Diversity In Marginal Populations Research Articles

Genetic Diversity in Marginal Populations of Nitraria schoberi L. from Romania

Nitraria schoberi L. (Nitrariaceae) is a halophytic plant with a continuous range in Central Asia and with only two populations in the westernmost distribution limit of species, in Romania. Currently, there is no documented explanation for the species’ presence in Europe, outside the main distribution area. Considering that marginal populations genetics are important in establishing range limits and species adaptative potential, genetic diversity was assessed using Inter-simple sequence repeat markers (ISSR). Both the Shannon’s Information Index (I) and Expected Heterozygosity (He) suggested a relatively low level of genetic diversity within the two populations. However, the Unweighted Pair Group Method with Arithmetic Mean (UPGMA) dendrogram and Principal Coordinates Analysis clearly distinguished the two populations. Our presumptions, based on current results, are that the marginal westernmost population of N. schoberi was established due to the unique conditions from the “islands of desert” developed in a temperate continental climate. The European establishment of this species was likely accidental and probably due to ornithochory. Genetic relatedness between populations could be a consequence of their common origin, presumably from proximal Asian N. schoberi populations, while the separation can be explained by the lack of genetic material exchange between the two populations.

Genetic evidence of peripheral isolation and low diversity in marginal populations of the Acropora hyacinthus complex

Zooxanthellate corals are found throughout the tropics, but also extend into subtropical and marginal locations due to the presence of warm ocean currents. The population history of corals in marginal locations is of great interest in relation to changing global climatic conditions, as species edge zones might play an important role in evolutionary innovation. Here, we examine the genetic structure of a widely distributed coral species complex, Acropora hyacinthus, from tropical to high subtropical regions along the Kuroshio Current in Taiwan and Japan. Population genetic analysis of 307 specimens from 18 locations (7 reefal and 11 marginal) identified at least four genetic lineages within the A. hyacinthus complex: HyaA, HyaB, HyaC (dominating reefal locations) and HyaD dominating marginal locations in mainland Japan and Taiwan, except the upper Penghu Islands, which were dominated by HyaC. Crossing experiments suggested semi-incompatibility and hybridization between HyaC and D from reefal locations, implying that the existence of hybridization partners enhances diversification and genetic diversity. An incomplete barrier between the HyaC and HyaD dominations was found along the two straits in the Ryukyu Islands, where Kuroshio Current flows constantly. Despite geographical distance, the genetic composition of populations in mainland Japan was comparable to that in mainland Taiwan, which may reflect a region-specific connectivity around the northern limit of A. hyacinthus in the Pacific. In contrast, populations in the Ryukyu Islands were not significantly different from those of Palau and the Great Barrier Reef. While the precise taxonomic nature of the lineages found around the Kuroshio Current remains to be elucidated, these results indicate that, despite the presence of four lineages in the Kuroshio triangle, low genetic diversity populations of the two main lines might be isolating and differentiating in the marginal region.

Structure of genetic diversity in marginal populations of black poplar (Populus nigra L.)

It has been hypothesized that populations at the margins of the distributional range of a species show reduced genetic diversity and increased inter-population differentiation compared to central populations. Here, we test this hypothesis by examining the structure of genetic diversity in marginal populations of black poplar, Populus nigra L. (Salicaceae). This species occurs mainly in Europe but its range extends to central Asia. We collected 117 individuals from 10 populations at the edge of the distributional range of the species in central Asia to examine the structure of genetic diversity based on genetic polymorphisms at 20 microsatellite markers. As expected, the genetic diversity within these marginal populations is relatively low, with an average observed heterozygosity Ho of 0.337 and an average expected heterozygosity He of 0.466, compared to the genetic diversity of populations from central distributions. However, we recovered very low genetic differentiation between populations, with an average Fst of 0.0745, a value similar to those reported for central populations. AMOVA analyses confirmed this result, showing that only 9.2% of the total variation could be attributed to between-population variance (P < 0.001). Our findings do not fully support hypotheses about the structure of genetic diversity in marginal populations formed from observations on other species. We suggest that a high rate of outcrossing and possible postglacial colonization at the edge of the distributional range of this long-lived poplar may explain the observed structure of the genetic diversity.

Low genetic diversity in marginal populations of Bletilla striata (Orchidaceae) in southern Korea: Insights into population history and implications for conservation

The warm-temperate vegetation of Korea, currently limited to southern coastal areas, shifted southward during the Last Glacial Maximum towards glacial refugia putatively located in southern Japan and/or southern China. We hypothesized two scenarios of post-glacial re-colonization of warm-temperate species: (i) that extant Korean populations originated from a single source or (ii) that they are derived from multiple source populations. To test which of these scenarios is more likely, we investigated the patterns of genetic diversity in 16 populations of the warm-temperate terrestrial orchid Bletilla striata, employing 20 allozyme loci. Levels of genetic variation in B. striata were substantially lower than those reported in most other terrestrial orchids. However, the degree of genetic differentiation among populations was moderate. These results suggest a pattern of post-glacial re-colonization fitting the first scenario. Both in situ and ex situ conservation strategies are suggested to preserve the genetic variation of B. striata in Korea.

The importance of phenotypic differentiation and plasticity in the range expansion of the annual Atriplex tatarica (Amaranthaceae)

The ability of a species to adapt to sub-optimal conditions at the margin of its distribution range and to cope with environmental stress is considered to be important for its successful geographic expansion. To ascertain the roles of phenotypic differentiation and plasticity in the expansion of the annual Atriplex tatarica, we compared plants from populations found in Marginal and Central areas of the species’ range. We grew these plants under marginal climatic conditions in pots with different types of substrate. We assessed the population genetic structure at five putatively neutral allozyme loci to evaluate whether there was any evidence of reduced genetic diversity in Marginal populations compared to Central ones. We used the QST vs. FST approach (while FST gives a standardised measure of the genetic differentiation among populations for a genetic locus, QST measures the amount of genetic variance among populations relative to the total genetic variance) to ascertain the roles of adaptive vs. non-adaptive processes on phenotypic differentiation. Plants native to the Marginal area of the species’ range flowered earlier and had a lower shoot mass and a higher reproductive allocation than plants native to the Central part of the species’ range. The Marginal populations of Atriplex tatarica showed lower genetic diversity at allozyme loci and higher phenotypic differentiation than the Central populations. We recorded similar plastic responses to substrates in plants native to both regions. Our results indicate that Marginal populations of expanding A. tatarica maintain the ability to adapt locally and to elicit a plastic response to environmental stress, despite loss of genetic diversity.

Genetic structure in northeastern populations of the Alpine newt (Triturus alpestris): evidence for post‐Pleistocene differentiation

Genetic variation in 13 populations of the Alpine newt, Triturus alpestris, was assessed at the northeastern margin of its range (southern Poland). Variation at six microsatellite loci was scored in 354 newts, and two mitochondrial DNA fragments (c. 2000 bp) were sequenced in a subset of 27 individuals. Significant differences in allele frequencies and the presence of private alleles determined genetic units corresponding to three separate mountain ranges, i.e. the Carpathian, Sudetes and Holy Cross Mountains. F(ST)'s were three times greater in among than in within mountain range pairwise comparisons. An assignment test and pairwise F(ST)'s suggested relatively high levels of gene flow at the local level, although the Sudetes populations revealed some subtle structuring. Genetic variation was lower in the Carpathians and Holy Cross Mountains. The geographic pattern of mitochondrial DNA variation indicated that these newt populations originated from a single glacial refugium/founder population, and that the colonization of southern Poland took place in an easterly direction. The data show that substantial neutral variation and between group divergence has accumulated relatively quickly in these low-vagility organisms. The Alpine newt case exemplifies species history as a factor determining patterns of genetic diversity in marginal populations.

Tree populations bordering on extinction: A case study in the endemic Canary Island pine

Marginal populations are usually located at the edge of the species’ range, constituting small isolates that grow in suboptimal environments. In widely distributed conifers, such as pines, marginal populations have often been considered recent or rare colonizing events without ecological relevance. The Canary Island pine provides an excellent case to study the significance of critically endangered marginal populations (from 20 to a few hundred trees) in an endemic conifer that is also extensively used in commercial plantations. Our chloroplast microsatellite (cpSSR) survey showed contrasting values of haplotypic genetic diversity in marginal populations of Canary Island pine, from very low at Garabato (northern La Gomera) and Anaga (Tenerife), located within broad-leaved tree formations, to very high in Gran Canaria marginal populations (Arguineguin and Galdar). Competition with broad-leaved vegetation, after rare long-distance colonization events, might explain present-day low levels of variation in Garabato and Anaga. Gran Canaria and southern La Gomera relict populations, which contain high haplotypic diversity, are probably remnants of larger populations severely reduced by adverse climatic conditions and human activities, the latter starting shortly after the European colonization of the islands in the XV century. Given the extremely low population numbers and a general lack of natural regeneration, the risk of extinction in the near future is high in marginal populations of Canary Island pine. Conservation activities are urgently required and should focus on the highly diverse Gran Canaria and southern La Gomera pine marginal populations. Here, the establishment of in situ genetic reserves (Arguineguin and Imada), and multifunctional ex situ collections (Arguineguin and Galdar), such as seed orchards, is highly recommended. In addition, common garden experiments to test the performance of potentially drought-tolerant genotypes from Arguineguin (southernmost Gran Canaria’s relict) and potential risks due to inbreeding and/or outbreeding depression should be established.

Genetic diversity in butterflies: interactive effects of habitat fragmentation and climate-driven range expansion

Some species are expanding their ranges polewards during current climate warming. However, anthropogenic fragmentation of suitable habitat is affecting expansion rates and here we investigate interactions between range expansion, habitat fragmentation and genetic diversity. We examined three closely related Satyrinae butterflies, which differ in their habitat associations, from six sites along a transect in England from distribution core to expanding range margin. There was a significant decline in allozyme variation towards an expanding range margin in Pararge aegeria, which has the most restricted habitat availability, but not in Pyronia tithonus whose habitat is more widely available, or in a non-expanding 'control species' (Maniola jurtina). Moreover, data from another transect in Scotland indicated that declines in genetic diversity in P. aegeria were evident only on the transect in England, which had greater habitat fragmentation. Our results indicate that fragmentation of breeding habitats leads to more severe founder events during colonization, resulting in reduced diversity in marginal populations in more specialist species. The continued widespread loss of suitable habitats in the future may increase the likelihood of loss of genetic diversity in expanding species, which may affect whether or not species can adapt to future environmental change.

Alozyme Diversity and Population Genetic Structure in Haplopappus gracilis (Compositae)

Levels of allozyme diversity and patterns of genetic structure within and among populations of Haplopappus gracilis were assessed using data from starch gel electrophoresis. Several indices of genetic diversity were used to compare H. gracilis with other species possessing similar life form and distributional attributes. F statistics using allozyme data indicated only a small degree of differentiation among populations, and an Nm value of 3.60 suggested a relatively high level of gene flow among populations. Uniformly high genetic identities (mean 0.989) between populations are in the range of other conspecific populations. Genetic diversity values at the species (Hes) and population (H,P) levels were in general agreement with those of similar species, with little reduction of diversity in marginal populations. Unlike previously reported differentiation of H. gracilis pop- ulations into distinct ecological races based on life history and morphological traits, there was no apparent correlation of ecological or geographical patterns to allozyme variational patterns. Studies of genetic variation in natural pop- ulations have produced important information about evolutionary processes and the nature of species. Potential for evolutionary change is de-

PATTERNS OF ALLOZYMIC SIMILARITY IN ECOLOGICALLY CENTRAL AND MARGINAL POPULATIONS OF HORDEUM JUBATUM IN UTAH.

Populations at the margins of a species range often appear to be living in an environment stressful for the species. Individuals may show signs of physiological stress, such as stunting, dehydration, or starvation, while the populations may be smaller, exhibit a lower density of individuals, and have a higher probability of extinction than populations at the center of the range. In contrast, central populations often appear to be in a favorable environment, one to which the species is highly adapted (Mayr, 1963). Whereas central populations may inhabit a variety of habitats, those at the margin may be restricted to a single type (Bantock and Price, 1975). Such stressful environments can occur not only at the species border but also locally, as conditions along some environmental gradient become less and less favorable until the species is no longer able to maintain a population (Lewontin, 1974). In such cases, both ecologically marginal populations (those existing near the minimum level of subsistence) and ecologically central ones (those in favorable conditions) may be located close to one another away from the geographic periphery of the species range. Da Cunha and Dobzhansky (in Dobzhansky, 1970) have suggested that a restriction in available habitats will lead to a reduction in genetic diversity in marginal populations. Previous work on the genetic structures of marginal populations in animals has focused primarily on quantifying levels of variability among chromosomal polymorphisms (e.g., Carson, 1955; da Cunha et al., 1959; Soule, 1973) and allozymes (e.g.,