Does the phenotypic selection affect the genetic structure and diversity? A study case on Walnut in eastern central Italy (the region of Marche)

The Persian walnut (Juglans regia L.) is widely planted in western Europe, either for fruit either for high quality timber production. This tree is generally considered non authoctonous, probably introduced from East some 7000 years ago and spread by several ancient civilisations. The possible artificial origin seems confirmed by the low intra-specific variation and the higher individual variability recorded by several Authors as well as by the lack of natural populations. Indeed, only wider fruit cultivation areas or small groups, lines or isolated walnut trees can be recorded in Italy. The occurrence of walnuts in forest, escaped from cultivation areas, is very rare. Due to the increased interest of planters, walnut plantations have been extended several ten thousands hectares throughout all western Europe. As a consequence of that it was evident the necessity of selected suitable basic populations in order to supply high quality reproductive materials. The conventional method based on the organisation of a wide and exhaustive seed procurement from the native range to establish provenance tests is at the present impossible. Thus it is necessary to study methods of selection which consider basic materials growing within the western European range. This study is aimed to test the efficiency of the multi-trait Selection Index method, in preserving levels of genetic diversity and structures compatible with the standards observed within a reference system of extended Italian populations. As a consequence of the relatively recent introduction, the genetic structure of the species shows individual variation higher than inter-population diversity. Those genetic structure characteristics were revealed also during a survey of walnut resources in the region of Marche, central Italy. The survey was the starting point for selecting and preserving basic materials for high quality woody production, possibly interesting for forest nurseries in the region. The genetic variation of Marche’s population, compared to a reference system of 7 other Italian provenances, was used as a base to establish a possible improvement strategy together with basic guidelines to manage those genetic resources. Indeed, the very important individual component of the genetic variation suggested to select directly superior phenotypes in view of establishing a comparative multisite progeny test network. No substantial differences were detected concerning the genetic structure of the Italian population, neither within the Marche population, neither in the phenotypically selected material. Homozygosity was always high, probably due to genetic erosion, isolation and adaptation to extreme conditions. Given this general situation in the Marche area, a special care should be payed in the management of walnut reproductive materials, in order to maintain sufficient levels of variation in plantations

Black Locust (Robinia pseudoacacia L.) Root Cuttings: Diversity and Identity Revealed by SSR Genotyping: A Case Study

Background and Purpose: Black locust (Robinia pseudoacacia L.) is a valuable species native to North America and today widely planted throughout the world for biomass production. In Hungary, where Robinia has great importance in the forest management, the clones have been selected for plantations on good, medium and poor quality sites. To conserve the identity, superior clones are vegetatively propagated by root cuttings. At times the collection of root cuttings can cause uncertainty for clonal identity because of the overlap of roots from neighboring plants. This can occur especially when the repository is damaged from severe environmental accidents and the planting layout has been lost. The aim of this study has been to verify by molecular markers the diversity or identity of black locust clones by root cuttings harvested in a damaged trial. Materials and Methods: Root cuttings of 91 clones belonging to five cultivars were collected in a trial severely damaged by storms and flooding periods. The obtained plantlets were analyzed with nine microsatellite (SSR) markers and the genetic identity/diversity within and among the plants was tested using the software GenAlEx version 6. Results: Multilocus genotypes (MLG) and the Paetkau’s assignation test (1985) revealed genetic variability among the samples: the analyzed plantlets were grouped in four classes instead of the five expected. In addition, 6 unique genotypes have been detected. Conclusions: This study remarks problems that may arise during the harvest of Robinia’s root cuttings, especially when the planting layout has been confused. Molecular analyses can be successfully used to control the germplasm before its sale as guaranty for nurseries, farmers and stakeholders

Rethinking the history of common walnut (Juglans regia L.) in Europe: Its origins and human interactions

Common walnut (Juglans regia L) is an economically important species cultivated worldwide for its high-quality wood and nuts. It is generally accepted that after the last glaciation J. regia survived and grew in almost completely isolated stands in Asia, and that ancient humans dispersed walnuts across Asia and into new habitats via trade and cultural expansion. The history of walnut in Europe is a matter of debate, however. In this study, we estimated the genetic diversity and structure of 91 Eurasian walnut populations using 14 neutral microsatellites. By integrating fossil pollen, cultural, and historical data with population genetics, and approximate Bayesian analysis, we reconstructed the demographic history of walnut and its routes of dispersal across Europe. The genetic data confirmed the presence of walnut in glacial refugia in the Balkans and western Europe. We conclude that human-mediated admixture between Anatolian and Balkan walnut germplasm started in the Early Bronze Age, and between western Europe and the Balkans in eastern Europe during the Roman Empire. A population size expansion and subsequent decline in northeastern and western Europe was detected in the last five centuries. The actual distribution of walnut in Europe resulted from the combined effects of expansion/contraction from multiple refugia after the Last Glacial Maximum and its human exploitation over the last 5,000 years

Ancient humans influenced the current spatial genetic structure of common walnut populations in Asia

Common walnut (Juglans regia L) is an economically important species cultivated worldwide for its wood and nuts. It is generally accepted that J. regia survived and grew spontaneously in almost completely isolated stands in its Asian native range after the Last Glacial Maximum. Despite its natural geographic isolation, J. regia evolved over many centuries under the influence of human management and exploitation. We evaluated the hypothesis that the current distribution of natural genetic resources of common walnut in Asia is, at least in part, the product of ancient anthropogenic dispersal, human cultural interactions, and afforestation. Genetic analysis combined with ethno-linguistic and historical data indicated that ancient trade routes such as the Persian Royal Road and Silk Road enabled long-distance dispersal of J. regia from Iran and Trans-Caucasus to Central Asia, and from Western to Eastern China. Ancient commerce also disrupted the local spatial genetic structure of autochthonous walnut populations between Tashkent and Samarkand (Central- Eastern Uzbekistan), where the northern and central routes of the Northern Silk Road converged. A significant association between ancient language phyla and the genetic structure of walnut populations is reported even after adjustment for geographic distances that could have affected both walnut gene flow and human commerce over the centuries. Beyond the economic importance of common walnut, our study delineates an alternative approach for understanding how the genetic resources of long-lived perennial tree species may be affected by the interaction of geography and human history

Does the phenotypic selection affect the genetic structure and diversity? A study case on Walnut in eastern central Italy (the region of Marche)

The Persian walnut (Juglans regia L.) is widely planted in western Europe, either for fruit either for high quality timber production. This tree is generally considered non authoctonous, probably introduced from East some 7000 years ago and spread by several ancient civilisations. The possible artificial origin seems confirmed by the low intra-specific variation and the higher individual variability recorded by several Authors as well as by the lack of natural populations. Indeed, only wider fruit cultivation areas or small groups, lines or isolated walnut trees can be recorded in Italy. The occurrence of walnuts in forest, escaped from cultivation areas, is very rare. Due to the increased interest of planters, walnut plantations have been extended several ten thousands hectares throughout all western Europe. As a consequence of that it was evident the necessity of selected suitable basic populations in order to supply high quality reproductive materials. The conventional method based on the organisation of a wide and exhaustive seed procurement from the native range to establish provenance tests is at the present impossible. Thus it is necessary to study methods of selection which consider basic materials growing within the western European range. This study is aimed to test the efficiency of the multi-trait Selection Index method, in preserving levels of genetic diversity and structures compatible with the standards observed within a reference system of extended Italian populations. As a consequence of the relatively recent introduction, the genetic structure of the species shows individual variation higher than inter-population diversity. Those genetic structure characteristics were revealed also during a survey of walnut resources in the region of Marche, central Italy. The survey was the starting point for selecting and preserving basic materials for high quality woody production, possibly interesting for forest nurseries in the region. The genetic variation of Marche’s population, compared to a reference system of 7 other Italian provenances, was used as a base to establish a possible improvement strategy together with basic guidelines to manage those genetic resources. Indeed, the very important individual component of the genetic variation suggested to select directly superior phenotypes in view of establishing a comparative multisite progeny test network. No substantial differences were detected concerning the genetic structure of the Italian population, neither within the Marche population, neither in the phenotypically selected material. Homozygosity was always high, probably due to genetic erosion, isolation and adaptation to extreme conditions. Given this general situation in the Marche area, a special care should be payed in the management of walnut reproductive materials, in order to maintain sufficient levels of variation in plantations.</p

Rethinking the history of common walnut (<i>Juglans regia</i> L.) in Europe: Its origins and human interactions

<div><p>Common walnut (<i>Juglans regia</i> L) is an economically important species cultivated worldwide for its high-quality wood and nuts. It is generally accepted that after the last glaciation <i>J</i>. <i>regia</i> survived and grew in almost completely isolated stands in Asia, and that ancient humans dispersed walnuts across Asia and into new habitats via trade and cultural expansion. The history of walnut in Europe is a matter of debate, however. In this study, we estimated the genetic diversity and structure of 91 Eurasian walnut populations using 14 neutral microsatellites. By integrating fossil pollen, cultural, and historical data with population genetics, and approximate Bayesian analysis, we reconstructed the demographic history of walnut and its routes of dispersal across Europe. The genetic data confirmed the presence of walnut in glacial refugia in the Balkans and western Europe. We conclude that human-mediated admixture between Anatolian and Balkan walnut germplasm started in the Early Bronze Age, and between western Europe and the Balkans in eastern Europe during the Roman Empire. A population size expansion and subsequent decline in northeastern and western Europe was detected in the last five centuries. The actual distribution of walnut in Europe resulted from the combined effects of expansion/contraction from multiple refugia after the Last Glacial Maximum and its human exploitation over the last 5,000 years.</p></div

Delaunay connections associated with linguistic distance (D_LAN) and crossed by a statistically significant genetic barrier.

<p>^a Statistically significant genetic barriers were calculated using the Monmonier’s maximum difference algorithm as implemented in BARRIER software 2.2 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0135980#pone.0135980.ref062" target="_blank">62</a>].</p><p>Delaunay connections associated with linguistic distance (D_LAN) and crossed by a statistically significant genetic barrier.</p

Common walnut population graph for 39 study sites in the Asian range.

<p>Nodes represent geographic sites with diameter proportional to within-site heterozygosity and length of edges connecting nodes equivalent to genetic differentiation among the sites calculated using 14 SSR markers. The color of each node represents the language phylum spoken by human communities living in the geographic sampling sites.</p

Spatial genetic structure of 91 walnut populations in Eurasia.

<p>Population structure inference for 91 walnut populations by Bayesian assignment using STRUCTURE for K = 4. Synthetic map of Inverse Distance Weighted (IDW) interpolations of the estimated mean population membership values (<i>Q</i>_<i>i</i>) (a) and bar plot showing assignment probabilities of individuals to K clusters (b). Abbreviations: CN = China, UZ = Uzbekistan, KG = Kyrgyzstan, TJ = Tajikistan, PK = Pakistan, IR = Iran, GE = Georgia, TR = Turkey, MD = Moldova, RO = Romania, HU = Hungary, SK = Slovakia, GR = Greece, IT = Italy, FR = France, ES = Spain.</p