Karyotype and Heterochromatin Pattern of the Fat Sandrat Psammomys obesus obesus with Special Reference to the Sex Chromosomes and Their Behaviour During Meiosis

Abstract

The karyotype of the fat sandrat Psammomys obesus obesus from Egypt has been investigated. The diploid number of its chromosomes is 2n = 48. According to the relative chromosome length and the centromeric index, the autosomes were classified into 14 pairs of metacentrics and 9 pairs of telocentrics. The X-chromosome is represented by an outstandingly large metacentric element. The Y-chromosome is also a large metacentric falling in length between the second and the third pairs of autosomes. The constitutive heterochromatin, as revealed by C-banding technique is localized at the centromeric regions of the autosomes, different regions along the length of the X-chromosome (about 6.7-7.4% of its length) and one entire arm of the Y chromosome. The other arm showed heterochromatin bands homologous to that of the short arm of the X chromosome. In meiosis, the behaviour of the sex chromosomes was followed including types of association and the mode of segregation of the chromosomes. It has been suggested that the increase in the size of the sex chromosomes is mainly due to autosomal translocation.