Presumptive X Monosomy in Black Rats from Malaya

DURING a cytogenetic study of the Malayan species of Rattus Fischer, black rats, Rattus rattus diardii (Jentink), were trapped at Kuala Lumpur, Selangor, and Batu Pahat, Johore, between November 1968 and July 1970. Ninety animals were examined cytologically and seventeen gave chromosome counts that differed from the normal diploid number of 42 (ref. 1). Fourteen had between one and three additional small metacentric chromosomes which were not distinguished from normal members of the complement. Nevertheless, they are assumed to be equivalent to the “inert” supernumerary (or B) chromosomes of many invertebrate and plant species. Counts of 41 chromosomes were recorded repeatedly in preparations from four of the rats trapped at Kuala Lumpur. Karyotype analysis indicated that “Robertsonian fusion” between two acrocentric chromosomes had occurred in a male specimen2. In the remaining three specimens (all females) an acrocentric chromosome was absent. Observations were made on preparations from bone marrow