Mitochondrial Analysis of Gene Flow between New Zealand Mallards (anas Platyrhynchos) and Grey Ducks (a. Superciliosa)

•One of the more well-known examples of hybridization in birds is the frequently documented occurrence between sexually dimorphic Mallards (Anas platyrhynchos) and several closely related nondimorphic species in the mallard complex. In New Zealand, the Grey Duck (Anas superciliosa superciliosa) is the indigenous, nondimorphic Mallardlike species, and extensive hybridization with introduced Mallards has been implicated in the population decline of Grey Ducks. Individuals from throughout the country were classified phenotypically as parentals or hybrids based on variation in plumage, bill color, and leg color. We confirmed species-specific mitochondrial DNA haplotypes by comparing restrictionenzyme fragment patterns in Grey Ducks and New Zealand Mallards to those of Pacific Black Ducks (A, superciliosa rogersi) from Australia and Mallards from North America, respectively. Our data indicate that hybridization has led not only to introgression of Grey Duck mtDNA into Mallard populations (the predicted direction of gene flow), but also to significant introgression of Mallard mtDNA into Grey Duck populations. Thus, the contention that hybridization between Mallards and nondimorphic species involves primarily Mallard males with females of the other species is not upheld for this example from New Zealand. The speciation process appears to be undergoing reversal. Received 1 April 1993, accepted 2 July 1993. THE INCIDENCE OF interspecific and intergeneric hybridization in the order Anseriformes is higher than in any other order of birds, reaching 30 to 40% by some estimates (Grant and Grant 1992). In addition, a substantial proportion of interspecific hybrids (20%) in this order have been reported to be fertile (Scherer and Hilsberg 1982), so there is potential for extensive gene ííovf and introgression between some species. Among the more well-known examples is the frequently documented incidence of hybridization between sexually dimorphic Mallards {Anas platyrhynchos) and several closely related, nondimorphic species. For instance, in North America, hybridization with Mallards has been implicated as one factor in the population decline of American Black Ducks (A. rubrifes; Johnsgard 1967, Heusmann 1974, Ankney et al. 1987), Hawaiian Ducks {A. wyvilliana; Griffin et al. 1989), and Mexican Ducks (A. platyrhynchos diazi; Hubbard 1977). Increasing numbers of ' Present address: Department of Biological Sciences, Clemson University, Clemson, South Carolina 29634, USA. Mallard/Mottled Duck (A. fulvigula) hybrids also are being reported in some areas of Florida (Mazourek and Gray 1994). In fact, the AOU (1983) declared the Mexican Duck to be conspecific with the Mallard because of extensive hybridization between them. In New Zealand, the Grey Duck (A. superciliosa superciliosa) is the indigenous, nondimorphic. Mallardlike species. Grey Ducks are thought to have colonized from Australia, like much of New Zealand's avifauna (Baker 1991), and the Pacific Black Duck (A. superciliosa rogersi) in Australia is virtually identical phenotypically to the New Zealand Grey Duck (Frith 1982). Mallards were introduced by the Otago Acclimatization Society into the southern region of South Island, New Zealand in the mid-1800s from European game-farm stock and into North Island by the Auckland Acclimatization Society in the 1930s from North America (Williams 1981). Over the last few decades. Mallard populations have increased dramatically, while Grey Duck populations have declined. Extensive hybridization with Mallards, facilitated by the loss of natural habitats to agriculture, has been im-