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.

Background: Under cultivation, many outcrossing fruit tree species have switched from sexual reproduction to vegetative propagation. Traditional production systems have persisted, where cultivar propagation is based on a mixed reproductive system. For millenia, almond, Prunus dulcis, has been propagated by 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 cultivars and associated practices, on the structure of the genetic diversity among and within the two main Lebanese cultivars.

Results: As expected, the sexually propagated cultivar Khachabi exhibited more genotypic and genetic diversity than the vegetatively propagated cultivar Halwani. It also exhibited lower differentiation among populations. The distribution of clones showed that propagation modes were not exclusive: farmers have introduced clonal propagation in the seed-propagated cultivar while 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 pools separated by the Lebanese mountains. As to 'Halwani', two different gene pools were introduced. The most ancient one shares the same geographic range as 'Khachabi'; longtime coexistence and sexual reproduction have resulted in admixture with 'Khachabi'. In contrast, the more recent introduction of the second gene pool in the Bekaa region followed an evolution towards more extensive clonal propagation of '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.

Conclusions: Almond diversification in Lebanon is clearly related to the evolution of propagation practices adapted to self-incompatible cultivars. The comparison between both cultivars demonstrated the genetic effects of the introduction of a new cultivar and the associated grafting propagation practices. Our study provided information to develop a strategy for in situ conservation of cultivars and to limit gene flow from introduced material to ancient orchards.