Neutral Genetic Markers (neutral + genetic_marker)

Distribution by Scientific Domains

Selected Abstracts


EVOLUTION, Issue 12 2006
Josh Van Buskirk
Abstract Inbreeding is known to reduce heterozygosity of neutral genetic markers, but its impact on quantitative genetic variation is debated. Theory predicts a linear decline in additive genetic variance (VA) with increasing inbreeding coefficient (F) when loci underlying the trait act additively, but a nonlinear hump-shaped relationship when dominance and epistasis are important. Predictions for heritability (h2) are similar, although the exact shape depends on the value of h2 in the absence of inbreeding. We located 22 published studies in which the level of genetic variation in [source]

Complete lack of mitochondrial divergence between two species of NE Atlantic marine intertidal gastropods

Abstract Some mitochondrial introgression is common between closely related species, but distinct species rarely show substantial introgression in their entire distribution range. In this study, however, we report a complete lack of mitochondrial divergence between two sympatric species of flat periwinkles (Littorina fabalis and Littorina obtusata) which, based on previous allozyme studies, diverged approximately 1 Ma. We re-examined their species status using both morphology (morphometric analysis) and neutral genetic markers (microsatellites) and our results confirmed that these species are well separated. Despite this, the two species shared all common cytochrome-b haplotypes throughout their NE Atlantic distribution and no deep split between typical L. fabalis and L. obtusata haplotypes could be found. We suggest that incomplete lineage sorting explains most of the lack of mitochondrial divergence between these species. However, coalescent-based analyses and the sympatric sharing of unique haplotypes suggest that introgressive hybridization also has occurred. [source]

Five questions on ecological speciation addressed with individual-based simulations

Abstract We use an individual-based simulation model to investigate factors influencing progress toward ecological speciation. We find that environmental differences can quickly lead to the evolution of substantial reproductive barriers between a population colonizing a new environment and the ancestral population in the old environment. Natural selection against immigrants and hybrids was a major contributor to this isolation, but the evolution of sexual preference was also important. Increasing dispersal had both positive and negative effects on population size in the new environment and had positive effects on natural selection against immigrants and hybrids. Genetic divergence at unlinked, neutral genetic markers was low, except when environmental differences were large and sexual preference was present. Our results highlight the importance of divergent selection and adaptive divergence for ecological speciation. At the same time, they reveal several interesting nonlinearities in interactions between environmental differences, sexual preference, dispersal and population size. [source]

Testing alternative mechanisms of evolutionary divergence in an African rain forest passerine bird

T. B. Smith
Abstract Models of speciation in African rain forests have stressed either the role of isolation or ecological gradients. Here we contrast patterns of morphological and genetic divergence in parapatric and allopatric populations of the Little Greenbul, Andropadus virens, within different and similar habitats. We sampled 263 individuals from 18 sites and four different habitat types in Upper and Lower Guinea. We show that despite relatively high rates of gene flow among populations, A. virens has undergone significant morphological divergence across the savanna,forest ecotone and mountain,forest boundaries. These data support a central component of the divergence-with-gene-flow model of speciation by suggesting that despite large amounts of gene flow, selection is sufficiently intense to cause morphological divergence. Despite evidence of isolation based on neutral genetic markers, we find little evidence of morphological divergence in fitness-related traits between hypothesized refugial areas. Although genetic evidence suggests populations in Upper and Lower Guinea have been isolated for over 2 million years, morphological divergence appears to be driven more by habitat differences than geographic isolation and suggests that selection in parapatry may be more important than geographic isolation in causing adaptive divergence in morphology. [source]

Contrasting genetic structures of two parasitic nematodes, determined on the basis of neutral microsatellite markers and selected anthelmintic resistance markers

Abstract For the first time, the neutral genetic relatedness of natural populations of Trichostrongylid nematodes was investigated in relation to polymorphism of the ,-tubulin gene, which is selected for anthelminthic treatments. The aim of the study was to assess the contribution of several evolutionary processes: migration and genetic drift by neutral genetic markers and selection by anthelminthic treatments on the presence of resistance alleles at ,-tubulin. We studied two nematode species (Teladorsagia circumcincta and Haemonchus contortus) common in temperate climatic zones; these species are characterized by contrasting life history traits. We studied 10 isolated populations of goat nematode parasites: no infected adult goat had been exchanged after the herds were established. Beta-tubulin polymorphism was similar in these two species. One and two ,-tubulin alleles from T. circumcincta and H. contortus respectively were shared by several populations. Most of the ,-tubulin alleles were ,private' alleles. No recombination between alleles was detected in BZ-resistant alleles from T. circumcincta and H. contortus. The T. circumcincta populations have not diverged much since their isolation (FST <0.08), whereas H. contortus displayed marked local genetic differentiation (FST ranging from 0.08 to 0.18). These findings suggest that there are severe bottlenecks in the H. contortus populations, possibly because of their reduced abundance during unfavourable periods and their high reproductive rate, which allows the species to persist even after severe population reduction. Overall, the data reported contradict the hypothesis of the origin of ,-tubulin resistance alleles in these populations from a single mutational event, but two other hypotheses (recurrent mutation generating new alleles in isolated populations and the introduction of existing alleles) emerge as equally likely. [source]

Does habitat fragmentation reduce fitness and adaptability?

A case study of the common frog (Rana temporaria)
Abstract Studies examining the effects of anthropogenic habitat fragmentation on both neutral and adaptive genetic variability are still scarce. We compared tadpole fitness-related traits (viz. survival probability and body size) among populations of the common frog (Rana temporaria) from fragmented (F) and continuous (C) habitats that differed significantly in population sizes (C > F) and genetic diversity (C > F) in neutral genetic markers. Using data from common garden experiments, we found a significant positive relationship between the mean values of the fitness related traits and the amount of microsatellite variation in a given population. While genetic differentiation in neutral marker loci (FST) tended to be more pronounced in the fragmented than in the continuous habitat, genetic differentiation in quantitative traits (QST) exceeded that in neutral marker traits in the continuous habitat (i.e. QST > FST), but not in the fragmented habitat (i.e. QST , FST). These results suggest that the impact of random genetic drift relative to natural selection was higher in the fragmented landscape where populations were small, and had lower genetic diversity and fitness as compared to populations in the more continuous landscape. The findings highlight the potential importance of habitat fragmentation in impairing future adaptive potential of natural populations. [source]

Site-specific genetic divergence in parallel hybrid zones suggests nonallopatric evolution of reproductive barriers

Abstract The evolution of reproductive isolation in the presence of gene flow is supported by theoretical models but rarely by data. Empirical support might be gained from studies of parallel hybrid zones between interbreeding taxa. We analysed gene flow over two hybrid zones separating ecotypes of Littorina saxatilis to test the expectation that neutral genetic markers will show site-specific differences if barriers have evolved in situ. Distinct ecotypes found in contrasting shore habitats are separated by divergent selection and poor dispersal, but hybrid zones appear between them. Swedish islands formed by postglacial uplift 5000 years ago provide opportunities to assess genetic structure in a recently evolved system. Each island houses a discrete population containing subpopulations of different ecotypes. Hybrid zones between ecotypes may be a product of ecological divergence occurring on each island or a consequence of secondary overlap of ecotypes of allopatric origin that have spread among the islands. We used six microsatellite loci to assess gene flow and genetic profiles of hybrid zones on two islands. We found reduced gene flow over both hybrid zones, indicating the presence of local reproductive barriers between ecotypes. Nevertheless, subpopulations of different ecotypes from the same island were genetically more similar to each other than were subpopulations of the same ecotype from different islands. Moreover, neutral genetic traits separating the two ecotypes across hybrid zones were site-specific. This supports a scenario of in situ origin of ecotypes by ecological divergence and nonallopatric evolution of reproductive barriers. [source]

Lack of mitochondrial genetic structure in hamlets (Hypoplectrus spp.): recent speciation or ongoing hybridization?

Marina L. Ramon
Abstract Species in the genus Hypoplectrus (hamlet fish) have been recognized primarily on the basis of colour morphology, which varies substantially. Limited differentiation in other morphological characters, however, has led to ongoing debate about their taxonomy. Our analysis of mtDNA sequences demonstrates neither reciprocal monophyly nor significant genetic differentiation among hamlet colour morphs. These data are potentially consistent with a model of recent speciation due to sexual selection on colour morphology. The presence within hamlets of two divergent mtDNA clades, however, suggests a longer history during which hybridization and gene flow have prevented the differentiation of hamlet colour morphs, at least in neutral genetic markers. [source]

Pattern of geographical variation in petal shape in wild populations of Primula sieboldii E. Morren

Abstract The petal shape of Primula sieboldii E. Morren (Primulaceae) is diverse in wild populations. In this study, we investigated population differentiation in the petal shape of P. sieboldii using image analysis. Flowers were sampled from 160 genets from eight wild populations in the western to north-eastern parts of the Japanese archipelago. Principal component (PC) analysis of 40 coefficients of elliptic Fourier descriptors (EFDs) detected three major characteristics of petal shape variation: the ratio of length to width (PC1), the depth of the head notch (PC2) and the position of the center of gravity (PC3). To test the association between divergence in petal shape and geographical and genetic distances, we calculated two types of pairwise population distances for petal shape: Mahalanobis distances for the 40 EFD coefficients and for the first three PCs. The existence of an association between neutral genetic markers and petal shape was revealed by the Mahalanobis distances based on the 40 EFD coefficients, suggesting that evolutionary forces, such as founder effect and isolation by distance effect, are probably the main causes of differentiation in petal shape. In contrast, we found no association between Mahalanobis distances for the first three PCs and geographical and genetic distances. The discrepancy between the two petal shape distances indicated that the population differentiation promoted by the founder effects and isolation by distance effect appears mainly as subtle changes in petal shape rather than in major characteristics of petal shape variation. [source]