Genetic Characterization of the Male/Female Determination Region in Date Palm Phoenix dactylifera

Date palm Phoenix dactylifera is a widely distributed and well-adapted crop in the Middle East, and is considered an important product in the history and diet of the Arab countries. Cultivation of P. dactilyfera is also considered as an option for anti-desertification programs. Date fruits are one of the primary exportable agricultural products in the gulf contries, and current efforts are under way to improve its production. Traditionally, clonal propagation is used to maintain selected varieties with desirable agricultural traits, which has led to the establishment of uniform date palm plantations. As a result, the genetic diversity between date palm orchards is usually highly reduced, increasing their potential susceptibility to pests and diseases. Use of date palm breeding programs as an option to improve date production through is highly challenging, especially because of the long regeneration times and the absence of knowledge that exists about the genetic markers responsible for desirable agricultural traits. In commercial plantations, one hundred fruit bearing female trees can be pollinated with a single non-bearing male tree. However, half of the date palm offspring planted from seeds are expected to be male individuals. One of the biggest challenges that date production encounters is that female trees can only be distinguished after flowering, which occurs five to eight years after planting.

Preliminary cytological studies suggest the presence of heteromorphic sex chromosomes in date palm, but enzymatic approaches utilized to determine plant gender have been unsuccessful. Recent genome sequencing of the commercial varieties Khalas, Deglet Noor and Medjool, indicated the presence of a gender-linked genomic region. This single nucleotide polymorphism (SNP)-rich region was shown to segregate with gender, and is predicted to extend to 24 scaffolds. Limited information about this region is currently available, and our goal is to use of a deep-sequencing approach to significantly improve the date palm genome assembly, with an emphasis on this male/female segregating region. A bacterial artificial chromosome (BAC) library, with a 12x genome coverage and an average insert size of 125 Kb was constructed. We propose that BAC-end sequencing of multiple clones related to this polymorphic sex-linked region will generate a complete physical and sequence map of the sex-determination region. Our main goal is to detect the critical mutation that gave rise to male and female separation. This is the first time such level of detail in a dioecious plant will be achieved.  Furthermore, standardization of this sequencing procedure, will also allow us to characterize other genomic regions associated with desirable commercial properties of these fruits. This in turn, will provide a valuable tool for the development of marker-assisted selection programs, for the improvement of date palm production.