Sunday, September 11, 2011

New publications on the molecular ecology and population structure of Ischnura elegans

Over the last two years, we have started to look more into the molecular population structure of the blue-tailed damselfly Ischnura elegans, and have tried to relate the molecular structure to ecological patterns and processes. As a result of our efforts, we have recently published a number of papers which we would like to draw your attention to. The first study investigates the large scale molecular and ecological structure of I. elegans populations across Europe. The second paper studies the frequencies of different colour morphs in Ischnura spp. populations across Europe, and investigated the processes that could explain the different patterns among regions. Lastly, the third study examines contemporary hybridisation between the sister species I. elegans and I. graellsii in Europe, and quantifies the effect of introgression on the colour morph frequencies in both species. 

Photograph by Erik Svensson: The blue-tailed damselfly

ABSTRACT: Identifying environmental factors that structure intraspecific genetic diversity is of interest for both habitat preservation and biodiversity conservation. Recent advances in statistical and geographical genetics make it possible to investigate how environmental factors affect geographic organisation and population structure of molecular genetic diversity within species. Here we present a study on a common and wide ranging insect, the blue tailed damselfly Ischnura elegans, which has been the target of many ecological and evolutionary studies. We addressed the following questions: (i) Is the population structure affected by longitudinal or latitudinal gradients?; (ii) Do geographic boundaries limit gene flow?; (iii) Does geographic distance affect connectivity and is there a signature of past bottlenecks?; (iv) Is there evidence of a recent range expansion and (vi) what is the effect of geography and climatic factors on population structure? We found low to moderate genetic sub-structuring between populations (mean FST = 0.06, Dest = 0.12), and an effect of longitude, but not latitude, on genetic diversity. No significant effects of geographic boundaries (e.g. water bodies) were found. FST-and Dest-values increased with geographic distance; however, there was no evidence for recent bottlenecks. Finally, we did not detect any molecular signatures of range expansions or an effect of geographic suitability, although local precipitation had a strong effect on genetic differentiation. The population structure of this small insect has probably been shaped by ecological factors that are correlated with longitudinal gradients, geographic distances, and local precipitation. The relatively weak global population structure and high degree of genetic variation within populations suggest that I. elegans has high dispersal ability, which is consistent with this species being an effective and early coloniser of new habitats. 

Sánchez-Guillén, R., B.Hansson, M. Wellenreuther, E. I. Svensson, and A. T. Cordero-Rivera. 2011a. Theinfluence of stochastic and selective forces in the population divergence offemale colour polymorphism in damselflies of the genus Ischnura. Heredity.

ABSTRACT: Disentangling the relative importance and potential interactions of selection and genetic drift in driving phenotypic divergence of species is a classical research topic in population genetics and evolutionary biology. Here, we evaluate the role of stochastic and selective forces on population divergence of a colour polymorphism in seven damselfly species of the genus Ischnura, with a particular focus on I. elegans and I. graellsii. Colour-morph frequencies in Spanish I. elegans populations varied greatly, even at a local scale, whereas more similar frequencies were found among populations in eastern Europe. In contrast, I. graellsii and the other five Ischnura species showed little variation in colour-morph frequencies between populations. FST-outlier analyses revealed that the colour locus deviated strongly from neutral expectations in Spanish populations of I. elegans, contrasting the pattern found in eastern European populations, and in I. graellsii, where no such discrepancy between morph divergence and neutral divergence could be detected. This suggests that divergent selection has been operating on the colour locus in Spanish populations of I. elegans, whereas processes such as genetic drift, possibly in combination with other forms of selection (such as negative frequency-dependent selection), appear to have been present in other regions, such as eastern Europe. Overall, the results indicate that both selective and stochastic processes operate on these colour polymorphisms, and suggest that the relative importance of factors varies between geographical regions. 

Sánchez-Guillén, R. A., M.Wellenreuther, A. Cordero-Rivera, and B. Hansson. 2011b. Admixture analysisreveals introgression in Ischnura damselflies in a recently establishedsympatric region. BMC Evol. Biol. 11:210.

ABSTRACT: Studying contemporary hybridization increases our understanding of introgression, adaptation and, ultimately, speciation. The sister species Ischnura elegans and I. graellsii (Odonata: Coenagrionidae) are ecologically, morphologically and genetically similar and hybridize. Recently, I. elegans has colonized northern Spain, creating a broad sympatric region with I. graellsii. Here, we review the distribution of both species in Iberia and evaluate the degree of introgression of I. graellsii into I. elegans using six microsatellite markers (442 individuals from 26 populations) and five mitochondrial genes in sympatric and allopatric localities. Furthermore, we quantify the effect of hybridization on the frequencies of the genetically controlled colour polymorphism in females of both species.

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