So-called aberrant gonosomic formulas in placental mammals. The special case of the sloth Choloepus hoffmanni Peters (Edentata, Xenartha, family Bradypodidae)

Abstract

The chromosome complement of the sloth Choloepus hoffmanni Peters has been investigated in mitosis and also in male meiosis. The karyotype for both males and females is characterized by a diploid number of 49 chromosomes. In the male the Y-material is translocated on an autosome but the meiotic behavior of the gonosomes is normal and therefore the sex determining mechanism may be normal too, despite the translocation. The females have an XO sex-chromosome constitution in somatic cells. An hypothesis, based on a slight deviation of a normal phenomenon is proposed to explain as regular such a formula in normal animals. — Relating to these conclusions, other known deviations of the standard XX/XY sex chromosome constitution in placental mammals are discussed (multiple sexchromosomes, “composite” gonosomes and XO female formula). The general conclusion is that despite an apparent variability of sex chromosome morphology, all placental mammals seem to retain a truly XX/XY sex constitution.