#Guest post by Francesco Janzen.
Have you ever wondered how much work and time has been put into producing the food you eat today: that juicy apple, or that fresh loaf of bread? In modern times, we can easily recognize fruits and vegetables such as tomatoes, corn, and bananas, but would it surprise you that these foods have not always looked the way they do? Like all parts of the living world, food crops have changed much over time, and this change is directly linked to human efforts (Purseglove, 1965; Allaby et al., 2015).
Agriculture began approximately 11,000-12,000 years ago, and has originated in several parts of the world (National Geographic, 2015). Humans domesticated wheat in the Fertile Crescent, or Near East approximately 8,000-9,000 years ago. (Nevo, 2014; National Geographic, 2015). In China, rice is proposed to have been domesticated 10,000-20,000 years ago (Gross & Zhao, 2014; National Geographic, 2015). Across the ocean, squash was domesticated about 10,000 years ago in what is known today as Mexico, and the beginning of sunflower cultivation began in North America around 5,000 years ago (Janick, 2013; National Geographic, 2015). All of these domestications began with wild progenitors of today’s crop species (Gross et al., 2014; Allaby et al., 2015).
But how did the wild crops of ancient times develop into the modern ones we know today? John William Purseglove, a former tropical agricultural officer and director of the Singapore Botanic Gardens, discussed the ways in which humans have changed crop species over time in a chapter of “The Genetics of Colonizing Species” (1965). In his chapter, “The Spread of Tropical Crops”, Purseglove (1965) states that humans would have begun the first agricultural crops with a subset of desired plants from the original wild population. This subset would not possess the genetic diversity of the original population, essentially producing a genetic bottleneck effect (Purseglove, 1965). Furthermore, certain desired traits would be selected for in this new population, so breeding strategies would overtime change the traits expressed, such as larger fruit, seedless fruit, lack of defense mechanisms, etc. (Purseglove, 1965). Although they benefit humans, these changes could potentially decrease the competitive ability of these new plants. This intrinsically ties their survival to human assistance (Purseglove, 1965).
Humans have not only changed the physical characteristics of crop plants; they have altered their geographic distributions as well. Compared to their wild ancestors, most crop plants are now grown in areas far removed from their origin, such as with vanilla (Vanilla planifolia). Vanilla originated in Mexico, but is now grown in large numbers in Madagascar (Purseglove, 1965). In fact, vanilla and most other crops are much more successful in their new environments, but why is this so? Purseglove (1965) proposed that by moving a crop plant into a new habitat where predators or disease are absent, little would control population sizes, and increase crop yields.
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Visible difference between a wild strawberry (Fragaria virginiana, left) and a domestic strawberry (Fragaria x ananassa, right), from http://www.jamesandthegiantcorn.com/tag/domestication/.
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The new environments that domestic crops are exposed to may further increase the genetic gap with their wild ancestors. Under new, adverse environmental conditions, a population of a crop may be culled, save for a few individuals possessing recessive genes that confer a benefit to coping with the altered conditions (Purseglove, 1965). The remaining individuals reproduce, which shifts the next generation’s genotypic frequency (Purseglove, 1965). In addition, this can effectively expand the range of the domestic crop, whereas the wild type remains restricted to its original range (Purseglove, 1965).
Science has come a long way since Purseglove proposed his ideas 50 years ago, and the advent of DNA has helped improve our understanding of evolution. With respect to the evolution of crops, DNA allows for testing of certain theories proposed, one such being the bottleneck effect. A study conducted by Gross et al. (2014) investigated whether perennial crop species, specifically the apple (Malus x domestica) showed a decrease in genetic diversity when compared to closely related wild species. They expected that there would have been a narrowing of genetic diversity at two moments in history. Firstly, during a domestication bottleneck, similar to that proposed by Purseglove (1965), and secondly during an improvement bottleneck, where desirable traits in the crop species were selected for to produce elite cultivars (Gross et al., 2014).
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A visual depiction of the bottleneck effect, where the bottleneck represents stochastic (random) events, from http://bio1151.nicerweb.com/Locked/media/ch23/bottleneck.html.
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By sequencing specific DNA regions of 11 varieties of apple cultivar (both ancient and modern), and that of three wild species, Gross et a. (2014) sought to demonstrate that domesticated cultivars show less genetic diversity than wild species. The regions selected were areas where each species show a variable amount of repeated sequence length, known as microsatellites, allowing for easy comparison of genetic quality (Gross et al., 2014). What they found, contrary to what was expected, was that domestic apples have not undergone a significant reduction of genetic diversity, either at the domestication or improvement phases (Gross et al., 2014). This evidence shows that not all theories produced 50 or more years ago withstand the test of time, especially when new tools to test these theories become available.
So how does any of this information impact management practice of controlling invasive species? Purseglove (1965) stated in his chapter that by understanding the evolution of crop species, we gain insight into the success of introduced weed species. Although weeds do not require any human assistance in survival, the forces acting on them may be the similar to those acting on agricultural crops. Just as crops experience a release from predators and disease when removed from their native habitats, weeds may also undergo this release, contributing to their widespread success (Purseglove, 1965). This parallel could be quite useful in the understanding and management of weedy species.
References:
Allaby, R.G., Gutaker R., Clarke, A.C., Pearson, N., Ware, R., Palmer, S.A., Kitchen,
J.L., and Smith, O. 2015. Using archaeogenomic and computational approaches to unravel the history of local adaptation in crops. Philosophical Transactions Royal Society 370: 20130377.
Gross, B.L., Henk, A.D., Richards, C.M., Fazio, G., and Volk, G.M. 2014. Genetic
diversity in Malus × Domestica (Rosaceae) through time in response to domestication. American Journal of Botany 101(10): 1770-1779.
Gross, B.L. & Zhao, Z. 2014. Archaeological and genetic insights into the origins of
domesticated rice. Proceedings of the National Academy of Sciences 111(17): 6190-6197.
Janick, J. 2013. Development of New World crops by indigenous Americans.
Horticultural Science 48(4): 406-412.
National Geographic Society. 2015. The development of agriculture. Retrieved from
Nevo, E. 2014. Evolution of wild emmer wheat and crop improvement. Journal of
Systematics and Evolution 52(6): 673-696.
Purseglove, J.W. (1965). The spread of tropical crops. In H.G. Baker, and G.L Stebbins
(Eds.). The Genetics of Colonizing Species. New York: Academic Press.
