Genetic Patchiness Research Articles

Population structure of the Dover sole, Solea solea L., in a background of high gene flow

To investigate the genetic population structure of the Dover sole, Solea solea L., allozyme electrophoresis was performed on 303 fish collected from seven locations ranging from Cumbria, Great Britain, to Greece. A total of 22 enzyme systems were analysed, coded by 33 loci. Of these, 27 loci were polymorphic using the P 99 criterion. A phenogram using Prevosti's Distance generated by the Wagner method exhibited a geographic pattern in the clustering of populations. Estimates of N m (effective number of migrants per generation between populations) were sufficiently high to imply near-panmixia between the North Sea, Bay of Biscay and the Irish Coast populations, indicating a probable movement of migrants through the English Channel. However, despite this high level of gene flow, striking patterns of geographic differentiation were observed at a few loci. Allele frequencies at loci ACOH, EST-I-1, PEP-I-2 exhibited genetic patchiness on both local and range-wide (within the northern and southern European basins) scales. This pattern of genetic patchiness could be the result of localised selection, genetic drift or single-generation sampling effects. Estimates of mean heterozygosity ( H) were inversely related to latitude. Evolutionary processes such as genetic drift and founder effect, and/or selection, may have produced the observed difference in the number of alleles between the basins. Moreover, the absence of isolation by distance provides support for a model of geographic isolation. Such a pattern of genetic patchiness, revealing a slight reduction of genetic variability in the northern European basin, may suggest a population bottleneck, or local reduction in population size. Various physical parameters, especially water temperature during the reproductive period, vary within the range of the species, and may produce or maintain this genetic differentiation. These results indicate the role of both ecological and evolutionary structuring mechanisms in determining the genetic population structure of S. solea.

Genetic structure of natural populations of black locust (Robinia pseudoacacia L.) at Coweeta, North Carolina

Natural population of black locust maintains very high levels of genetic diversity within populations without any noticeable geographic patterns. In order to assess the species' potential for manipulation through breeding programs, more detailed spatial pattern analyses of fine-scale population structure was attempted in the two study sites (watershed 2 and 21) in the Coweeta River Basin of southern Appalachian mountains using allozyme markers. Of the 200 and 420 plants examined in watershed 21 and 2 respectively, 13 and 15 major genotypes of clone were distinguished. Within watersheds 2 and 21, 60% of the pairwise clone combinations had three or four loci differences out of the 8 loci investigated. No correlation between distance between clones and the number of allele difference was found. The similarity values (16 highest and 0 lowest value) among clones for watershed 2 and 21 were 12.82 and 10.19 respectively. Typical distances between clones on both watershed 2 and 21 were from 90 and 190 m, but the range varied from several meters up to 300 m. An average distance among clones were 140.9±66.0 m and 239.4±108.6 m in watershed 2 and 21 respectively, whereas an average distance within clone was 38.7±36.2 m and 145.0±123.6 m in watershed 2 and 21. Genotype diversity indices (DG) for watershed 2 and 21 were 0.89 and 0.86 respectively. The largest clones on both watersheds covered more than 100 m×100 m. On the other hand, some clones on watershed 2, which had a few fallen trees with many young sprouts (up to 200 individuals), were 45 m×30 m in size under the open habitat. Although clonal substructure and limited seed dispersal may cause near neighbor mating interaction, significant genetic patchiness among clones of black locust was not evident. Although the first establishment of a black locust population would have relied on an off-site seed source, it is more likely that asexual reproduction has occurred for many generations in these black locust populations because of the presence of many different sizes of trees (or ages) in one clone despite widespread opportunities for sexual reproduction. Presence of three or four different sizes of trees within a clone implies that the trees has developed from a clone at each different time. The current study suggested that the amount of sexual reproduction and dispersal rate (especially seed dispersal) should be determined. It would be useful to examine whether or not clonal strategies and patterns of clonal growth of black locust are influenced by density dependent regulation of ramet recruitment and death.

Gene Flow and Larval Duration in Seven Species of Fish from the Great Barrier Reef

The Great Barrier Reef (GBR) is a continental archipelagic system of 5000 reefs and shoals stretching >2000 km along the east Australia coast. The interconnectivity of these reefs should determine the choice of biological management units, which for most biota will reflect the dispersal of their eggs and/or larvae. A comparative approach using population genetics was used to ask whether the along—shore dispersal of coral reef fishes is influenced by the duration of this mobile phase. Seven species of coral reef fish, selected from three families to provide a range of taxonomic diversity and pelagic larval durations, were tested for genetic homogeneity between two regions of the GBR separated by 1000 km. A spectrum of potential dispersal capabilities was analyzed from that of Acanthochromis polyacanthus, a damselfish with brood care that uniquely lacks pelagic larvae, to that of Ctenochaetus striatus, a surgeonfish with large, specialized larvae that spend several months in the plankton. A total of 19 enzyme systems and general proteins were examined from multiple populations in each region to provide a base of 32 loci for these comparisons. With one exception, species sampled from different coral reefs within regions showed statistically significant heterogeneities across multiple loci, indicative of chaotic genetic patchiness among the samples. The exception was an anemonefish, Amphiprion melanopus, that had to be collected from large areas on each reef because of its low densities. The homogeneity of allele frequencies at local scales for this species suggests that the genetic patchiness observed in others may be a within—reef phenomenon that was manifested at the reef scale by our pseudoreplicated sampling strategy. After pooling local variability, all but two species showed significant regional differences. The exceptions were the pair (Ctenochaetus striatus, Pterocaesio chrysozona) with the longest larval durations. Acanthochromis polyacanthus showed increased variation at this larger scale, consistent with a major stock division between the two regions. The logarithm of genetic variation between northern and southern populations (measured by Weir and Cockerham's Fst was correlated with mean larval duration by an inverse linear relationship that explained 85% of the variance in the global data set. Comparison with an outgroup (Amphiprion melanopus from the Chesterfield Reefs, 1000 km east in the Coral Sea) confirmed the genetic cohesion of mainland populations for the species with shortest larval duration and shows that our empirical relationship applies only within the context of the highly connected GBR. On this basis, calculations of gene flow (Nem, the number of effective migrants per generation) between geographic regions predict panmixis for species with larval durations exceeding 1 mo. Many common species have shorter dispersal times, from which classical isolation—by—distance" models predict differentiation between northern and southern populations at genetic equilibrium. Given that modern populations on the GBR are <10 000 yr old, however, there has not been sufficient time for such differences to evolve in situ and we consider alternative scenarios for the observed heterogeneities. Comparisons with invertebrate taxa sampled over the same spatial scales imply lower gene flows in fish despite longer pelagic durations. This suggests that fish larvae may use their greater mobility to retard, rather than enhance, dispersal due to hydrodynamic advection.

Population genetics of the blue crabCallinectes sapidus: modest population structuring in a background of high gene flow

To determine the genetic population structure of blue crabs (Callinectes sapidus Rathbun), electrophoretic allozyme analysis was performed on 750 individuals collected from 16 nearshore locations ranging from New York to Texas, USA. Twenty enzymes and non-enzymatic proteins coded by 31 presumptive loci were examined. Twenty-two loci were either monomorphic or polymorphic at less than theP95 level; alleles for these polymorphic loci were geographically dispersed. Allele frequencies for three of the remaining polymorphic loci were homogeneous over all populations, as were levels of polymorphism and heterozygosity. Phenograms generated by the UPGMA (unweighted pair-group method using arithmetic averages) and distance Wagner methods exhibited no geographic pattern in the clustering of populations. Estimates ofNem (effective number of migrants per generation between populations) indicated substantial gene flow, with aalues sufficiently high to infer panmixia between all blue crab populations from New York to Texas. However, despite this high level of gene flow, two striking patterns of geographic differentiation occurred: genetic patchiness and clinal variation. Allele frequencies atEST-2, GP-1, IDHP-2, DPEP-1, DPEP-2, andTPEP exhibited genetic patchiness on local and range-wide geographic scales, and allele frequencies atEST-2 varied temporally. Genetic patchiness in blue crabs is likely to be the result of the pre-settlement formation and subsequent settlement of genetically heterogeneous patches of larvae; allele frequencies of those larval patches may then be further modified through ontogeny by localized selection. In the Atlantic Ocean, a regional latitudinal cline ofEST-2 allele frequencies was superimposed on the range-wide genetic patchiness exhibited by that locus. This pattern against a background of high gene flow is highly likely to be maintained by selection. In estuaries along the Atlantic Ocean coast, a combination of low adult long-distance migration and a high retention rate of locally spawned larvae could serve to segregate populations and allow for the development of the geographic cline inEST-2. The Gulf of Mexico showed no apparent cline, perhaps due to long-distance migration of females in some regions of the Gulf, or to masking by genetic patchiness. These results emphasize the importance of both ecological and evolutionary time scales and structuring mechanisms in determining genetic population structure.

Population subdivision in Australian temperate marine invertebrates: Larval connections versus historical factors*

AbstractThis paper reviews the use of genetic data, in combination with manipulative experimentation, to infer the mode of reproduction and the extent and directionality of dispersal for a range of Australian temperate marine invertebrates. Local populations of obligately sexually reproducing species have been inferred to be strongly interconnected by larval dispersal, over distances of thousands of kilometres. Their larvae may be subject to strong post‐settlement selection, but this selection is independent of obvious geographic or intertidal gradients. Within local populations selection may therefore result in apparently chaotic genetic patchiness which is eliminated by the effects of sexual reproduction and the widespread dispersal and mixing of the colonizing larvae of each generation. In partial contrast, local populations of species which rely on asexual reproduction for the maintenance of populations show evidence of similar larval connections, but no recent settlement of their sexually generated larvae has been demonstrated. The apparent connectedness of these populations may reflect either historical events or a more episodic pattern of settlement by sexually generated larvae. Local populations of these species are more highly differentiated as a result of the continued asexual replication of a limited number of genotypes. In one of these species, reciprocal transplantation of the clones within and among populations has revealed that resident clones can be highly locally adapted (as reflected by much higher asexual fecundity), which implies that selection is an important determinant of the composition of local populations. Nevertheless, the failure to detect continuing sexual recruitment into these populations obscures the evolutionary significance of this finding.

Effects of recruitment on genetic patchiness in the urchinEchinometra mathaei in Western Australia

Enzyme polymorphisms in the sea urchinEchinometra mathaei were examined to test the relative influences of population turnover and patchiness in recruitment on genetic heterogeneity. We found that the total variance in allelic frequency among three populations separated by approximately 4 km at Rottnest Island, Western Australia (collected in February 1985) is as large as that among five additional samples collected over a distance of 1 300 km along the Western Australian coast in August 1987. This suggests that the forces causing genetic differentiation act on a local scale and occur in a single generation. A comparison of sites with different histories of recruitment indicates that the observed genetic differences among age groups are the result of prerecruitment effects, and that differences among sites reflect their individual histories of recruitment.

Populations in clonal plants

Population phenomena in higher plants are reviewed critically, particularly in relation to clonality. An array of concepts used in the field are discussed. In contrast to animals, higher plants are modular in structure. Plant populations show hierarchy at two levels: ramets and genets. In addition, their demography is far more complicated, since even the direction of development of a ramet may change by rejuvenation. Therefore, formulae concerning animal populations often require modification for plants. Furthermore, at the zygotic stage, higher plants are generally less mobile than animals. Accordingly, their population processes tend to be more local. Most populations of plants have a genetic structure: alleles and genotypes are spatially aggregated. Due to the short-ranged foraging behaviour of pollinators, genetically non-random pollination prevails. A generalized formula for parent-offspring dispersal variance is derived. It is used to analyze the effect of clonality on genetic patchiness in populations. In self-compatible species, an increase in clonality will tend to increase the degree of patchiness, while in self-incompatible species a decrease may result. Examples of population structure studies in different species are presented. A considerable degree of genetic variation appears to be found also in the populations of species with a strong allocation of resources to clonal growth or apomictic seed production. Some consequences of clonality are considered from the point of view of genetic conservation and plant breeding.

Pattern Beneath the Chaos: The Effect of Recruitment on Genetic Patchiness in an Intertidal Limpet

Pattern Beneath the Chaos: The Effect of Recruitment on Genetic Patchiness in an Intertidal Limpet

Polymorphism in Pomatoceros

Allelic variation at 13 sites for the intertidal Pomatoceros lamarckii (Quatrefages) and at six sites for the sublittoral P. triqueter (L.) in North-West Europe, when examined at 19 metabolic enzyme loci, showed significant differences between sites in the frequencies of the various enzyme morphs. There was no readily interpretable geographic or environmental trend. The Hardy-Weinberg equilibrium was maintained at almost all loci and all sites. The genetic distance between populations was very small in comparison with the difference between the two species. Colour morphs of P. triqueter showed a clear latitudinal cline with blue and brown morphs predominating at more northerly latitudes, and grey, orange and red morphs at more southerly latitudes. Interlocality variations in allele frequency, though statistically significant, were never sufficient to alter the order of frequency of the two or three commonest alleles, so that the frequency of all but rare alleles showed a consensus throughout the ranges studied for each species. Accounting for local genetic patchiness in gene frequencies in species with pelagic larvae raises problems if the alleles are selectively neutral; accounting for wide geographic uniformity raises problems if alleles are selected for or against by environmental factors.

The Wahlund effect and the geographical scale of variation in the intertidal limpet Siphonaria sp.

Samples of Siphonaria sp. were collected between 1978 and 1982 from sites covering its known geographic range, from Kalbarri, Western Australia to Port Robe, South Australia. Geographic variation of 7 polymorphic enzymes was examined in this intertidal pulmonate limpet, and was found to be consistently small, indicating a large-scale influence of gene flow due to planktonic dispersal. Despite this large-scale uniformity, there is fine-scale genetic patchiness, which is repeated, rather than accumulated, on the larger scale. Throughout its geographic range, Siphonaria sp. shows deficits of heterozygotes for all 7 loci. The consistency among loci indicates that the causes of the deficits are populational, rather than locus-specific. A Wahlund effect, the departure from Hardy-Weinberg equilibrium due to mixing of individuals from groups with different allelic frequencies, is the simplest explanation of such deficits. The limited geographic variation of allelic frequencies, however, is grossly inadequate to produce these deficits through a Wahlund effect. Similarly, temporal variation in allelic frequencies in recruits does not explain the deficits. The largest contributor to a Wahlund effect appears to be binomial sampling variance among small local breeding groups. Thus, mixing of larvae on a scale of metres, rather than among geographical areas, apparently produces the deficits of heterozygotes.

Chaotic genetic patchiness in an intertidal limpet, Siphonaria sp.

Variation of 4 polymorphic enzymes was studied for 2 yr (1978 and 1979) in an undescribed species of Siphonaria, a pulmonate limpet, from a rocky shore at Rottnest Island, Western Australia. Depending on the locus, significant genetic differences were found among sites along 50 m of shore, between high and low portions of the shore within sites, between adults and recruits, and between recruits in the 2 yr. This genetic heterogeneity does not follow a simple, consistent pattern, but forms a shifting, ephemeral genetic patchiness best described as chaotic. This patchiness may result from temporal variation of numbers and genotypes of recruits, which leads to the proposal that planktonic dispersal, although causing uniformity on a large scale, can give rise to fine-scale genetic patchiness.

Allozyme and mtDNA variation of white seabream Diplodus sargus populations in a transition area between western and eastern Mediterranean basins (Siculo-Tunisian Strait)

To investigate the possible influence of the Siculo-Tunisian Strait on the genetic structure of white seabream Diplodus sargus, 13 polymorphic allozyme loci and a fragment of the cytochrome b mito-chondrial DNA were analysed. Allozyme data indicated a moderate but significant differentiation between some north-eastern (Bizerta, Ghar El Melh Lagoon and Mahdia) and southern (Gabes Gulf and El Biban Lagoon) samples. This heterogeneity was also highlighted after removing PGM* and PGI-1* loci which may be under selection. These results can be explained by the chaotic genetic patchiness hypothesis. In contrast, the mtDNA data indicated genetic homogeneity among localities showing the absence of structure in white seabream populations across the Siculo-Tunisian Strait. Historical demography of this species suggests that it has undergone a recent population expansion as a consequence of a bottleneck event during the Pleistocene glaciations.