Genetic structure and core collection of the World

Authors: Hicham Haouane Ahmed El Bakkali Abdelmajid Moukhli Christine Tollon Sylvain Santoni Ahmed Oukabli Cherkaoui El Modafar Bouchaib Khadari

Publish Date: 2011/09/30

Volume: 139, Issue: 9, Pages: 1083-1094

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Abstract

The conservation of cultivated plants in exsitu collections is essential for the optimal management and use of their genetic resources For the olive tree two world germplasm banks OWGB are presently established in Córdoba Spain and Marrakech Morocco This latter was recently founded and includes 561 accessions from 14 Mediterranean countries Using 12 nuclear microsatellites SSRs and three chloroplast DNA markers this collection was characterised to examine the structure of the genetic diversity and propose a set of olive accessions encompassing the whole Mediterranean allelic diversity range We identified 505 SSR profiles based on a total of 210 alleles Based on these markers the genetic diversity was similar to that of cultivars and wild olives which were previously characterised in another study indicating that OWGB Marrakech is representative of Mediterranean olive germplasm Using a modelbased Bayesian clustering method and principal components analysis this OWGB was structured into three main gene pools corresponding to eastern central and western parts of the Mediterranean Basin We proposed 10 cores of 67 accessions capturing all detected alleles and 10 cores of 58 accessions capturing the 186 alleles observed more than once In each of the 10 cores a set of 40 accessions was identical whereas the remaining accessions were different indicating the need to include complementary criteria such as phenotypic adaptive and agronomic traits Our study generated a molecular database for the entire OWGB Marrakech that may be used to optimise a strategy for the management of olive genetic resources and their use for subsequent genetic and genomic olive breedingThe olive tree Olea europaea L is one of the most important fruit crops of the Mediterranean area Zohary and SpiegelRoy 1975 Palynological and anthracological fossil charcoal studies have shown that wild olive populations were present in eastern and western Mediterranean zones before the Neolithic Terral et al 2004 Carrión et al 2010 Early domesticated forms were probably disseminated during successive human migrations especially from east to west throughout the Mediterranean Basin but olive selection from local western populations has also been revealed by genetic studies Besnard et al 2002 Breton et al 2006 Khadari et al 2008Today the area devoted to olive growing worldwide is estimated at 88 Mha IOC 2007 It is one of the most economically important trees in Mediterranean areas largely due to its multiple uses eg oil canned fruit wood ornamental uses medicinal applications Over 2000 cultivars have been described exhibiting significant levels of variation in oil content fruit size and adaptation to local environmental conditions Bartolini et al 1998 but this number is probably underestimated since there is a lack of information on minor local varieties Cantini et al 1999Olive growing is undergoing a sharp transition from traditional to modern orchards with a reduced number of main varieties featuring interesting agronomic traits such as yield oil quality and adaptive traits related to biotic and abiotic stresses For instance the “Picual” and “Arbequina” varieties have been massively planted over the last two decades in Andalusia and Catalonia respectively Belaj et al 2010 A similar trend was noted in Portugal with the main cultivar “Galega” grown in about 80 of the olive groves Gemas et al 2004 and in Morocco where “Picholine Marocaine” is the dominant cultivar throughout the country Khadari et al 2008 Hence despite the high initial varietal diversity a recent trend towards establishing modern orchards based on the most productive cultivars leads to high erosion of this germplasm Several olive germplasm collections have been created at national and regional levels to manage exsitu olive genetic resources for conservation purposes and eventual use in subsequent breeding programs Bartolini 2008 The first major attempt to conserve and characterise the most important cultivars from all olive growing countries led to the establishment of the World Olive Germplasm Bank in Córdoba Spain OWGB Córdoba This bank was initiated by FAOINIA in 1970 with the contribution of the International Olive Oil Council IOOC Caballero et al 2006 It includes Spanish cultivars that were collected by Barranco and Rallo 2000 and varieties originating from other Mediterranean countries OWGB Córdoba has served for many studies using morphological descriptors Caballero and del Río 2002 and molecular markers such as random amplified polymorphism DNA RAPDs and single sequence repeat SSRs Belaj et al 2003 2004Over the course of the RESGEN project numerous surveys and studies on conservation and characterisation have been conducted by each of the following 15 participating countries Algeria Croatia Cyprus Egypt France Greece Israel Italy Morocco Portugal SerbiaMontenegro Slovenia Spain Syria Tunisia Caballero et al 2006 These partners have completed their sampling by collecting local olive cultivars based on a morphological characterisation As genetic redundancies homonymy and synonymy cases are common problems in the management of exsitu collections Engels and Visser 2003 morphological description has been complemented by the use of molecular markers Recently the development of SSRs in olive eg Sefc et al 2000 de La Rosa et al 2002 has significantly enhanced the possibility of individual olive cultivar identification After testing 37 SSR loci for their reproducibility and discriminating power in four independent laboratories Baldoni et al 2009 proposed a consensus list of SSRs for genotyping of cultivated oliveMore recently a second international germplasm bank was set up at the experimental orchard of Tessaout National Institute of Agronomic Research Marrakech Morocco OWGB Marrakech in 2003 Compared to OWGB Córdoba OWGB Marrakech was set up in a different scientific context with more knowledge available about the plant material To optimise olive germplasm sampling local genetic resources had been characterised in several Mediterranean countries using standardised morphological descriptors The bank was established by introducing previously characterised genetic resources from each Mediterranean area For some partner countries eg Spain Barranco and Rallo 2000 a set of accessions representative of the local diversity was proposed following morphological or molecular characterisation In 2010 OWGB Marrakech included 561 accessions originating from 14 Mediterranean countries and further introduction of additional olive germplasm is ongoingTo optimise the management and use of the large olive exsitu collections it is essential to select a subsample of accessions socalled core collections displaying the overall genetic diversity and phenotypic variability as first proposed by Frankel and Brown 1984 Several strategies have been proposed to facilitate the construction of core collections which can be classified into two groups according to the allocation methods The first one is based on maximising the variability including the MMethod strategy developed by Schoen and Brown 1993 and implemented in the MSTRAT software package Gouesnard et al 2001 and the second group known as the stratified method is based on similarity clustering Escribano et al 2008 Among numerous potential applications core collections can be used as a first step in genetic association studies for detecting quantitative trait loci QTLs related to agromonic traits Barnaud et al 2006 Le Cunff et al 2008 Aranzana et al 2010

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