Scientists have decoded the genome of the shea tree (Vitellaria paradoxa), a unique agroforestry tree species central to sustaining local livelihoods and the farming environments of rural communities across Africa’s Sudano-Sahelian agroclimactic belt.
Shea trees, the source for the popular ingredient shea butter, grow in a shea parkland during the lush rainy season near the town of Bole in northern Ghana. Image credit: Iago Hale / University of New Hampshire.
The shea tree is a small to medium-sized deciduous tree usually growing between 7 and 15 m tall with occasional specimens to 25 m.
Cited as the second most important oil crop in Africa after oil palm, it is likely among the most economically and culturally important indigenous tree species in the Sudano-Sahelian region of Africa where oil palm does not grow.
Rural families in hundreds of thousands of villages across the so-called ‘shea belt,’ a 500-750 km wide semi-arid area stretching 6,000 km and spanning 21 countries from Senegal to South Sudan, use shea in their daily lives as an edible butter/oil, soap, cosmetic, and medicine.
Shea is also a multimillion dollar export commodity as an ingredient in luxury cosmetic and personal care and pharmaceutical products.
The largest export demand (90%) for shea is, however, linked to the extraction of edible stearin used in the formulation of cocoa butter equivalents for chocolate confectionary.
Based on population and daily shea butter consumption data, the number of women taking part in shea nut collection across the shea belt is estimated at more than 18 million.
Indeed, shea has been called ‘women’s gold’ because it is one of only a few resources that female members of rural households in the region have control over, from harvesting to commercialization.
The sale of shea products allows women to secure additional food for themselves and their children once cereal harvests are exhausted and to generate cash for household expenses including clothing, medicine, and school fees.
The seasonality of shea availability is also critical to farming households’ nutrition. Shea fruits are the only widely available, energy-rich food source at the time when land is tilled and crops are planted at the end of the dry season.
The fruits supply significant amounts of protein, sugar, calcium, potassium, and essential fatty acids during this annual ‘hungry season’ when cereal stocks in granaries are lowest and labor requirements for field preparations with the coming of the rains are highest.
Despite its increasing demand, the slow-growing shea tree is being threatened by other cash crops and its preservation most likely lies in its genetic improvement.
“A shea tree can take 25 years or more to come into production so it can be very costly for a farmer to wait that long and wonder if a tree is worth keeping,” said Dr. Iago Hale, a researcher in the Department of Agriculture, Nutrition, and Food Systems at the University of New Hampshire.
“With such a long growing period, traditional breeding strategies simply aren’t viable, which is one reason there aren’t any high-performing shea varieties.”
“The shea genome will enable researchers to gauge the potential of a seedling as soon as it germinates and through genome-enabled tree selection, we can start moving the needle on this difficult species.”
In the new study, Dr. Hale and colleagues assembled a chromosome-scale reference genome for the shea tree.
Genome annotation by the team led to the identification of 38,505 coding genes.
Further comparative sequencing with a diverse collection of trees revealed nearly 3.5 million natural genetic variations, known as single nucleotide polymorphisms (SNPs), that can be used to distinguish trees and begin to understand the genetics underlying important attributes.
“The shea parklands are in decline due to the threat of alternative, typically male-controlled cash crops like cashew and mango and the immediate pay-off of cutting questionably productive trees for firewood,” Dr. Hale said.
“Women need to travel longer distances to collect shea nuts, which means more labor and less profits.”
“To counteract these trends, we need to enhance the value of shea in the landscape. Genome-assisted breeding presents a path for achieving that.”
Using the newly-mapped genome, the team also identified forty-five fatty acid biosynthesis genes in shea that likely govern stearin content.
Being able to identify these enzymes could help researchers develop better selection strategies for seedlings.
“With these resources, the hope is to support national programs across the shea belt in the strategic, genome-enabled conservation and long-term improvement of the shea tree for Sahelo-Sudanian Africa,” the researchers said.
The results appear in the journal Frontiers in Plant Science.
Iago Hale et al. Genomic Resources to Guide Improvement of the Shea Tree. Front. Plant Sci, published online September 9, 2021; doi: 10.3389/fpls.2021.720670