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Drosophila buzzatii is native to Argentina and Brazil and was introduced to Australia, probably between 193 1 and 1936, during the biological control program for prickly pear cacti (Opuntia spp.) (Mann 1970; Barker 1982; Barker et al. 1985). It lives and breeds in rotting tissues of prickly pear and apparently is restricted to this niche (Barker and Mulley 1976 ) . Populations of D. buzzatii in Australia occur in islands corresponding to patches of Opuntia. Before the introduction of the moth Cactoblastis cactorum as a biological control agent, prickly pears formed a nearly continuous distribution throughout much of Queensland and northern New South Wales ( fig. 1) . Cactoblastis cactorum rapidly reduced the distribution of Opuntia to isolated patches (Dodd 1940; Mann 1970); consequently, the distribution of D. buzzatii is restricted to these same patches. The sizes of the populations, as well as their degree of isolation, vary widely. Barker and Mulley ( 1976) and Barker ( 198 1) found low levels of allozyme variation in Australian D. buzzatii. They compared their results with those for related species and concluded that low levels of allozyme variation may be typical of the mulleri subgroup and that their data did not necessarily indicate a founder effect at the time of introduction to Australia. In this report we present the results of a survey of mitochondrial DNA (mtDNA) restriction-site variation in Australian D. buzzatii and conclude that lack of variation is probably due to the founder effect. We studied 25 isofemale lines established from wild caught females. The 25 lines represented nine populations from throughout the Australian range of D. buzzatii (fig. 1). Flies were maintained in the laboratory on cactus-sucrose-yeast medium (Starmer and Barker 1986). mtDNA extraction followed the procedure of Tamura and Aotsuka ( 1988 ) , with minor modifications. mtDNA was digested with 12 restriction enzymes ( AvaII, BarnHI, BgZII, ClaI, EcoRI, EcoRV, HaeIII, HindIII, Hid, HpaII, PstI, and XhoI). Fragments were separated in 0.8% agarose gels, were transferred to charged nylon membranes by Southern blotting, and were probed with mtDNA isolated as above and labeled with digoxigenin-dUTP ( Boehringer Mannheim) . Bands were visualized by using the DNA Detection Kit (Boehringer Mannheim) . Sizes of bands were estimated either graphically or by using the computer program of Halliburton ( 199 1). Forty-two restriction sites were detected, representing 1.3% of the mtDNA molecule. The sum of fragment sizes was 15.8-16.5 kb for different enzymes. The smaller figures probably indicate undetected small ( ~300 bp) fragments. No size variation or restriction-site variation was detected. All lines showed the same pattern for a given enzyme. The most reasonable explanation for the lack of variation in Australian D. buzzatii is that it indicates a founder effect at the time when the species colonized Australia. Other evidence is consistent with this hypothesis. Allozyme studies (for the same set

Lack of mitochondrial DNA variation in Australian Drosophila buzzatii.