Endemic insular and coastal Tunisian date palm genetic diversity

Spectroscopic and Molecular Methods to Differentiate Gender in Immature Date Palm (Phoenix dactylifera L.)

Phoenix dactylifera (date palm) is a well-known nutritious and economically important fruit tree found in arid regions of the Middle East and North Africa. Being diploid, it has extremely high divergence in gender, where sex differentiation in immature date palms (Phoenix dactylifera L.) has remained an enigma in recent years. Herein, new robust infrared (near-infrared reflectance spectroscopy (NIRS) and Fourier transform infrared attenuated total reflectance (FTIR/ATR)) and nuclear magnetic resonance (NMR) spectroscopy methods coupled with extensive chemometric analysis were used to identify the sex differentiation in immature date palm leaves. NIRS/FTIR reflectance and 1H-NMR profiling suggested that the signals of monosaccharides (glucose and fructose) and/or disaccharides (maltose and sucrose) play key roles in sex differentiation. The three kinds of spectroscopic data were clearly differentiated among known and unknown male and female leaves via principal component and partial least square discriminant analyses. Furthermore, sex-specific genes and molecular markers obtained from the lower halves of LG12 chromosomes showed enhanced transcript accumulation of mPdIRDP52, mPdIRDP50, and PDK101 in females compared with in males. The phylogeny showed that the mPdIRD033, mPdIRD031, and mPdCIR032 markers formed distinctive clades with more than 70% similarity in gender differentiation. The three robust analyses provide an alternative tool to differentiate sex in date palm trees, which offers a solution to the long-standing challenge of dioecism and could enhance in situ tree propagation programs.

Cross-species hybridization and the origin of North African date palms

Date palm (Phoenix dactylifera L.) is a major fruit crop of arid regions that were domesticated ∼7,000 y ago in the Near or Middle East. This species is cultivated widely in the Middle East and North Africa, and previous population genetic studies have shown genetic differentiation between these regions. We investigated the evolutionary history of P. dactylifera and its wild relatives by resequencing the genomes of date palm varieties and five of its closest relatives. Our results indicate that the North African population has mixed ancestry with components from Middle Eastern P. dactylifera and Phoenix theophrasti, a wild relative endemic to the Eastern Mediterranean. Introgressive hybridization is supported by tests of admixture, reduced subdivision between North African date palm and P. theophrasti, sharing of haplotypes in introgressed regions, and a population model that incorporates gene flow between these populations. Analysis of ancestry proportions indicates that as much as 18% of the genome of North African varieties can be traced to P. theophrasti and a large percentage of loci in this population are segregating for single-nucleotide polymorphisms (SNPs) that are fixed in P. theophrasti and absent from date palm in the Middle East. We present a survey of Phoenix remains in the archaeobotanical record which supports a late arrival of date palm to North Africa. Our results suggest that hybridization with P. theophrasti was of central importance in the diversification history of the cultivated date palm.

Long and attenuated: comparative trends in the domestication of tree fruits

This paper asks whether we can identify a recurrent domestication syndrome for tree crops (fruits, nuts) and track archaeologically the evolution of domestication of fruits from woody perennials. While archaeobotany has made major contributions to documenting the domestication process in cereals and other annual grains, long-lived perennials have received less comparative attention. Drawing on examples from across Eurasia, comparisons suggest a tendency for the larger domesticated fruits to contain seeds that are proportionally longer, thinner and with more pointed (acute to attenuated) apices. Therefore, although changes in flavour, such as increased sweetness, are not recoverable, seed metrics and shape provide an archaeological basis for tracking domestication episodes in fruits from woody perennials. Where available, metrical data suggest length increases, as well as size diversification over time, with examples drawn from the Jomon of Japan (Castanea crenata), Neolithic China (Prunus persica) and the later Neolithic of the Near East (Olea europaea, Phoenix dactylifera) to estimate rates of change. More limited data allow us to also compare Mesoamerica avocado (Persea americana) and western Pacific Canarium sp. nuts and Spondias sp. fruits. Data from modern Indian jujube (Ziziphus mauritiana) are also considered in relation to seed length:width trends in relation to fruit contents (flesh proportion, sugar content). Despite the long generation time in tree fruits, rates of change in their seeds are generally comparable to rates of phenotypic evolution in annual grain crops, suggesting that gradual evolution via unconscious selection played a key role in initial processes of tree domestication, and that this had begun in the later Neolithic once annual crops had been domesticated, in both west and east Asia.