MOLECULAR RELATIONSHIPS OF THE AUSTRALIAN BANDICOOT GENERA ISOODON AND PERAMELES (MARSUPIALIA: PERAMELINA) M. WESTERMAN AND C. KRAJEWSKI Westerman M and Krajewski C 2000. Molecular relationships of the Australian bandicoot genera Isoodon and Perameles (Marsupialia: Peramelina). Australian Mammalogy 22: 1-8. 12S rRNA sequences resolve the two Australian bandicoot genera Perameles and Isoodon as monophyletic clades which diverged from one another in the middle Miocene. Perameles bougainville, the most divergent species of this genus, appears to have split from the P. gunnii + P. nasuta lineage in the late Miocene, whilst subsequent speciation events occurred in the latter half of the Pliocene. Within Isoodon, although there was a clear recognition of an I. macrourus group of taxa, little support could be found for the continued recognition of the Tasmanian I. obesulus and the Barrow Island form of I. auratus as separate subspecies. Major radiations within Isoodon appear to have occurred in the last 3 million years as Australia became more arid. Key words: 12S rRNA, molecular phylogeny, evolution, mammals, marsupials, bandicoots M. Westerman, Genetics Department, LaTrobe University, Bundoora, Vic 3083, Australia. E- mail: genwm1@lure.latrobe.edu.au. C. Krajewski, Department of Zoology, and Centre for Systematic Biology, Southern Illinois University, Carbondale, Illinois 62901-6501, USA. E- mail: careyk@siu.edu. Manuscript received 9 September 1999; accepted 20 March 2000. TAXONOMIC relationships of Australian bandicoots are still rather confused. The family Peramelidae is often used to include all bandicoot genera, though their precise interrelationships are the subject of much debate. It now seems that not only Macrotis (Archer and Kirsch 1977; Mahoney and Ride 1988; Kirsch et al. 1999) but also Chaeropus ( Tate 1948; Westerman et al. 1999) may, in fact, be distinct from all other bandicoots and constitute a separate family, Thylacomyidae. Even the relationships of the Australian bandicoot genera Isoodon and Perameles are by no means clear. With the exception of a single species, Isoodon macrourus, which is found in southern New Guinea (and which may represent a Pleistocene isolate), all members of the family are endemic to Australia (Strahan 1995). Members of Perameles share few or no derived character states with Isoodon (Groves and Flannery 1990). Six species are recognised: four modern (P. bougainville, P. eremiana, P. gunnii, and P. nasuta; Seebeck et al. 1990) and two fossil (P. allinghamensis and P. bowensis; Muirhead et al. 1997). P. bougainville, the western barred bandicoot, though formerly much more widespread, is now probably restricted to the islands of Shark Bay, WA. It is considered to have the most plesiomorphic dentition of the modern species, and until recently included four recognised subspecies (Friend 1990). The desert bandicoot, P. eremiana, now thought to be extinct, is sometimes considered only an arid-adapted subspecies/form of P. bougainville (Strahan 1995). Only the eastern barred (P. gunnii) and the long- nosed (P. nasuta) bandicoots seem to be reasonably common today, and for P. gunnii this designation is limited to the Tasmanian populations. Indeed, little is known about the genetic relationships between Tasmanian and mainland populations of P. gunnii, though the mtDNA RFLP study of Robinson (1995) suggested that they were genetically dissimilar. For P. nasuta, two subspecies are often recognised: P. nasuta nasuta from the Australian east coast south of Townsville (Qld.), and P. n. pallescens from Townsville north to Ravenshoe (Strahan 1995). Taxonomic relationships within the genus Isoodon are much more complex. Although Isoodon is commonly divided into three species (I. auratus, I. macrourus and I. obesulus) (see Tate 1948; Ride 1970; Kirsch and Calaby 1977; Strahan 1995), morphological variation within the genus has, from time to time, led to the recognition of up to ten (Iredale and Troughton 1934; Lyne and Mort 1981; Close et al.1990). Most of these are now regarded as subspecies of one or other of the main three listed above, and even the specific status of I. auratus is sometimes questioned. Thus I. macrourus and I. auratus each have three subspecies, whilst I. obesulus has five (Strahan 1995). Based on a number of morphological characters, Tate (1948) distinguished two major species groups