Evolution of almond genetic diversity and farmer practices in Lebanon: impacts of the diffusion of a graft-propagated cultivar in a traditional system based on seed-propagation

Under cultivation, many outcrossing fruit treespecies have switched from sexual reproductionto vegetative propagation. Traditional production systems have persisted, where cultivar propagationis based on a mixed reproductive system. For millenia, almond, Prunus dulcis, has been propagatedby seeds. Almond grafting remained of little importance until recently. In Lebanon, both sexual and clonal reproductions are used for almond propagation. We used 15 microsatellite markers to investigate the effect of introducing graft- propagated cultivarsand associated practices, on the structure of the genetic diversity among and within the two main Lebanese cultivars. As expected, the sexually propagated cultivarKhachabi exhibited more genotypic and genetic diversity than the vegetatively propagated cultivarHalwani. It also exhibited lower differentiation among populations. The distribution of clones showed that propagationmodes were not exclusive: farmers have introduced clonal propagationin the seed- propagated cultivarwhile they have maintained a diversity of genotypes within populations that were mostly graft- propagated. These practices are also important to avoid mate limitations that hamper fruit production in a self-incompatible species. ‘Khachabi’ is structured into two gene poolsseparated by the Lebanese mountains. As to ‘Halwani’, two different gene poolswere introduced. The most ancient one shares the same geographic range as ‘Khachabi’; longtime coexistence and sexual reproductionhave resulted in admixture with ‘Khachabi’. In contrast, the more recent introduction of the second gene poolin the Bekaa region followed an evolution towards more extensive clonal propagationof ‘Halwani’ limiting hybridizations. Furthermore, some pairs of geographically distant ‘Halwani’ orchards, exhibited low genetic distances, suggesting that a network of exchanges between farmers was effective on a large scale and/or that farmers brought clonal plant material from a common source. Almond diversification in Lebanon is clearly related to the evolution of propagationpractices adapted to self-incompatible cultivars. The comparison between both cultivarsdemonstrated the genetic effects of the introduction of a new cultivarand the associated grafting propagationpractices. Our study provided information to develop a strategy for in situ conservationof cultivarsand to limit gene flow from introduced material to ancient orchards.