Evolution of Crop Farming IV: Wheat

by Emma J Devereux

Citation: Devereux, E.J.,(2021), “Sustainable Crops: The Evolution & Genetics of Wheat” , EcoFoodDev, https://www.ecofooddev.com/sustainable-crops-the-evolution-genetics-of-wheat/

Wheat is one of the most important economic cereal crops in the world. It is grown all over the globe as a source of food and fodder. Its primary cultivars (plants or “versions” produced by selective breeding) include Triticum aestivum (our common bread wheat), Triticum spelta (the increasingly popular spelt wheat), and Triticum durum (durum wheat).  

Almost 215 million hectares of land are used annually to grow wheat, and almost US$50 billion is traded each year as part of the global wheat trade. 

Roughly 2.5 billion people across 89 countries eat wheat as a food- beating both maize and rice. Wheat is a valuable source of protein- particularly in developing nations- providing the bulk of calories alongside rice. Wheat provides 50% of the calories in the modern western diet, and such consumption is due to continue and increase in the face of rapid and rampant globalization. This increase in consumption, alongside the threat of resource change and resource depletion due to global climate change, requires us to find sustainable, environmentally-sound, economically viable and less labour-intensive ways to produce wheat, as well as find viable food alternatives to avoid a future wheat near-monoculture.  

Geography of Ancient Wheat 

   

Our modern, domesticated wheats began their lives thousands of years ago as different types of wild wheat. These wild wheats were located in what is now modern southwest Asia- an area known as the Fertile Crescent (read my blog on ancient crop domestication here: https://bit.ly/3ezw7nj). Many of the wild ancestors of wheat still exist in this region today.  

Nikolai Vavilov was a Russian botanist and geneticist who pioneered work on the different domestication centres of modern plants, especially crops plants. He recognized that each of the three ploidy levels (ploidy refers to the number of chromosomes in a cell, and essentially refers to different versions of wheat that we grow today) of wheat had its own geographical centre of diversity, and hence a separate geographical place of origin. He placed the origin of einkorn wheat (T. monococcum) to Asia Minor, durum wheat (T. turgidum ssp. durum), and bread wheat (T. aestivum) to a region extending over an area from Afghanistan and Turkmenistan to Transcaucasia (Vavilov, 1926). He placed the centre of diversity of wild emmer (T. turgidum ssp. dicoccoides) from Israel to western Iran (Vavilov, 1926).    

Molecular studies were later conducted that confirmed Vavilov’s assumptions. These studies placed the origin of:  

• einkorn wheat to south-eastern Turkey/Asia Minor,  
• the origin of durum wheat to the eastern Mediterranean and north-east Africa,  
• the origin of  bread wheat to Transcaucasia and north-western Iran,  
• and the centre of diversity of wild emmer was indeed found to be in the south-western tip of the Fertile Crescent, in a region including modern Israel, Jordan, Lebanon, and south-western Syria (Dvorak et al. 2011; Garrard, 1999; Kilian et al. 2009; Luo et al. 2007). 

Archaeological Evidence for Cultivated/Domesticated Wheat 

There are many examples of botanical evidence for the origins of wheat worldwide, especially in the Middle East & Southwest Asia. Once example of such evidence for early domesticated emmer wheat (“domesticated” meaning changed or modified through cultivation) has been found at Tell Aswad, Damascus, dating from 7800 BC in the form of domesticated grains (in the archaeobotanical record, domesticated grains generally mean enlarged grains). Pre-domestication use of cereal crops is widely accepted to have taken place in this area of southwest Asia, and while debates continue over the nature of early agriculture at Tell Aswad, what we can be certain of is the wealth of grain identified at this site, indicating that Tell Aswad was part of a growing, regional, social, and agricultural development. The earliest date for domesticated einkorn wheat is later, from Jericho in the West Bank, dating from 7300-6500 BC. 

This, combined with many other examples of the presence of domesticated/cultivated wheat in the ancient world supports the idea that, even at this early stage in the development of agriculture and cereal domestication, humans were modifying wheat grasses to get useful characteristics from the crop.  

Characteristics Selected for Breeding 

The types of characteristics selected for the domestication of wheat from its wild ancestors during early cereal domestication include:  

• grain retention and threshability (ability to remove grains from the chaff),  
• yield improvement,  
• changes to photoperiod sensitivity (meaning the ability of plants to respond to light, which impacts flowering time and therefore yield and durability),  
• and nutritional value.  

These are the most pronounced differences between the wild and domesticated forms of wheat, and of most cereals. (Haas et al., 2019). Further traits that are looked for in modern cereals include: 

• timing of crop maturation,  
• disease resistance,  
• height,  
• and robustness in the face of environmental stress.  

An Overview of Wheat Genetic Evolution  

Next-generation genetic sequencing of cereal crops is a vital tool in crop improvement and development, and hence for the future of food. Yet, it is a relatively recent technological innovation to be applied in cereal research. Originally, molecular markers were used to trace the geographic origin of crops and calculate distance (how related they were to wild ancestors), genetic diversity, and to generate genetic maps which are vital to crop breeding programs. Both wheat and barley have complex, large genomes. Genetic diversity is key to crop improvement, and there have been competitive pushes to sequence the whole genome of cereal crops such as wheat and barley.  

The genetic history of wheat is very complex. Triticum aestivum ssp. Aestivum is the most commonly grown hexaploid bread wheat (hexaploid meaning an organism made up of cells containing six sets of chromosomes). It obtains its three genomes (A, B and D) from three wild ancestors: Triticum urartu; a relative of Aegilops speltoides; and Aegilops tauschii. These three ancestors are related to one another via their own evolutionary events.  

•  The result of the initial hybridization event of these three ancestors led to the production of wild tetraploid (emmer) wheat (T. turgidum ssp. Dicoccoides). Cultivated emmer wheat (T. dicoccum) and durum wheat (T. turgidum ssp. durum) eventually arose from this wild ancestor.  
• Cultivated einkorn, T. monococcum ssp. monococcum, was derived from the wild T. monococcum ssp. boeoticum, a close relative of T. urartu. Today, einkorn is considered a relic crop since it has largely been supplanted by agronomically superior wheats. 
• About 8,000 years ago, a second hybridization event took place between domesticated emmer wheat and the wild Ae. tauschii, giving rise to modern bread wheat (Peng et al. 2011; Wang et al. 2013). It is believed that this event occurred when cultivation of domesticated emmer wheat spread into the natural range of Ae. Tauschii. 

Reference genomes for wheat, as well as barley, have allowed the investigation of the domestication of these crops, providing useful information on the origins and spread of cereals and insights into the genetic diversity and history of crop domestication. This information can aid sustainability and biodiversity efforts in modern agriculture, and the development of cereal crops compatible with sustainable agriculture (read here about Crop to Wild Relative research: https://bit.ly/3Fj6iD6).  

Archaeobotany of Emmer Wheat & It’s Importance for Future Crop Development  

Wild emmer wheat (T.dicoccoides) is the ancestor of most cultivated bread wheat. It is an ecological specialist and an ideal model to study the evolutionary history- and future- of wheat. Wild emmer is good species from which to model future wheat improvement scenarios. It grows naturally all over the Fertile Crescent of southwest Asia. The modern, domesticated form of wild emmer is Triticum dicoccum (emmer).  

Two different forms of wild T. dicoccoides have been identified through analysis: one in the western Levant, (Israel, Syria, Lebanon and Jordan); and one in the eastern Levant, (Turkey mainly, and Iraq and Iran). It is this eastern form that is the ancestor of the modern emmer wheat (Peng et al, 2011).  

Archaeological evidence suggests that wild emmer wheat was first cultivated in the southern Levant. Botanical remains from Jilat, Jordan include wild and domesticated forms of einkorn wheat, wild and domesticated barley, domesticated emmer wheat, along with pulses and lentils. This assemblage has been dated to the early Epipalaeolithic period, (~14,000BC).  

Archaeobotanical studies suggest that wild emmer wheat was cultivated more than once in the Levant region. Possible reasons for this include that the late-ripening forms of emmer wheat from southeast Turkey would have been better suited to the climate of Anatolia, Central Asia and Europe than the hardier, heat and drought tolerant early ripening lines from the south. Another possible reason is that most wild emmer from the southern Levant have thick glumes (leaf-like structures surrounding the seed) meaning it may not have had much of a chance to disperse out of this area. The spread of domesticated emmer would have been extremely complex and may not represent a simple linear process (Ozkan et al, 2009; Peleg et al, 2011; Fuller et al, 2012).  

Wild emmer wheat is highly adaptive at the DNA and protein levels in response to biotic and abiotic environmental stresses. Wild emmer genes are an important resource for conferring drought tolerance, salt tolerance and disease resistance to modern wheat lines. Hence, wild emmer wheat is extremely important in crop improvement, and in crop to wild relative research (read here about Crop to Wild Relative research: https://bit.ly/3Fj6iD6). As a result, in situ and ex situ conservation of wild emmer wheat is a top priority for food security worldwide. 

Spelt Wheat: Ancient and Gluten-free? 

Another species of domesticated wheat is spelt wheat (T. aestivum ssp. spelta). Spelt wheat was previously cultivated in Europe much more widely than today, being replaced during the 20^th century with bread wheat. Spelt wheat is gaining popularity due to the idea that it is an “ancient grain”, often being marketed in this way. There is also a popular idea that spelt wheat is gluten-free, however, spelt is wheat, and does contain gluten, and thus should not be consumed by those allergic to gluten, though the gluten structure of spelt is weaker than that of bread wheat. Spelt is grown more commonly in Central Europe, being a more marginal crop.  

The idea of spelt being an ancient grain came from the fact that it is not a free threshing wheat. Free threshing wheats are also known as “naked” wheats. Naked wheats have a weak glume, the seed being contained within the glume. Fragile glumes readily release the seed during threshing/harvesting, which is the trait that makes them “free-threshing” and is a favourable trait that was selected for during crop development. The two genes that govern this characteristic are tenacious glumes (Tg) and the domestication locus Q (which also affects other domestication traits) (Haas et al., 2019).  

Using Crop Genetic Information 

Crop genetic research, particularly the availability of reference genomes, allows scientists to answer important questions in crop evolution and domestication genomics. Additional reference sequences of both wheat and barley will allow scientists to tackle complex questions in crop research- and has already been achieved with rice. The vast reduction in the cost of genetic sequencing is fuelling genetic research even further. This allows crop scientists to capture the fullness of genetic diversity of agricultural crop communities, which is incredibly important for future sustainable food production and environmental conservation. Additional reference sequences allow us to investigate the impact of domestication on crop genetics, discover new, sustainable ways of growing food crops, and are vital in the area of crop to wild relative research. These approaches and insights will prove invaluable in the face of increasing population levels, global warming, climate change, and a changing resource base- the effects of which are already being seen in the form of global crop failures followed by famine, migration and civil unrest. Therefore, unravelling the archaeological and evolutionary history of crops and their relatives, and applying these insights to modern Agri science is no longer academic, but is urgent.  

References 

• Dietrich L, Meister J, Dietrich O, Notroff J, Kiep J, Heeb J, et al. (2019) “Cereal processing at Early Neolithic Göbekli Tepe, southeastern Turkey”. PLoS ONE 14(5): e0215214. https://doi.org/10.1371/journal.pone.0215214  
• Douché, C. and Willcox, G., (2018) “New archaeobotanical data from the Early Neolithic sites of Dja’de el-Mughara and Tell Aswad (Syria) A comparison between the Northern and the Southern Levant”. Paléorient, 44(2), pp.45-58. 
• Dvorak, J., Luo, M.C. and Akhunov, E.D., (2011) “NI Vavilov’s theory of centres of diversity in the light of current understanding of wheat diversity, domestication and evolution.” Czech J Genet Plant Breed, 47, pp.S20-S27.
• Fuller, D., Willcox, G., Allaby, R., (2012)” Early agricultural pathways: moving outside the ‘core area’ hypothesis in Southwest Asia”, Journal of Experimental Botany, pp617-633  
• Garrard, A, 1999, “Charting the Emergence of Cereal Domestication in South-west Asia”, Environmental Archaeology 4, pp67-86.  
• Giraldo, P., Benavente, E., Manzano-Agugliaro, F. and Gimenez, E., (2019) “Worldwide research trends on wheat and barley: A bibliometric comparative analysis.” Agronomy, 9(7), p.352. 
• Haas, M., Schreiber, M. and Mascher, M., (2019) “Domestication and crop evolution of wheat and barley: Genes, genomics, and future directions”. Journal of Integrative Plant Biology, 61(3), pp.204-225. 
• Kilian, B., Özkan, H., Pozzi, C. and Salamini, F., (2009) “Domestication of the Triticeae in the Fertile Crescent.” In Genetics and Genomics of the Triticeae (pp. 81-119). Springer, New York, NY.
• Lister, D.L., Jones, H., Oliveira, H.R., Petrie, C.A., Liu, X., Cockram, J., Kneale, C.J., Kovaleva, O. and Jones, M.K., (2018) “Barley heads east: Genetic analyses reveal routes of spread through diverse Eurasian landscapes.” PloS ONE, 13(7), p.e0196652. 
• Luo, M.C., Yang, Z.L., You, F.M., Kawahara, T., Waines, J.G. and Dvorak, J., (2007) “The structure of wild and domesticated emmer wheat populations, gene flow between them, and the site of emmer domestication.” Theoretical and Applied Genetics, 114(6), pp.947-959.
• Lu, H., Tang, L., Spengler III, R.N., Boivin, N., Song, J., Wangdue, S., Chen, X., Liu, X. and Zhang, Z., (2021) “The transition to a barley-dominant cultivation system in Tibet: First millennium BC archaeobotanical evidence from Bangga.” Journal of Anthropological Archaeology, 61, p.101242. 
• Mohammadi, S.A., Sisi, N.A. and Sadeghzadeh, B., (2020) “The influence of breeding history, origin and growth type on population structure of barley as revealed by SSR markers.” Scientific Reports, 10(1), pp.1-13. 
• Morrell, P.L. and Clegg, M.T., (2007) “Genetic evidence for a second domestication of barley (Hordeum vulgare) east of the Fertile Crescent.” Proceedings of the national academy of sciences, 104(9), pp.3289-3294. 
• Nevo, E., (2014) “Evolution of wild emmer wheat and crop improvement.” Journal of Systematics and Evolution, 52(6), pp.673-696. 
• Ozkan, H. and Feldman, M., (2009) “Rapid cytological diploidization in newly formed allopolyploids of the wheat (Aegilops-Triticum) group.” Genome, 52(11), pp.926-934.
• Peleg, Z.V.I., Fahima, T., Krugman, T., Abbo, S., Yakir, D.A.N., Korol, A.B. and Saranga, Y., 2009. “Genomic dissection of drought resistance in durum wheat× wild emmer wheat recombinant inbreed line population.” Plant, cell & environment, 32(7), pp.758-779.
• Peng, J., Sun, D. and Nevo, E., (2011) “Wild emmer wheat,’Triticum dicoccoides’, occupies a pivotal position in wheat domestication process.” Australian Journal of Crop Science, 5(9), pp.1127-1143.
• Scanes, C.G., (2018) “The Neolithic revolution, animal domestication, and early forms of animal agriculture.” In Animals and Human Society (pp. 103-131). Academic Press.  
• Shewry, P.R., (2009) “Wheat.” Journal of Experimental Botany, 60(6), pp.1537-1553 
• Vavilov, N., (1926) “Center of origin of cultivated plants.” Papers on Applied Botany, Genetics and Plant Breeding.
• Wang, Y., Ren, X., Sun, D. and Sun, G., (2015) “Origin of worldwide cultivated barley revealed by NAM-1 gene and grain protein content.” Frontiers in Plant Science, 6, p.803.  
• Weiss, E. and Zohary, D., (2011) “The Neolithic Southwest Asian founder crops: their biology and archaeobotany.” Current Anthropology, 52(S4), pp. S237-S254. 
• Willcox, G., (2013) “The roots of cultivation in southwestern Asia.” Science, 341(6141), pp.39-40. 
• Willcox, G. and Stordeur, D., (2012) “Large-scale cereal processing before domestication during the tenth millennium cal BC in northern Syria.” Antiquity, 86(331), pp.99-114.