Scientists Unravel Evolutionary History of Date Palm
Where did dates begin? Research offers two possible theories.
By Matthew Corcoran, Writer & Multimedia Producer
The origin of the date palm has long been a matter of debate by farmers, historians, and scientists. Like one’s affinity for a football team, allegiances can be determined by geographic proximity. Those with ties to North Africa claim the shores of the Mediterranean as the origin, while others argue that the advent of the sweet fruit occurred in the soil between the Tigris and Euphrates.
But a research group at the Center for Genomics and Systems Biology at NYUAD has offered a solution that both sides may be able to support. Dates from the Middle East and North Africa seem to be genetically different. One of the possible explanations is that there could have been two separate domestication events, one in North Africa, and one in the Middle East, with a mixing between the two strains in Egypt and Sudan.
The research, published in Nature Communications, is part of the 100 Dates! genome sequencing project led by NYU Dean of Science Michael Purugganan. The project’s goal is to learn more about the traits and evolution of the date palm through analysis of the plant’s genome. And this research-intensive and data-heavy work may offer practical benefits for farmers and date-lovers alike: "The data on diversity in the genomes helps us to identify genes that may help develop better date palms," Purugganan said. "It also tells us how date palms evolve, and provides clues as to where date palms came from."
The 100 Dates! team analyzed the genome of 62 varieties of date palm from 12 countries. Seventeen samples came from North Africa; 36 inhabit the Middle East. The work was a collaboration between NYUAD and UAE University, as well as other researchers in Dubai, Iraq, US, Syria, Tunisia and Qatar.
Researchers have documented the cultivation history of other crops, like rice. They know that contemporary rice developed into what it is today because humans began cultivating a wild rice ancestor thousands of years ago. The wild ancestor of the date palm is elusive, but identifying one would provide scientists with valuable information. "It is important to know the identity and geographic origin of the wild progenitor of a domesticated species because it will help us understand the evolutionary process underlying domestication and the nature of the genetic changes underlying domestication," said Khaled Hazzouri, senior research scientist at NYUAD and lead author on the paper.
Evidence from archeological digs suggest that the origin of domesticated dates is in the Gulf. Seeds have been found on Dalma Island, Abu Dhabi that are more than 7,000 years old. Cultivated dates seem to appear about 3,000 years later in North Africa, according to excavation of ancient sites.
The team's research, coupled with archeological data, offers two possible explanations. Since Middle Eastern and North African date palms are relatively dissimilar genetically, their paper suggest that contemporary date palms descend from two distinct domestication events — an early event in the Middle East, and a later one in North Africa. A second hypothesis suggests that date palms were first cultivated in the Middle East and later spread to North Africa, but somewhere along the way North African dates were crossed with a wild predecessor.
This is particularly important for traits like water consumption, since some varieties of date palms can live on water with high salinity. Farmers who grow crops in areas that have little water could plant date palm varieties that could survive in that kind of climate.
The researchers analyzed the genomes of the different date palm varieties, creating long strings of data that represent the sequences of the plant’s DNA. They then look at sections of the DNA called Single Nucleotide Polymorphisms, or SNPs. (SNP is a case where two different samples of DNA have a difference in a single nucleotide in the same sequence. For example, a sequence in one variety could be AATGC, while in another variety it could be AATGG.) Comparing SNPs allows scientists to make estimates about the heredity closeness of different varieties, and helps them group varieties that have similar genetic make up.
Purugganan, Hazzouri, and the research team also discovered a genetic mutation that causes the trees to produce either yellow or red fruit. Interestingly, the date palm shares this genetic mutation with its very distant cousin, the oil palm. These two plants are separated by approximately 60 million years of evolution, so it’s surprising that genes in both species would code for the same trait. "This similarity tells us that evolution uses the same genes in different species to get the same result," Purugganan said.
It’s possible that information like this could be used by plant breeders to engineer date varieties that have particular traits. "This is particularly important for traits like water consumption, since some varieties of date palms can live on water with high salinity," Hazzouri said. "Farmers who grow crops in areas that have little water could plant date palm varieties that could survive in that kind of climate."
Purugganan said that the 100 Dates! project is still in its first phase, and there is a lot of research yet to be done. "The next phase includes getting many more samples and mapping important genes," he said. "Our collaborations with many researchers in the region, including at UAE University in Al Ain and the University of Baghdad, will help use launch the next phase of the project."