Phylogeographic Patterns in Mitochondrial Dna of the Ostrich (struthio Camelus)

--We assayed restriction-site differences in mitochondrial DNA (mtDNA) within and among populations of the Ostrich (Struthio camelus) throughout much of its African distribution. Little genetic diversity was evident among samples drawn from localities throughout southern Africa (S.c. australis), while deep divisions in the mtDNA gene tree exist between representatives of the eastern (S.c. molybdophanes and S.c. massaicus) and northern African subspecies (S.c. camelus). The low mtDNA variability within australis and the presence of widespread mtDNA genotypes in this subspecies uggest considerable historical interconnectedness among populations, either through gene flow and/or recent colonization from smaller source populations. The strong phylogeographic structuring evident in eastern and northern Africa aligns with the currently accepted subspecies designations. Data indicate that the Ethiopian system of the Great Rift Valley has been effective in disrupting east-west gene flow between molybdophanes and camelus, while ecological differences and behavioral/reproductive cues have contributed to maintaining the genetic and phenotypic discreteness of molybdophanes and massaicus in east Africa. Although contemporary Ostrich populations are effectively divided into southern and northern populations by a belt of Brachystegia woodland, arid-corridor links in the recent evolutionary past appear to have allowed for periodic contact between australis and massaicus populations. Consequently, the development of subspecific differences between these two taxa has occurred within the context of shallow evolutionary separation. Received 28 July 1992, accepted 25 November 1992. THE OSTRICH (Struthio camelus) is currently regarded as comprising four extant subspecies separated by fairly marked phenotypic differences; a fifth, S.c. syriacus, whose range once reportedly extended into Arabia, is now considered extinct (Brown et al. 1982). As presently understood, the species' natural range is restricted to the African continent, generally south of the Sahara. This distribution is disrupted by a belt of Brachystegia ("miombo") woodland in south-central Africa (Hamilton 1982) that effectively divides the Ostrich into northern and southern populations with the former incorporating S.c. camelus, S.c. molybdophanes and S. c. rnassaicus, while S.c. australis is confined to southern Africa (Brown et al. 1982). Restriction-enzyme analysis of mitochondrial DNA (mtDNA) has been useful in the study of evolutionary relationships over microevolutionary time scales in a variety of avian species (e.g. Mack et al. 1986, Ovenden et al. 1987, Shields and Wilson 1987a, Avise and Zink 1988, Avise et al. 1990), often providing resolution beyond that yielded by more conventional approaches such as protein electrophoresis (Barrowclough 1983). Part of the utility of mtDNA analyses as a means for determining the magnitude of intraspecific polymorphism in organisms stems from its high rate of nucleotide-sequence evolution by base substitution, which can give rise to extensive sequence heterogeneity among conspecific individuals. Population genetic variation in mtDNA depends both on the maternal effective population size and life-history processes, with accumulating evidence clearly pointing to the dominant role that historic demographic and zoogeographic factors play in shaping intraspecific phylogenies (Avise 1989). Geographic variation in size, plumage, eggshell porosity and assorted phenotypic characteristics, underpinnings of the intraspecific classification of the Ostrich (Brown et al. 1982), may be indicative of relatively low interpopulation gene flow across obvious geographic barriers, possibly promoted to some extent by the species' inability to fly. Given the tendency for mtDNA discontinuities to align with geographic boundaries in other species (Avise et al. 1987), we sought to determine whether geographic structuring in mtDNA haplotypes among the Ostrich subspecies and/or the northern and southern Ostrich populations may exist. We present a geographic survey of mtDNA variation in the Ostrich that includes representatives of the four extant subspecies drawn from lo-