The XY Sex Chromosome in Date Palm (Phoenix dactylifera): From Discovery and Mapping to Sequencing

The date palm is a dioecious fruit tree of importance to many arid land cultures. Its historical importance is revealed by deep archaeological evidence of multiple uses including agriculture and religion for thousands of years. The date fruit is highly nutritious and energy rich making it a critical food source in early human civilization. The date palm female tree produces the nutritious date fruit and they are frequently propagated by offshoot to maintain consistent fruit quality. Among the critical questions in the past was how date palm sex was determined and multiple theories were proposed. We recently showed that date palm employs an XY sex chromosome system by identifying sequence scaffolds segregating with gender. We then mapped the scaffolds to the end of linkage group 12 of the date palm genetic map. Using these resources we have begun identifying and sequencing bacterial artificial clones containing sequences from the sex-linked region. Here we show the results of sequencing clones harboring fragments from both the X and Y alleles in date palm. These results give a much clearer picture of the genetic changes occurring between the two alleles including single nucleotide and large-scale polymorphisms. Some changes are ancient and occur in multiple other Phoenix species while others are correlated with the geography of date palm cultivation. Sequencing the date palm X and Y alleles will give new insight into the development of sex in the Phoenix species and a better understanding of the domestication history of date palm.