Genetic Substructuring As a Result of Barriers to Gene Flow In
Total Page：16
File Type：pdf, Size：1020Kb
Heredity 79 (1997) 117—127 Received 9 May 1996 Geneticsubstructuring as a result of barriers to gene flow in urban Rana temporaria (common frog) populations: implications for biodiversity conservation SUSAN P. HITCHINGS & TREVOR J. C. BEEBEE* Department of Biochemistry, University of Sussex, Fa/mer, Brighton BN1 9QG, U.K The ability to maintain small populations in quasi-natural settings is an issue of considerable importance in biodiversity conservation. The genetic structure of urban common frog (Rana temporaria) populations was determined by allozyme electrophoresis and used to evaluate the effects of restricted intersite migration. Despite the lack of any absolute barrier to movement between ponds, substantial genetic differentiation was found between sites separated by an average of only 2.3 km. Genetic distances between these town ponds correlated positively with geographical distances and were almost twice as great as those found between rural sites separated by an average of 41 km. Measures of genetic diversity and fitness were always lowest in the town, where the degree of subpopulation differentiation (FST =0.388)was high. Popula- tion decline was not evident in the town, but molecular and fitness data indicated the presence of genetic drift and inbreeding depression. The long-term survival of artificially restricted populations, particularly of relatively sedentary species, may require molecular monitoring, if genetic diversity is not to be lost by chance when facets of the species niche prove to be poorly understood. Keywords:allozymes,biodiversity, conservation genetics, fitness, genetic drift, Rana temporaria. Introduction (Melitaea cinxia), in which the probability of extinc- tion was greatest in small, isolated habitat patches Continuinganthropogenic disruption of natural where immigration rates were comparatively low population structure has become a major issue (Hanski et al., 1995). Such findings have resulted in among conservationists (Diamond, 1989; Tilman et a variety of initiatives to reduce the present and a!., 1994; Kruess & Tscharntke, 1994). Fragmented predicted attrition of species numbers. In addition to populations, especially those of small size, are likely the creation of nature reserves, these include: plans to encounter genetic drift as the major mechanism for wildlife corridors to facilitate colonization and of genetic change, and this is essentially a random link partitioned ranges (Lindenmayer & Nix, 1992; process of loss equivalent to sampling error (see, for Mann & Plummer, 1995); governmental policies for example, Falconer, 1989). Such effects have been biodiversity conservation (EEC, 1992); legislation demonstrated in the natural mosaic habitat of the for the protection of endangered species and habi- Ozark Mountains of the United States, where tats; and inauguration of reintroduction schemes genetic isolation in local populations of a variety of (Martin, 1994). The majority of these measures, ectothermic animals, with concomitant reduction in however, involve limitations on population size, subpopulation polymorphism, resulted from histori- immigration and emigration. cal, geographical and climatic influences (Templeton To quantify the effects of barriers to gene flow in et al., 1990). Local habitat fragmentation can also animal populations on a local scale, we have affect populations adversely if connectivity is exploited the quasi-natural environment of an urban disrupted, as was shown in the Glanville fritillary town and used the relatively abundant and wide- spread common frog Rana temporaria (Arnold, *Correspondence. E-mail: t.j [email protected] 1995) as an example. Concerns over declines in this 1997 The Genetical Society of Great Britain. 117 118 S. P. HITCHINGS & T. J. C. BEEBEE amphibian have been expressed for some time (e.g. populations of the same species existed, which were Cooke, 1972), although not in the context of urban also sampled and provided a control against which gardens where provision of new habitat (i.e. garden the urban data could be evaluated. The effect on ponds) was associated with increasing numbers genetic structure of barriers to gene flow is a ques- (Beebee, 1979).Ranatemporaria is a semiaquatic tion likely to be encountered with increasing species, which breeds communally in shallow ponds frequency in communities worldwide. in early spring (Cooke, 1975a) and exhibits faithful- ness to a summer home range (Smith, 1973; Haapa- Methods nen, 1974) that is usually within 750 m of the breeding pond (Glandt, 1986). Rana temporaria lays Site selection, sampling and rearing an average of 1300 eggs (Cooke, 1975b) in a single Thirteen breeding sites were selected to assess spawn clump. This facilitates study, as subsamples of genetic diversity and relative fitness in their R. the progeny of each individual can be taken without temporaria populations (Table 1). The criteria for confusion of identity or diminution of the popula- selection were that ponds were long-established, tion. It was hypothesized that urban Brighton traditional breeding sites hosting populations of R. (Sussex, UK) would demonstrate the effects on frog temporaria and the toad Bufo bufo (the subject of a populations of isolating factors consisting of separate study) every year. Eight sites were urban, of inhospitable intersite terrain created by man and which six were owned privately and two were public imposed within a comparatively short time period. parks where, over many years, humans are known to In 1799 Brighton was an isolated fishing village but have removed and deposited spawn. Five rural today is part of an intensively developed, extensive Sussex sites with large frog populations were also conurbation with many transecting major roads and assayed. With the exception of the parks, no site had railways. It was estimated (Beebee, 1979) that any known record of introductions within the past 35 Brighton and its adjoining coastal towns contained years. > 7000 R. temporaria breeding ponds. By selecting The number of breeding anurans was recorded as ponds within the town with no recent history of twice the direct count of spawn clumps made at all introductions, it was possible to sample the gene urban sites sampled, supplemented by rural popula- pools of a range of subpopulations. Raising spawn tion size estimates (this survey and Banks & Elliott, and tadpoles in captivity permitted measures of 1990). Female R. temporai-ia produce one spawn fitness to be obtained, which could in turn be related clump per year, and the operational sex ratio is to genetic diversity and the developmental history of generally 1:1 (Cooke, 1975a). Embryos were Brighton. In the surrounding county, large rural collected by gently removing a small portion of every TabJe I Details of sampling sites and populations of Rana temporaria Site Type Sample number Population size Urban I Private crematorium 70 140 2 Private garden 7 40 3 Private garden 7 14 4 Public park 32 64 5 Public park 33 66 6 Private garden 5 10 7 Private garden 80 160 8 Private garden 11 22 Rural A Wooded heathland 100 > 1000 B River valley marsh 20 100—1000 C Coastal marsh 84 >1000 D Old parkland 100 2000—4000 E Lake in agricultural setting 60 >1000 Sample number is the number of spawn clumps from which samples were taken. Population size is twice the spawn clump count. The Genetical Society of Great Britain, Heredity, 79, 117—127. GENETICSUBSTRUCTURINGINR. TEMPORARIA 119 accessible spawn clump at each site, up to a exhibited atypically high mortality and develop- maximum of 100 such samples. In all but one urban mental abnormality of a type associated with nonge- site (2, where 13/20 spawn clumps sank), every netic factors (possibly caused by parasite infection or spawn clump at every site was sampled. These were pesticide pollution) and was therefore excluded from stored temporarily in water from the breeding site the analysis of survival. and returned immediately to the laboratory, where a total of 100 embryos were carefully removed, by Allozymeanalysis gentle separation of adjacent capsules, from the maximum possible number of these spawn sub- Proteinextracts were homogenates of entire indivi- samples. Thus, each final sample represented the dual larvae, thawed sufficiently to permit crushing fullest possible range of genotypes at the breeding and then liquidized on ice in 150 1uL or 200 1iL location. Spawn was examined briefly under a lower (depending on tadpole size) of ice-cold 50 mt Tris- power light microscope at 7—40 x magnification to HC1 (pH 8) extraction buffer. Homogenates were confirm that it was undamaged, and the embryos frozen at —20°C for 15 mm to ensure lysis of transferred to containers with approximately 8 L of subcellular organelles, spun for 15 mm at 8000 g in a dechlorinated tap water for culture under uniform chilled MicroCentaur microcentrifuge to thaw and conditions. pellet debris, and supernatants either used immedia- After resorption of gills, free-swimming larvae tely or re-stored in liquid nitrogen. An extensive were maintained at densities of <7 L1 and fed series of trials using standard methodologies (as rabbit pellets and blanched lettuce ad libitum. described, for example, in Richardson et a!., 1986) Developmental abnormalities visible to the naked identified 16 enzymes from 19 loci which produced eye and survival were recorded during growth. At reliable banding patterns on gels during repeated about Gosner stage 37 (Gosner, 1960), 24 indi- assays. These loci could be resolved using three viduals were