Evidence for hybridization and introgression within a species-rich oak (Quercus spp.) community

BMC Evolutionary Biology, Nov 2007

Background Analysis of interspecific gene flow is crucial for the understanding of speciation processes and maintenance of species integrity. Oaks (genus Quercus, Fagaceae) are among the model species for the study of hybridization. Natural co-occurrence of four closely related oak species is a very rare case in the temperate forests of Europe. We used both morphological characters and genetic markers to characterize hybridization in a natural community situated in west-central Romania and which consists of Quercus robur, Q. petraea, Q. pubescens, and Q. frainetto, respectively. Results On the basis of pubescence and leaf morphological characters ~94% of the sampled individuals were assigned to pure species. Only 16 (~6%) individual trees exhibited intermediate morphologies or a combination of characters of different species. Four chloroplast DNA haplotypes were identified in the study area. The distribution of haplotypes within the white oak complex showed substantial differences among species. However, the most common haplotypes were present in all four species. Furthermore, based on a set of 7 isozyme and 6 microsatellite markers and using a Bayesian admixture analysis without any a priori information on morphology we found that four genetic clusters best fit the data. There was a very good correspondence of each species with one of the inferred genetic clusters. The estimated introgression level varied markedly between pairs of species ranging from 1.7% between Q. robur and Q. frainetto to 16.2% between Q. pubescens and Q. frainetto. Only nine individuals (3.4%) appeared to be first-generation hybrids. Conclusion Our data indicate that natural hybridization has occurred at relatively low rates. The different levels of gene flow among species might be explained by differences in flowering time and spatial position within the stand. In addition, a partial congruence between phenotypically and genetically intermediate individuals was found, suggesting that intermediate appearance does not necessarily mean hybridization. However, it appears that natural hybridization did not seriously affect the species identity in this area of sympatry.

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Evidence for hybridization and introgression within a species-rich oak (Quercus spp.) community

BMC Evolutionary Biology Evidence for hybridization and introgression within a species-rich oak (Quercus spp.) community Alexandru L Curtu 0 1 Oliver Gailing 1 Reiner Finkeldey 1 0 Department of Forest Sciences, Transilvania University Brasov , Sirul Beethoven 1, Brasov, 500123 , Romania 1 Department of Forest Genetics and Forest Tree Breeding, Busgen-Institute Georg - August University Gottingen , Busgenweg 2, Gottingen, , 37077 , Germany Background: Analysis of interspecific gene flow is crucial for the understanding of speciation processes and maintenance of species integrity. Oaks (genus Quercus, Fagaceae) are among the model species for the study of hybridization. Natural co-occurrence of four closely related oak species is a very rare case in the temperate forests of Europe. We used both morphological characters and genetic markers to characterize hybridization in a natural community situated in west-central Romania and which consists of Quercus robur, Q. petraea, Q. pubescens, and Q. frainetto, respectively. Results: On the basis of pubescence and leaf morphological characters ~94% of the sampled individuals were assigned to pure species. Only 16 (~6%) individual trees exhibited intermediate morphologies or a combination of characters of different species. Four chloroplast DNA haplotypes were identified in the study area. The distribution of haplotypes within the white oak complex showed substantial differences among species. However, the most common haplotypes were present in all four species. Furthermore, based on a set of 7 isozyme and 6 microsatellite markers and using a Bayesian admixture analysis without any a priori information on morphology we found that four genetic clusters best fit the data. There was a very good correspondence of each species with one of the inferred genetic clusters. The estimated introgression level varied markedly between pairs of species ranging from 1.7% between Q. robur and Q. frainetto to 16.2% between Q. pubescens and Q. frainetto. Only nine individuals (3.4%) appeared to be first-generation hybrids. Conclusion: Our data indicate that natural hybridization has occurred at relatively low rates. The different levels of gene flow among species might be explained by differences in flowering time and spatial position within the stand. In addition, a partial congruence between phenotypically and genetically intermediate individuals was found, suggesting that intermediate appearance does not necessarily mean hybridization. However, it appears that natural hybridization did not seriously affect the species identity in this area of sympatry. - Background Natural hybridization and introgression can play an important role in evolution, e.g. by formation of new species or increasing genetic variation within species [1-3]. Hybridization is a quite common phenomenon in many organismal groups, particularly in plants [4]. However, the occurrence of natural hybridization is not universal, but concentrated in a small fraction of plant families and genera [4]. A well-known example is the genus Quercus (the oaks), in which many species are known to hybridize [5]. Because of their propensity to hybridize the biological species concept based largely on effective genetic isolation can not be applied to Quercus [6]. To overcome the problems with the reproductive species concept, another concept that relies on ecology was proposed with reference to oaks [7]. The occurrence of rare natural forms with intermediate morphologies was often interpreted as the result of a hybridization event. However, within oak species, morphology alone does not allow to detect putative hybrids, since the parental species are not sufficiently distinct and possess a wide variability [5]. For instance, no single morphological feature can unambiguously distinguish Q. robur from Q. petraea. However, it is possible to differentiate two distinct groups and thus to identify intermediate forms by using various multivariate analyses [e.g., [8,9]]. Furthermore, defining the limits between the 'typical' and 'intermediate' individuals is often more or less arbitrary [9]. Morphologically intermediate forms which are suspected to be hybrids are regularly observed in natural mixed populations (e.g., [10,11]). Various types of genetic markers have been applied in studies on hybridization of oaks. Chloroplast DNA studies revealed that the most frequent chloroplast DNA variants are shared among related oak species which was interpreted as evidence for hybridization and introgression between taxa (e.g., [12-14]). Hybridization as a mechanism of invasion by one species (Q. petraea) into the range occupied by another species (Q. robur) through pollen swamping was proposed to account for the lack of differentiation between these two species [15,16]. However, a relatively low level of hybridization between Q. robur and Q. petraea was detected by paternity analysis in a mixed stand [17]. Studies of hybrid zones using morphology (...truncated)


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Alexandru L Curtu, Oliver Gailing, Reiner Finkeldey. Evidence for hybridization and introgression within a species-rich oak (Quercus spp.) community, BMC Evolutionary Biology, 2007, pp. 218, 7, DOI: 10.1186/1471-2148-7-218