Genetic Erosion of Phoenix dactylifera L.: Perceptible, Probable, or Possible

Abstract

Genetic diversity of date palm (Phoenix dactylefera L.) encompasses genetic differences among and within species, subspecies, populations, cultivars, and individual clones in traditional oases and plantations. Components of this diversity can be estimated, throughout the tree’s ontogeny, at the phenotypic, phys- iological, biochemical, and molecular levels using quantitative, qualitative, and ecological parameters. Due to recent extensive expansion of its cultivation in plan- tations, P. dactylefera, as a species, may not be immediately threatened by genetic erosion in spite of documented isolated cases where some oasis agroecosystems passed ecological thresholds, leading to irreversible changes in the ecosystem and the loss of valuable genetic resources and associated ecosystem services. However, threats to genetic diversity increased during the last ~30 years partly due to the introduction of improved mass propagation methods of a limited number of elite date palm cultivars to the exclusion of many others; this widespread practice may have led to genetic vulnerability of the species to biotic (e.g., red palm weevil [(Rhynchophorus ferrugineus (Olivier)] and Bayoud, caused by Fusarium oxyspo- rum f. sp. albedinis) and abiotic (e.g., drought, heat, sand encroachment, deser- tification, aquifer depletion, and salinity) stresses, especially in view of climate change. Selecting a small number of resistant cultivars to biotic stresses is a fur- ther threat to the diversity of the species if the resistance to a particular disease or insect proved to be short lived because of changing climatic conditions or through a change in the virulence of the pest. Traditional propagation using offshoots of elite cultivars having desirable fruit quality traits may lead to the confinement of these cultivars to certain oases; its impact on genetic diversity will, most certainly,be influenced by plant quarantine regulations and the ease with which date palm propagules can be moved between oases, plantations, countries, and regions of the world; whereas the current massive propagation of a few elite date palm cultivars using tissue culture and other mass propagation methods, at the expense of less popular but genetically valuable adapted cultivars, may exacerbate the problem of genetic erosion. In order to combat potential genetic erosion, in-depth understand- ing of genetic diversity and genetic structure of the species’ gene pool complex, which have been shaped and greatly altered by human and natural selection, clonal propagation, and spatiotemporal exchange of germplasm, is indispensable. This chapter will present a comprehensive assessment of the factors with tangible or potential impact on date palm genetic diversity; appropriate research methodolo- gies to quantify and partition genetic diversity; and strategies that can be imple- mented at the oasis, plantation, regional, and global levels, to enhance sustainable use and conservation of maximum genetic diversity and combat genetic erosion in the date palm.