Abstract
Karyotype of the Australian wild rat, Rattus villosissimus, with 50 chromosomes were examined with conventional and G-banding techniques. Among 24 autosome pairs two were large metacentrics due to the Robertsonian fusion of pair nos. 2 and 3 and of pair nos. 6 and 10. Twelve extra small acrocentric pairs were confirmed to result from the Robertsonian fission of small metacentric pairs 14 to 19 occurring in the basic karyotype of Rattus.
