A comparative study of the chromosomes of the Norway rat (Rattus norvegicus ERXL.) and the black rat (Rattus rattus L.)

Publisher Website
Google Scholar
Abstract
By the use of a satisfactory technique, excellently preserved spermatogenetic tissue was had both for Rattus rattus and Rattus norvegicus. The careful examination of twelve spermatogonial cells of the former species and of twenty in the latter species shows that R. rattus has forty diploid chromosomes and R. norvegicus, forty‐two. A careful examination of the haploid cells of both species, both in the first and in the second spermatocyte divisions, confirms the diploid determinations.Both species have an unequal pair in the spermatogonial divisions and the finding of a similar unequal pair in the first spermatocyte division constitutes the evidence for an X‐Y mechanism in each. A comparison of the morphology of the first spermatocyte tetrads in the two species reveals the presence of a large K‐shaped chromosome in R. norvegicus which is not present in R. rattus. Furthermore, a comparison of the X‐Y complex in both the spermatogonial and first spermatocyte divisions shows that these are morphologically different in the two species, the Y in particular being markedly dissimilar in size. A short discussion as to the bearing of these findings on the questions of the origins of the two species and their known intersterility is presented. The marked similarity of the tetrads of the black rat to those described for the mouse is noted.