Karyotypic differences of black rats,Rattus rattus,collected in various localities of East and Southeast Asia and Oceania

Karyotypes of several subspecies of black rats, Rattus rattus, collected in different localities of Asia and Oceania were examined with special emphasis on the relationship between the chromosome polymorphism and differentiation of the subspecies. Subspecies of black rats (R. rattus) collected were as follows; tanezumi from Japan; flavipectus and sladeni from Hong Kong; diardii, jalorensis from Kuala Lumpur, Malaysia; argentiventer from Kuala Lumpur, and Java and Celebes, Indonesia; mindanensis from Luzon and Mindanao, Philippines; and rattus from Australia, New Zealand, and New Guinea. Subspecies in Formosa, Korea and Thailand were not determined. All black rats collected in the above Asian districts had 42 diploid chromosomes, while those in Oceania had 38. The rats collected in Japan (tanezumi), Korea, Formosa, Thailand and Malaysia (diardii) had A/A No. 1 pair or polymorphic No. 1 (A/A, A/S and S/S) pairs, while those collected in Java and Celebes (argentiventer), Luzon and Mindanao (mindanensis) showed a higher frequency of S/S No. 1 pair. From the higher occurrence of No. 1 A/A pair of black rats in the Asian continent where the black rats originated, it is suggested that the original type of No. 1 chromosome pair of the black rats is A/A, and a pericentric inversion occurred in the acrocentric No. 1 chromosome and thus rats with subtelocentric No. 1 pair formed.—Black rats with 38 chromosomes were observed in Australia, New Guinea and New Zealand. These karyotypes seem to have developed by Robertsonian fusion of 4 acrocentric pairs (No. 4 and 7, and No. 11 and 12) in black rats of the Asian type. A relationship between body size and chromosome constitution was observed in subspecies of the black rats.Contribution No. 831 from the National Institute of Genetics, Japan. Supported by a grant-in-aid from the Ministry of Education of Japan (Scientific Expedition in 1968, No. 8801 in 1969, and No. 9001 in 1970).