Asian karyotype (2n = 42) in the black rat (Rattus rattus) in Mexico

A cytogenetic survey in 10 black rats (Rattus rattus), captured in Guadalajara, Jalisco, Mexico, revealed a 42 chromosomal complement (Asian type) in all of them. Polymorphism due to pericentric inversions of chromosomes 9, 11 and 13, as well as a homomorphic subtelocentric pair 1, permitted further karyotypic distinction. An Asian origin of this rat with a subsequent isolated evolution in Mexico is concluded.