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Documentation:Artificial Selection in Crop Plants

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Humans have long been directing the evolution of crop plants via artificial selection to best meet their needs. In extreme cases of genetic bottlenecking, such as in monoculture fields, problems arise with disease susceptibility. With the advent of genetic modification, new varieties are created that may never have arisen through directed plant breeding. This new technology has potential to help solve food crisis problems, but could also be dangerous if transgenes find their way into non-target organisms in the ecosystem. With a changing climate, and ongoing bottleneck selection of crop varieties, it is essential to preserve the diversity that exists for use in future biotechnology endeavors and as an assurance of food security.

History

Agriculture has been a major part of human lifestyles since it was first practiced about 10,000 years ago [1]. Before the origin of agriculture humans gained their food by hunting and gathering. At this time people were able to collect their first choice foods, which were in great supply, without concern. It is believed that a gradual change to an unpredictable climate occurred at the end of the Pleistocene which affected the foods that were available to the hunter-gatherers. Humans then began to adapt to their available habitats. People began to combat the risk of unpredictable variation of food by broadening their diets to include more food, including a variety of plants. This is referred to as the broad-spectrum revolution [2].

The history of agriculture began when the first plant farmers took wild plants from their natural habitats and brought them closer to their dwellings and even to areas where the plants could be more productive [3]. When the hunter-gatherers selected various plants and brought them back to their abode, the wild and newly domesticated plants became separated geographically, causing gene flow to decrease and thus genetic drift to increase. Eventually differences arose between the domestic and wild plants [4], [5]. Humans learned to collect seeds from their crops of interest and plant them the following year. Therefore, likely without understanding, they were selecting the crops for plants with bigger and numerous seeds, as well as more erect or healthy plants. And so, our ancestors eventually were able to change the morphology and reproductive cycles of their food crops [6].

Our ancestors not only learned to cultivate crops for food, but also for fibers. Depending on the society, various fiber crops were developed to suit their needs. Hemp and linen found in Iraq and Iran were dated back to 8000-6000 B.C. These fiber crops were directed according to fiber quality, yield factors, and their ability to adapt to the climate [7]. The understanding that farmers were able to direct the development of their crops was a very beneficial idea. This artificial selection is thought to have first been practiced by the Romans. The Roman poet Virgil wrote a series on agriculture called Georgics where he shows an understanding that humans can affect the outcome of their crops: “the chosen seed through years and labor improved, We seem to run back, unless yearly Man selected by hand the largest and fullest ears” [8]. Although this was far before the discovery of genes, the Romans were nonetheless using them to their advantage.

In the 11th century, artificial selection was then theorized by Abū Rayḥān al-Bīrūnī, a Muslim scholar and polymath. In his works ‘On Vasudeva and the Wars of the Bharata’ he writes “The agriculturist selects his corn, letting grow as much as he requires, and tearing out the remainder. The forester leaves those branches which he perceives to be excellent, whilst he cuts away all others” [9]. He too showed an understanding of man’s capability to alter crops to his desire.

800 years later, Charles Darwin wrote his famous book “Origin of the Species”, where he coined the terms natural selection and artificial selection. Although he was not the first to realize that people could shape plants as desired, he studied this process in detail and made it known to the world. He states “Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection… We are profoundly ignorant of the causes producing slight and unimportant variations; and we are immediately made conscious of this by reflecting on the differences in the breeds of our domesticated animals in different countries, more especially in the less civilized countries where there has been but little artificial selection”[4]

Although the domestication of crops is recent with respect to human history, once instigated it rapidly spread to all corners of the globe. Farmers both unconsciously and consciously artificially selected their crops, regulating the phenotypic traits of interest. Eventually our ancestors learned to grow and improve the species that were of the greatest interest to them [6].

  • Charles Darwin
    Charles Darwin
  • Ancient Egyptian Agriculture
    Ancient Egyptian Agriculture
  • Georgics
    Georgics
  • Caption: Abū Rayḥān al-Bīrūnī
    Caption: Abū Rayḥān al-Bīrūnī

Monoculture

Broad, wide scale artificial selection has modified crop plants over time, through the consistent preference of desirable traits such as minimal grain shattering in rice crops [10]. Overtime, this has lead to the production of ideal crops that minimize production time and costs, while maximizing yields worldwide. Not only have individual organisms been selected within a population, but individual species have been selected over others, leading to fewer and fewer plants providing more and more of the worlds nutritional needs. 15 plant species alone make up 90 percent of the worlds plant-based diet, and two thirds of this is supplied from maize, wheat and rice [11]. This continued selection on crop species has lead to only a few plants being grown with a very limited gene pool, and evidence of genetic bottlenecking is becoming present [10].

Monoculture is the intensive farming practice of growing one crop over a large expanse of land, without any sort of intercropping or diversity of plant type [12], and is a prevailing farming method throughout the world. Most of the organisms grown in monoculture plots have been artificially selected and/or genetically modified, and ideally will provide large yield numbers, with a minimum energy input. Although there have been some advantages, such as a predictable growth pattern and standardized harvesting methods, which have been able to increase yield counts worldwide, there have been distinct disadvantages to the continued practice of continuously cultivating one crop. Some of these disadvantages include harm to the surrounding environment, such as erosion and nutrient depletion as well as genetic harm to the plants themselves [12].

With so many crops having a restricted gene pool, they do have a certain susceptability, even though they may have been selected for disease resistance [13] This is due to the rapidly evolving pesticide resistance in many known pathogens [14], [15] and the fact that there is little genetic difference in members of the crop stretched across a vast amount of land, in the case of monoculture farming. This can cause massive epidemics as none of these plants can defend themselves, being so alike to genetically [10]. Some examples where monoculture crops where lost include a maize epidemic in 1970 throughout the United States. Over 1 billion dollars worth of crop was lost to one blight. 18 million citrus trees were also destroyed in a similar incident in 1984 [13]. Having such large, genetically limited concentrations of crops leads to stronger pesticide and herbicide use, which could not only damage surrounding environments, but humans as well [13]. More and more modern farming implications are being developed to reduce the reliance on current monoculture practices in an attempt to rehabiliate the agroecosystems and to reduce the use on heavy chemicals [12].

Genetically Modified Crops

Genetically modified crops are plants grown and harvested for economic purposes which contain genes from other species or members of the same species [16]. These genes may confer traits such as disease resistance, herbicide resistance, pest resistance, tolerance of harsh abiotic stresses, production of pharmaceuticals, or improved nutritional quality [16]. Genetically modified crops with genes from different species are classified as transgenic, whereas genetically modified crops with genes from the same species are cisgenic [16].

By far, the majority of genetically modified crops on the market today are herbicide tolerant and insect resistant crops [17]. The ability to control weeds and reduce insecticide application greatly increase crop yield, and decrease production costs [17]. There are, however, many other interesting applications of transgenic crops that are somewhat less well known. Some plants are modified to increase oil content to be used in cosmetics, detergents, food products, or health products [17]. There are currently transgenic crops in phase I and phase II clinical trials as oral vaccines and antibodies [17]. Transgenic crops are also used to produce other products such as human collagen, fibrous spider protein, and biodegradable plastic [16].

Most commonly, genes are incorporated into the nuclear genome using Agrobacterium or particle bombardment [16]. The genes are inserted randomly without much control over placement [16]. This leads to positioning effects, and an inconsistency in gene expression [16]. Inserting genes into the nucleus of a plant will cause expression of proteins in the cytoplasm [18]. Genes may also be inserted into the chloroplast genome [18]. Because the chloroplast is inherited maternally and therefore not present in pollen, this technique reduces the risk of gene outflow and makes genetically modified populations easier to contain [18]. It is also easier to specify the insertion point of a gene into the chloroplast than it is into the nuclear genome, eliminating some positional effects seen in nuclear transformations [18]. Genetically modified chloroplast crops express proteins which are contained in the chloroplast [18]. This is especially useful for manufacturing some pharmaceuticals which are toxic to the plant if accumulated in the cytoplasm, but non-toxic if accumulated in the chloroplast [18].

Although much research concerning plant traits does exist, relatively little of it is implemented in biotechnology to create marketable crop products [19]. The process of taking a potentially advantageous trait and creating a marketable product is very costly and includes a number of highly regulated research and development challenges [19]. Transgenic crops that are successfully marketed are the product of both regulatory and consumer-based bottlenecks [19].

There is large public concern about the safety of transgenic crops, but for the most part they are substantially equivalent to their non-transgenic counterparts [16]. Transgenic crops intended for consumption must undergo food safety assessment and rigorous testing to verify the safety of transgenes [16]. Part of this testing involves the evaluation of proteins under food processing conditions [20]. Because proteins denature under non-physiological conditions, processed food products contain negligible levels of active protein and do not raise safety concerns [20].

Social and Ecological Consequences

Due to the increase in the human population the yield of the small number of crop plants has been the object of much research and controversy. Increasing the yield of food crops remains the key to a greater global carrying capacity, solving world hunger and poverty [21]. In most developing countries, food demands account for most of a person’s income, and the yield of a person’s crop is the difference between having enough food for one’s family and starvation of entire villages [21]. As populations increase the strain on crop yield is increasing every day. The yield of crop is measured as the size of fruit or inflorescences, and the amount of return, that is, how many seeds each plant yields. This is the basis of artificial selection and breeding and genetic modification of crop plants.

The risks associated with genetically modified food products include the potential for allergenicity and the decrease in food quality of these modified crops [22]. However to date that has been no scientific evidence to support these claims.

The ecological consequence of genetically modified crops is only beginning to be understood. These effects can be of two kinds, one is the unintentional negative effect on neighboring species and taxa and two is the greater persistence of hybridized populations in the native populations [22]. Of major concern is the increased rate of evolution of undesirable weeds, insects and other pests. Genetically modifying crops essentially speeds up the adaptation of plants to produce higher yield, these changes could also occur without genetic modification, just much slower [23]. This could come about not by the molecular techniques themselves but by the phenotypic traits that result from using recombinant DNA and whether or not the modified plant can hybridize with free-living populations [23]. The fear is that these modified genes will move from commercial plants to wild or weed populations of the same species or closely related species, through gene flow via insect pollinators or other routes of pollination or hybridization. These wild populations may then become herbicide-resistant or insect-resistant or are able to outcompete native species and choke them out [23]. Crop genes can also spread via seeds stored in seed banks.

The most concerning trait to be introduced into commercial crops is the ability to produce pesticide, such as that from the bacterium Bacillus thuringiensis. The fear is that these pesticide-producing plants will quickly cause selection of pesticide resistance in insects, and thereby shorten the lifespan of this environmentally friendly pesticide [23]. The same problems arise with herbicide tolerance. On the positive side herbicide tolerant crops are better for the environment (eliminate the need for chemical herbicides) but on the negative side they will allow crops to be grown in hazardous soils [23]. Another area of concern is that balance in the ecosystem. Natural enemies of many pest insects are dependant on the population density of these pests, if the genetically modified plants kill off all the pest insects, or weaken them, this will decrease the population density of the natural enemies [24]. There is also the danger of “prey-mediated” effects [24] on the natural enemies by ingestion of pollen or insects that have ingested pollen or plant material from the genetically modified plants.

With the advent of gene modification and recombinant DNA we are able to select specific genes and insert only the desirable ones into the crop species where as with classical selection many unknown and unwanted genes would also have been transferred to the crop species [25]. This also means we can now test the effects of these specific genes and know what the effects of each gene are.

Efforts to Maintain Genetic Diversity

Over the course of the last fifty years, genetic diversity in plants has been drastically reduced. Vegetables alone lose genetic resources at a scale of 1-2% per year, globally [26].

Organizations Promoting Plant Diversity

Many different organizations exist around the world aimed at protecting genetic diversity in plants, often aimed at increasing global food security. One key organization in this category is the Global Crop Diversity Trust. It was established as a joint project of the UN Food and Agriculture Organization (or the FAO) as well as Biodiversity International, a research center dedicated to the conservation of agricultural biodiversity.

The GCDT acts to ensure the conservation of crop diversity to maintain food security worldwide. Food security, the availability and accessibility of food, is becoming a progressively pressing issue globally [27], as an 800 million people are undernourished [28] and a food crisis looks increasingly more likely as climates continue to change worldwide.

One of the largest international agreements regarding the preservation of diversity is the International Treaty on Plant Genetic Resources for Food and Agriculture. The treaty aims to guarantee food security through increasing conservation efforts and promoting the sustainable use of plant genetic resources. It is one example of a responsible global governance that ensures both plant genetic resources and diversity as well as future food security internationally.

Seed Banks

To preserve the genetic information of many varieties of plant species, seed banks are utilized to store the seeds of a collection of plants, often native to the region of the bank, to ensure a ‘back-up’ supply of these species, often in case of a natural disaster. The need globally for these banks increases as industrial agriculture continues to create monocultures for higher yields and narrows the diversity of many plants. The banks create a store of seeds that may be better suited to growing in changing environmental conditions, and which may have otherwise been lost.

The most famous of these banks is the Svalbard Global Seed Vault, built into a mountain on an island in Norway. It is a highly secure underground facility fortified against every eventuality, from equipment failure to natural disaster. Even if the ice caps melt, the site will remain dry, and the permafrost ensures that the seeds will stay cool (ensuring low metabolic activity). Some of the seeds housed in the Svalbard Bank [28] will last thousands of years. The plan is that the seed bank will eventually house genetic samples of every crop on the planet. It cost the Norwegian government approximately 9 million dollars to construct.

In addition to this bank, The Vavilov Institute of Plant History was originally created as a seed bank and now serves as a research institute of plant genetics in Saint Petersburg, Russia. Nikolai Vavilov established it in 1921, and it currently houses the world’s largest collection of seeds, collected from all around the globe. It has survived the 28-month Siege of Leningrad during World War II, during which time one of Vavlinovs assistants guarding the bank starved to death inside it rather than eat the seeds he was protecting.

References

  1. ↑ Evans, L T (1976). “The two agricultures: renewable or resourceful”. J. Aust. Inst. Agric. Sci. 42(4): 222-31.
  2. ↑ Flannery, K V (1969). The Domestication of Plants and Animals. London: Ducksworth.
  3. ↑ Hillman, G C, Davies, M S (1990). "Measured domestication rates in wild wheats and barley under primitive cultivation, and their archaeological implications". J. World Prehist. 4:157-222.
  4. ↑ 4.0 4.1 Darwin, C (1859). "The Origin of Species by Means of Natural Selection (1st ed.)". London: John Murray.
  5. ↑ Rindos, D (1984). The Origins of Agriculture: An Evolutionary Perspective. San Diego: Academic.
  6. ↑ 6.0 6.1 Hancock, J.F (2007). "History of scientific agriculture: crop plants". eLS.
  7. ↑ Mohanty, A.K., Misra, M., Drzal, L.T. (2005). Natural fibers, biopolymers, and biocomposites. Boca Raton: CRC Press.
  8. ↑ Virgil (29 B.C.E.). Georgics I: 197.
  9. ↑ Wilczynski, J.Z. (1959). "On the Presumed Darwinism of Alberuni Eight Hundred Years before Darwin". Isis, 50(4), 459-66.
  10. ↑ 10.0 10.1 10.2 Kovach, M.J., and McCough, S.R. (2008). "Leveraging natural diversity: back through the bottleneck". Current Opinion in Plant Biology, 11(2), 193-200.
  11. ↑ "Staple foods: what do people eat", Dimensions of need, an atlas of food and agriculture
  12. ↑ 12.0 12.1 12.2 Smith, R.G.,Gross, K.L., and Robertson, G.P. (2008)."Effects of Crop Diversity on Agroecosystem Function: Crop Yield Response". Ecosystems, 11(3), 355-366.
  13. ↑ 13.0 13.1 13.2 Garcia, M.A., and Alteri, M.A. (2005). "Transgenic Crops: Implications for Biodiversity and Sustainable Agriculture", Bulletin of Science, Technology & Society, 25(4), 335-353.
  14. ↑ Alteri,M.A. (2007). "Modern Agriculture: Ecological impacts and the possibilities for truly sustainable farming". Agroecology in action.
  15. ↑ Poggio, S.L. (2005). "Structure of weed communities occurring in monoculture and intercropping of field pea and barley". Agriculture, Ecosystems & Environment, 109(1-2), 48-58.
  16. ↑ 16.0 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 Akhond, M.A.Y., and Machray, G.C. (2009). "Biotech crops: technologies, achievements and prospects". Euphytica, 166, 47-49.
  17. ↑ 17.0 17.1 17.2 17.3 Strivastava, N., et al. (2011). "Genetically Modified Crops – An Overview". Biotechnology, 10(2), 136-148.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 18.5 Grevich, J.J, and Daniell, H. (2005). "Chloroplast Genetic Engineering: Recent Advances and Future Perspectives". Critical Reviews in Plant Sciences, 24, 83–107.
  19. ↑ 19.0 19.1 19.2 Graff, G.D., Zilberman, D., and Bennett, A.B. (2010). "The commercialization of biotechnology traits". Plant Science, 179, 635-644.
  20. ↑ 20.0 20.1 Hammond, B.G., and Jez, J.M. (2011) "Impact of food processing on the safety assessment for proteins introduced into biotechnology-derived soybean and corn crops". Food and Chemical Toxicology, 49, 711-721.
  21. ↑ 21.0 21.1 Evans, L.T. (1993). "Crop Evolution Adaptation and Yield". Cambridge University. ISBN 0 521 29558 0. Retrieved 25 Nov 2011.
  22. ↑ 22.0 22.1 Pilson, D., and Prendeville, H. R. (2004). "Ecological Effects of Transgenic Crops and the Escape of Transgenes into Wild Populations". Annual Review of Ecology, Evolution, and Systematics, 35, 149-174. doi: 10.1146/annurev.ecolsys.34.011802.132406
  23. ↑ 23.0 23.1 23.2 23.3 23.4 Snow, A. A., and Palma, P.M. (Feb. 1997). "Commercialization of Transgenic Plants: Potential Ecological Risks". Bioscience, 47(2), 86-96. doi:10.2307/1313019
  24. ↑ 24.0 24.1 O’Callaghan, M., et al. (2005). "Effects of Plants Genetically Modified Insect Resistance on Nontarget Organisms". Annual Review of Entomology, 50, 271-92. doi: 10.1146/annurev.ento.50.071803.130352
  25. ↑ Prakash, C. S. (2001). "The Genetically Modified Crop Debate in the Context of Agricultural Evolution". Plant Physiology, 126, 8-15. doi: 10.1104/pp.126.1.8
  26. ↑ De Silva, D., and Joao,C. (2010)."Impact of Improved Vegetable cultivars in Overcoming Food Insecurity". Euphytica, 176(1),125-136. Web. 27 Nov. 2011.
  27. ↑ Keane, J., Page, S., and Kennan,J. (2009)."Climate Change, Agriculture and Aid for Trade". Draft paper. ICTSD-IPC Platform on Climate Change, Agriculture and Trade (2009): n.pag. Academic Search Complete. Web. 24 Nov 2011.
  28. ↑ 28.0 28.1 Walsh, B. (2008). "The Farmer's Bank". Time 171(6),56-57. Academic Search Complete. Web. 23 Nov 2011.
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