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Karyotypic studies were carried out on the Black Rat, Rattus rattus, with the aim of determining its chromosome number. The chromosomes were prepared from the bone marrow of a male rat treated with 0.04% colchicine. Chromosomes in well spread cells were counted and measured using Motic Images Plus - a computer software. Arm ratios and centromeric indices were determined from these measurements. The chromosomes were classified based on the centromeric indices obtained. Ideograms were also constructed from the measurements. Photomicrographs of well spread mitotic metaphase chromosomes were used to construct a standard karyotype for the species. A diploid chromosome number of 2n = 38 with a fundamental number (FN) of 76 was obtained for the local population of the Black Rat, Rattus rattus, here in the University of Nigeria, Nsukka. The karyotype shows a somewhat sharp break in chromosome lengths between chromosomes 4 and 5, and thereafter, a gradual decrease in size. From the constructed idiogram, there is a gradual decrease in length from one chromosome pair to another. The X and Y complements sharply deviated from this trend.

Studies on the Karyotype of the Black Rat, Rattus rattus, Collected at the University of Nigeria Zoological Garden