Abstract

This paper reports successful laboratory hybridization of R. lutreolus lutreolus (2n = 42) and R. fuscipes assimilis (2n = 38). Hybrid offspring grew rapidly (hybrid vigour) to age 24 days. Only one individual survived past 50 days but is fertile. Although intermediate for many characteristics the hybrids resemble R. lutreolus more than R. fuscipes. The hybridization indicates a closer relationship between these species than previously accepted, but there is no evidence of natural hybrids, perhaps due to an ecological separation observed in the field and to behavioural obstacles to mating observed in the laboratory. Hybrid animals (2n = 40) have single fused chromosomes from each of the two pairs of submeta- centrics in the R. fuscipes karyotype. Chromosome arm length measurements and rankings show that one centric fusion (CF1) comprises two longer chromosomes (4 + 8) and the second fusion (CF2) comprises the two shorter chromosomes (10 + 12). These fusions appear to be identical with those we observed in other R. fuscipes populations in New South Wales. The chromosome numbering determined by measurement of unbanded chromosomes in this study appears to differ from the numbering convention currently in use. This emphasizes the need to establish a standard sequence of chromosomes for Australian Rattus.