Indirect Estimates Of Gene Flow Research Articles

Genetic structure in the species‐pairSilene vulgarisandS. uniflora(Caryophyllaceae) on the Baltic island of Öland

Allozyme data were used to assess the genetic structure between 37 sympatric populations of the species‐pairSilene vulgarisandS. uniflorassp.petraea, and to infer levels of intra— and interspecific gene flow in the two species.Silene vulgarisis a geographically widespread weed of disturbed habitats whereasS. uniflorassp.petraeais endemic to the Baltic islands of Öland and Gotland. On Öland,Silene vulgarisforms extensive linear populations along roads whileS. uniflorassp.petraeaoccurs in sparse and spatially‐separated populations in open limestone habitats. Despite the differences in population size and structure between the two species, both species show extremely low levels of between‐population differentiation. Between‐site differences account for <2% of the total allozyme diversity within Öland inS. vulgaris, and <1% inS. uniflorassp.petraea.Indirect estimates of gene flow are high for both species (Nm = 11 and 27, respectively). There is no relationship between genetic distance and geographic distance within either species, and the lack of genetic structure is consistent with the pollination biology of the species — both of which are predominantly moth‐pollinated. The two species hybridize in intermediate habitats, and the geographic distribution of species‐characteristic alleles indicates a potential for spatially extensive interspecific gene flow. Nevertheless, there are significant differences in allele frequencies between the two species and multivariate analyses show no overlap between populations of the two species. The species are ecologically separated by their different habitat preferences and by differences in their flowering phenology. There is no evidence that the endemicS. uniflorassp.petraeais threatened by genetic contamination or assimilation by the widespread weed,S. vulgaris.

HIERARCHICAL GENETIC STRUCTURE AND GENE FLOW IN MACROGEOGRAPHIC POPULATIONS OF THE EASTERN TENT CATERPILLAR (MALACOSOMA AMERICANUM).

Genetic structure and inferred rates of gene flow in macrogeographic populations of the eastern tent caterpillar Malacosoma americanum were analyzed at two hierarchical scales: local demes and regional subpopulations. Wright's F-statistics were used to estimate population genetic structure using multilocus genotypic data generated electrophoretically. Estimated values of FST and the distribution of private alleles were then used to obtain indirect estimates of gene flow. We found modest, though significant, genetic structure at both spatial scales, a pattern consistent with high rates of gene flow over the large distances involved. Modest values obtained for Nei's genetic distance also suggested high levels of gene flow across the range of this species, although some gene-flow restriction resulting from isolation by distance was suggested by a positive regression of genetic distance on geographic distance. The observed homogeneity at enzyme loci across the range of M. americanum parallels the reported uniformity in morphology, suggesting a general absence of local genetic differentiation in this widely distributed species. The genetic homogeneity observed in this wide-ranging insect is discussed in terms of organism-specific environmental experience at different spatial scales. Some organisms occupying apparently heterogeneous environments may ameliorate unsuitable local conditions through microhabitat selection or behavioral modification of their microenvironment. This may be accomplished in M. americanum through group shelter construction and behavioral thermoregulation, closely tying thermoregulation to social biology in this species. If in this way the tent helps produce an effectively homogeneous environment for this species across its extensive range, this system may provide a unique example of how social behavior can influence the distribution of genetic variation in a population.

GENETIC AND MORPHOLOGICAL DIVERGENCE AMONG MORPHOTYPES OF THE ISOPOD EXCIROLANA ON THE TWO SIDES OF THE ISTHMUS OF PANAMA.

Excirolana braziliensis is a dioecious marine isopod that lives in the high intertidal zone of sandy beaches on both sides of Central and South America. It possesses no larval stage and has only limited means of adult dispersal. Indirect estimates of gene flow have indicated that populations from each beach exchange less than one propagule per generation. Multivariate morphometrics have discovered three morphs of this species in Panama, two of them closely related and found on opposite sides of Central America ("C morph" in the Caribbean and "C' morph" in the eastern Pacific), the third found predominantly in the eastern Pacific ("P morph"). Though the P and C' morphs are seldom found on the same beach, they have overlapping latitudinal ranges in the eastern Pacific. A related species, Excirolana chamensis, has been described from the Pacific coast of Panama. Each beach contains populations that remain morphologically and genetically stable, but a single drastic change in both isozymes and morphology has been documented. We studied isozymes and multivariate morphology of 10 populations of E. braziliensis and of one population of E. chamensis. Our objective was to assess the degree of genetic and morphological variation, the correlation of divergence on these two levels of integration, the phylogenetic relationships between morphs, and the possible contributions of low vagility, low gene flow, and occasional extinction and recolonization to the genetic structuring of populations. Genetic distance between the P morph, on one hand, and the other two morphotypes of E. braziliensis, on the other, was as high as the distance between E. braziliensis and E. chamensis. Several lines of evidence agree that E. chamensis and the P morph had diverged from other morphs of E. braziliensis before the rise of the Panama Isthmus separated the C and C' forms, and that the P morph constitutes a different species. A high degree of genetic differentiation also exists between populations of the same morph. On the isozyme level, every population can be differentiated from every other on the basis of at least one diagnostically different locus, regardless of geographical distance or morphological affiliation. Morphological and genetic distances between populations are highly correlated. However, despite the high degree of local variation, evolution of E. braziliensis as a whole has not been particularly rapid; divergence between the C and C' morphs isolated for 3 million yr by the Isthmus of Panama is not high by the standard of within-morph differentiation or by comparison with other organisms similarly separated. Alleles that are common in one population may be absent from another of the same morph, yet they appear in a different morph in a separate ocean. The high degree of local differentiation, the exclusive occupation of a beach by one genotype with rare arrival of foreign individuals that cannot interbreed freely with the residents, the genetic stability of populations with infrequent complete replacement by another genetic population, and the sharing by morphs of polymorphisms that are not shared by local populations, all suggest a mode of evolution concentrated in rare episodes of extinction and recolonization, possibly coupled with exceptional events of gene flow that help preserve ancestral variability in both oceans.

Protein polymorphism in Atlantic salmon within a Scottish river: evidence for selection and estimates of gene flow between tributaries

Significant heterogeneity was found for allelic variation at eight protein loci among Atlantic salmon (Salmo salar) from tributaries in the Kyles of Sutherland river system in northeast Scotland. Differentiation at the mMEP-2∗ locus was greatest and correlated with summer water temperature as found in other studies. This supports the contention that the polymorphism is affected by selection. Indirect estimates of gene flow among tributary populations based on Ncm, the effective number of individuals exchanged between tributary populatuons, and calculated from Wright's FST were 6.2 for mMEP-2∗ and varied from 9.3 to 79.8 for the other loci. The overall mean estimate was 15.1. An estimate of Ncm based on the mean frequency of “private” alleles was 2.0. The proportion of migrant salmon among each tributary's breeding population is likely to be in the order of 5% or less. The genetic substructuring of the Atlantic salmon observed within the river system provides the scope for genetic adaptation of populations to local environmental conditions.

Population Genetics and Gene Flow of the Common Tern

Genetic analysis using isoelectric focusing of blood proteins indicated that Common Terns, Sterna hirundo, from four colonies in Minnesota and Wisconsin had an observed average heterozygosity (Ho) of 0.044 and had 12 polymorphic loci of the 34 loci examined (P = 0.353). These values suggest that Common Terns of Minnesota and Wisconsin have approximately average levels of genetic variation and diversity for avian species. Interpopulational Fst values, four genetic distance estimates and an overall Fst value of 0.0022 revealed that the four colonies (subpopulations) were not genetically differentiated. Indirect estimates of gene flow, using the method of conditional average allelic frequency, indicated high levels of gene flow. Using the method of private alleles (alleles found in only one subpopulation), the estimated number of immigrants per generation (Nm) into each subpopulation ranged from seven to 16, again demonstrating significant levels of movement of terns among colonies. Direct estimates of dispersal from band recovery data corroborated these findings. By extending these results, I suggest that other Common Tern colonies in North America have at least as much gene flow among regional breeding populations as do these disjunct inland colonies.