of Sigmodon established on the basis of diploid and fundamental numbers and represent another expression of the differences in genomes between the two groups.