Lamia Krichen

Loss of genetic diversity as a signature of apricot domestication and diffusion into the Mediterranean Basin

BackgroundDomestication generally implies a loss of diversity in crop species relative to their wild ancestors because of genetic drift through bottleneck effects. Compared to native Mediterranean fruit species like olive and grape, the loss of genetic diversity is expected to be more substantial for fruit species introduced into Mediterranean areas such as apricot (Prunus armeniaca L.), which was probably primarily domesticated in China. By comparing genetic diversity among regional apricot gene pools in several Mediterranean areas, we investigated the loss of genetic diversity associated with apricot selection and diffusion into the Mediterranean Basin.ResultsAccording to the geographic origin of apricots and using Bayesian clustering of genotypes, Mediterranean apricot (207 genotypes) was structured into three main gene pools: ‘Irano-Caucasian’, ‘North Mediterranean Basin’ and ‘South Mediterranean Basin’. Among the 25 microsatellite markers used, only one displayed deviations from the frequencies expected under neutrality. Similar genetic diversity parameters were obtained within each of the three main clusters using both all SSR loci and only 24 SSR loci based on the assumption of neutrality. A significant loss of genetic diversity, as assessed by the allelic richness and private allelic richness, was revealed from the ‘Irano-Caucasian’ gene pool, considered as a secondary centre of diversification, to the northern and southwestern Mediterranean Basin. A substantial proportion of shared alleles was specifically detected when comparing gene pools from the ‘North Mediterranean Basin’ and ‘South Mediterranean Basin’ to the secondary centre of diversification.ConclusionsA marked domestication bottleneck was detected with microsatellite markers in the Mediterranean apricot material, depicting a global image of two diffusion routes from the ‘Irano-Caucasian’ gene pool: North Mediterranean and Southwest Mediterranean. This study generated genetic insight that will be useful for management of Mediterranean apricot germplasm as well as genetic selection programs related to adaptive traits.

Impact of Mapped SSR Markers on the Genetic Diversity of Apricot (Prunus armeniaca L.) in Tunisia

The impact of mapped microsatellites on the study of genetic diversity of Tunisian apricot accessions was assessed. The genetic variability of 47 traditional apricot cultivars originating from several areas in Tunisia was investigated with 32 polymorphic microsatellite loci selected for their location throughout the eight linkage groups of Prunus genome. The higher polymorphism and greater transportability of these markers among Prunus species were proved by the expected heterozygosity (He = 0.56) and Shannon’s index of diversity (I = 1.05), indicating that Tunisian apricot germplasm maintained a substantial level of genetic diversity. According to their geographical origin, the genetic differentiation among groups (north, center, and south; Fst = 0.04) was lower, while the gene flow among groups was consequent (Nm = 4.79), attesting a narrow genetic background of apricot in the country. Both unweighted pair-group method with arithmetic mean dendrogram, based on Nei’s genetic distances and factorial correspondence analysis, separated northern cultivars from central and southern cultivars, revealing the same molecular basis of apricot material in the Center and the South of Tunisia. These results revealed the efficiency of mapped markers for genetic variability measurements compared to randomly ones, however, no advantage was observed considering the genetic relationships among studied accessions.

Grafting versus seed propagated apricot populations: two main gene pools in Tunisia evidenced by SSR markers and model-based Bayesian clustering

Apricot was introduced into the Mediterranean Basin from China and Asian mountains through the Middle-East and the Central Europe. Traditionally present in Tunisia, we were interested in accessing the origin of apricot species in the country, and in particular in the number and the location of its introductions. A set of 82 representative apricot accessions including 49 grafted cultivars and 33 seed propagated ‘Bargougs’ were genotyped using 24 microsatellite loci revealing a total of 135 alleles. The model-based Bayesian clustering analysis using both Structure and InStruct programs as well as the multivariate method revealed five distinct genetic clusters. The genetic differentiation among clusters showed that cluster 1, with only four cultivars, was the most differentiated from the four remaining genetic clusters, which constituted the largest part of the studied germplasm. According to their geographic origin, the five identified groups (north, centre, south, Gafsa oasis and other oases groups) enclosed a similar variation within group, with a low level of differentiation. Overall results highlighted the distinction of two apricot gene pools in Tunisia related to the different mode of propagation of the cultivars: grafted and seed propagated apricot, which enclosed a narrow genetic basis. Our findings support the assumption that grafting and seed propagated apricots shared the same origin.

Relative efficiency of morphological characters and molecular markers in the establishment of an apricot core collection

In order to optimize the management of genetic resources, in most cases a representative sample of the germplasm collections needs to be developed. The establishment of a core collection is thus of major importance either to minimize the cost associated with the management of the associated germplasm or to apply analysis onto representative bases. In order to select a representative core collection among the Tunisian apricot germplasm of 110 accessions large, the Maximization strategy algorithm was used. This algorithm was shown to be the most convenient when using both morphological traits and molecular markers. Three core collections based on morphological characters, molecular markers or the combined data were compared. Our data indicate that both the molecular and the morphological markers have to be considered to obtain a core collection that represents the global diversity of the 110 accessions. Using this method, a subset of 34 selected accessions was found to represent accurately the 110 accessions present in the whole collection (75 to 100% for the morphological characters and 97% of the molecular markers). These results show that the combination of molecular and morphological markers is an efficient way to characterize the apricot core collection and provides an exhaustive coverage for the analyzed diversity on morphological and genetic bases.

Bottleneck and gene flow effects impact the genetic structure of seed-propagated apricot populations in Moroccan oasis agroecosystems

In order to highlight the genetic status and origin of Moroccan apricot populations, trees were collected from ten oasis agroecosystems and analysed with AFLP markers. A total of 87 accessions and 12 cultivars grown in Moroccan orchards, including ‘Canino’ and ‘Del Patriarca’ cultivars, were surveyed and compared with in situ Tunisian and ex situ Montfavet (France) collections. Our results highlighted a narrow genetic diversity in the Maghreb region (Tunisia and Morocco) associated with a strong differentiation from the other groups, which supports a bottleneck effect. A similar model was illustrated at a finer geographical scale, i.e. the Draa Valley in Morocco. Genetic structure appeared as two major clusters subdivided into six sub-clusters in which Moroccan germplasm constituted specific groups in comparison with other Mediterranean apricots. Moroccan germplasm was classified into three sub-clusters, two of which were formed by genotypes related to ‘Del Patriarca’ and ‘Canino’, respectively. The present study highlights the wide Moroccan apricots diversity in traditional agroecosystems, and also suggests a substantial gene flow occurring from recently introduced cultivars (‘Canino’ and ‘Del Patriarca’) to local apricot populations, thus leading to local germplasm diversification through seedling propagation. If we consider its geographical position, the historical diffusion of the species and farming practices, Morocco could be viewed as an additional centre of secondary diversification for apricot. Understanding the origin and specificity of local apricot populations is crucial for managing local collections in regard to adaptive traits for arid and Saharan conditions as well as for introducing local genetic resources into current breeding programmes.

Population structure and core collection construction of apricot ( Prunus armeniaca L.) in North Africa based on microsatellite markers

North Africa enclosed original apricot genetic resources with the cohabitation of grafting and seed-propagated accessions. In this study, we assessed the genetic diversity and population structure of 183 apricot accessions using 24 microsatellite markers distributed evenly in the Prunus genome. A total of 192 alleles and a high level of gene diversity (0.593) were detected among the whole panel. Genetic structure analysis revealed the presence of four genetic clusters. We also found that both geographical origin and mode of propagation are important factors structuring genetic diversity in apricot species. Results confirmed the presence of gene exchange between the northern and southern countries of the Mediterranean Basin. Subsequently, a core collection of 98 accessions based on M (maximization) strategy showing 99.47% of allele retention ratio was constructed. No significant differences for Shannons information index and Neis diversity index were observed between the core and entire collections. Our results provide an effective aid for future germplasm preservation and conservation strategies as well as genetic association studies development in relation to phenotypic data.