X-monosomy in the Indian black rat

The south Indian black rat, Rattus r. rufescens (Gray) has a diploid number of 38 chromosomes. A random sample from Hinakal (Mysore, S.W. India) revealed the presence of rats with aneuploid chromosome numbers, viz. 2n = 37, 39, 40, and 42. The 37-chromosome individual was a female and possessed the XO constitution. The diploid numbers higher than 38 were due to the presence of supernumerary (B) chromosomes. The origin and consequences of the XO condition in Rattus r. rufescens and other mammals is discussed.