Central concept(s) and/or questions: Germplasm: what is it, why is it important, why is it vulnerable?
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During the fifteen weeks of this course, we will explore plant germplasm nearly exclusively, but may mention particularly relevant studies of animal or microbial germplasm. As students in the plant sciences, you know basic botanical and/or genetic concepts. Nevertheless, there are certain elementary terms and questions that you may not have encountered before; these we should introduce now.
The term germplasm is slightly older than 100 years, having been coined in 1883 by the German zoölogist August Weißman to denote "the bearer of the characteristic nature of the species and of the individual." Weißman, an embryologist and cytologist, is also known for postulating four years later that a periodic reduction in chromosome number must occur during meiosis. So, to him, germplasm connoted tissues, cells, and organelles.
As an aside, 1883 was a most momentous year for germplasm. The Origin of Cultivated Plants, one of the most important scholarly studies of crop germplasm, was published in that year by the great Swiss botanist Alphonse de Candolle; we will discuss this seminal work in later classes. The ASTA or the American Seed Trade Association, was also founded in 1883 to represent the crop germplasm "business"--the seed industry in the United States. The ASTA has become embroiled in some rather acrimonious and controversial issues that we will also discuss later. But -back to germplasm. Well, what do you consider germplasm?
Today, Weißman's definition of germplasm still is useful but, because it pre-dates the field of genetics, it requires some modification. Germplasm is both the genetic material that controls heredity and the manifestations of variation in that genetic material. It is both the genotype, the phenotype, and the elements, factors, and processes that translate the hereditary blueprint of the former into the latter: GENOTYPE----------> PHENOTYPE. It is nucleic acid fragments, nucleic acids, genes, organelles, cells, tissues, organs, organisms, and colonies or populations of organisms. As far as I know, it has no synonyms. Potter Stewart, a Supreme Court justice, stated that although he could not define pornography precisely, he certainly knew it when he saw it. Such is the case with germplasm: it is defined more by context, use, or potential use. Much of this course stresses the germplasm of our most important domesticated plants--our major crops--although we will consider minor crops, weeds, and wild species whenever relevant.
To a certain extent, Ford-Lloyd and Jackson (1986) and the first chapter of Plucknett, Smith, Williams, and Anishetty (1987) serve as an excellent overview for this class. Donald Marshall, in his article in Brown et al. 1990, p. 369, also provides an excellent overview of the issues that we will address. At this point, you might just skim these references, because we will return to issues they discuss later. Many of the references will be on reserve in Parks Library at Iowa State University.
Marshall cites Sir Otto Frankel's and Erna Bennett's classification of germplasm into: 1) currently utilized or formerly used "elite" germplasm--the product of modern plant improvement; 2) landraces, or traditional varieties--the product of selection by traditional "non-scientific" cultures; 3) wild or weedy plants with the potential of providing useful genes for plant improvement and natural products useful to humans. These plants may be candidates for domestication; and 4) specialized genetic stocks for research. In this course, we shall discuss, for the most part, the first three categories.
Although these categories are widely used to classify germplasm and are useful tools for germplasm management, keep in mind that there are no absolutely clear boundaries among them. The categories form a rather imperfect continuum of wild-weedy-landrace-elite, according to increasing intensity of human selection. Many plant populations have characteristics of more than one of these categories.
A wild plant could be defined by what it is not. It has not been significantly altered by deliberate human selection, nor has it adapted specifically to habitats disturbed by humans. This definition does not imply that wild plants are not very useful--nor does it connote that a particular wild plant is not a crop. Like some other definitions, this one may be somewhat over simplified but, at present, it is convenient for us to categorize plants this way. Iltis (1988) presents two excellent examples of wild plants, one (a wild relative of tomato) has contributed extremely valuable genes to U. S. commercial tomato production; the other (teosinte, the wild ancestor or relative of maize) may someday provide useful genes to "scientific" maize breeding programs.
What, then, is a weed? Perhaps we could define it as the plants to whose study weed scientists have devoted their careers! Jack Harlan, a famous student of domesticated plants, has devoted an entire chapter of his book Crops and Man to this subject, and offers the disarmingly simple definition that weeds are plants adapted to habitats disturbed by humans. This definition will serve for the present, but we will see that the situation is far more complicated than that.
A domesticate, or domesticated plant, has been altered genetically via intentional human selection. Most crops (e. g., maize, soybeans) are domesticates, but it should be noted that crop implies only that the plant (or animal, for that matter--e.g., salmon) is harvested for human use. Some plants are crops, but are not domesticates--many trees species cut for timber are perhaps the most familiar examples, but there are many more, including fruits, nuts, forages, and industrial crops.
A domesticate's distinct genetic alteration--its evolution under human selection--is its hallmark. This evolution may be detected by direct measurement of genetic variation via genetic markers (isozymes, or DNA polymorphisms), but is also evident in particular morphological and ecological shifts which, in some cases, result in the domesticate depending completely on humanity for its survival. To cite familiar examples, a maize, soybean, or wheat field must be re-planted each year, as these domesticates can neither survive the severe winters of temperate regions, nor persist more than a year or so without human intervention to reduce competition from weeds.
The term "landrace", a somewhat misleading direct transliteration of the German "Landraße" (which connotes autochthonous or indigenous race or variety), is widely applied to local, often genetically highly variable, crop variants cultivated as part of traditional agriculture [Simmonds (1979) and Harlan (1992) contain useful discussions of landraces].
Generally, traditional horticultural/agronomic selection has adapted landraces to local climatic, edaphic, and biotic conditions so that, even under the most dire conditions, they will yield at least some food, medicine, or fiber. Landraces may also be selected for special, traditional uses. Frequently, a landrace includes a broad mixture of genotypes. In developed countries, this germplasm may be called a "heirloom variety," or a similar term. Both ecogeographic and human cultural contexts influence landrace evolution. Many landraces are distinguished by traditional farmers from other landraces by specific local names in indigenous languages.
Elite germplasm has been developed by modern, "Western" scientific breeding (i.e., post-G. Mendel and R. Fisher) programs pursued by seed companies, governments, international centers, and academia. Various intermediate, elite breeding populations, such as synthetics or composites, are often genetically rather variable. In contrast, the finished product of such breeding programs generally is a genetically stable, homogeneous variety, hybrid or clone which, under modern agronomic/horticultural practices, produces relatively high yields. When elite germplasm is no longer commercially competitive, it is said to be obsolete, although contemporary elite germplasm is often but the recombined genes or gene blocks of obsolete elite germplasm.
Genetic stocks often are elite germplasm in which have been incorporated specific genetic features, such as mutant alleles, or cytogenetic abnormalities (inversions, translocations, addition/subtraction lines). These stocks serve as tools for basic plant science research (e.g., as linkage testers for mapping genes).
The categories outlined in the section of this lecture on types of plant germplasm are primarily defined by degrees of human manipulation and genetic modification. Another system to categorize germplasm, the "gene pool concept" was outlined by Jack Harlan and Jan de Wet (1971) about 25 years ago. Their system is an informal, utilitarian scheme designed to organize the various types of germplasm from the perspective of plant breeders. A "gene pool" includes plants that can be crossed with, or contribute genes to, the crop(s) or species of interest. Harlan and de Wet proposed three informal categories of gene pools, based on the ease with which particular germplasm could be crossed:
(i) primary gene pool--germplasm that could be crossed easily to the crop or species of interest, with a highly fertile hybrid ensuing and, consequently, highly efficient gene transfer. The limits of the primary gene pool were considered congruent with the boundaries of a "biological species"--more about this in later lectures;
(ii) secondary gene pool--crossing and gene transfer are more difficult, and require more elaborate techniques. Hybrids among elements of the secondary gene pool and the crop or species of interest are often somewhat sterile and/or weak;
(iii) tertiary gene pool--although crosses can be made with great effort among elements of this gene pool and the crop or species of interest, the F1 plants are generally inviable or sterile.
This informal categorization appears frequently in the plant breeding literature, because it conveys succinctly the relative utility of particular germplasm for contributing genes to "conventional" crop improvement. As Harlan recognizes, the boundaries of the tertiary gene pool may be nebulous. Novel gene transfer methods may extend it vastly, even to the animal kingdom, e.g., if one considers the successful transfer of the firefly gene encoding luciferase to plants. The gene pool concept is less appropriate in an evolutionary/systematic context where the biological species concept has been considered out-dated during the last 15 years, because an evolutionary/phylogenetic species concept seems more useful.
The importance of plant germplasm can be measured on many different levels. From an ecological perspective, nearly all life is ultimately based on the photosynthetic ability of plants. The health and survival of terrestrial and aquatic ecosystems is linked to the ability of diverse plant populations to cope with environmental conditions and variability. From a human cultural perspective, modern human societies are based on reliable agricultural (agronomic, horticultural, and forest) production. Although the bulk of agricultural produce is derived from only a limited sample of the world's germplasm, the quality of human life is influenced by a much more diverse array of plants than just those species predominant in sustaining human life. As human populations grow and place increasingly heavy burdens on nature, ecological and human cultural perspectives converge. Diverse plant germplasm is needed to meet changes in both "natural" and cultivated plant communities.
Plant germplasm serves as a basis for plant science. Virtually all the phenomena we study as plant scientists, from the cellular to the community level, ultimately result from the interaction of genes and environment. Scientific studies designed to test hypotheses about biological systems must recognize both genetic and environmental variability. Ideally, plants that are the subjects of scientific research should be characterized and documented to the extent that such studies are repeatable. Managed germplasm collections can serve both as a source of genetic variability for research and as a repository for documented plants from previous studies.
Importance to "basic research"
The range of scientific disciplines that study, analyze, or otherwise use plant germplasm is extremely broad. In the social sciences, archaeologists often compare their finds to modern reference samples and anthropologists study the cultural aspects of germplasm. In the physical sciences, a panoply of natural plant products is analyzed by chemists. In medicine, nutritional and pharmacological activities of plant foods and drugs are evaluated: a specialized medical discipline, pharmacognosy, has historically been instrumental in germplasm preservation through the establishment of living collections of medicinal plants. In the biological and agricultural sciences, the utility of germplasm for basic research issues from both genetic diversity and documentation. For example, when studying the physiology of drought tolerance, you could examine a broad range of germplasm to identify both tolerant and intolerant populations. By studying examples of both types of populations, you can design experiments to identify different mechanisms for drought tolerance, and to examine the relative importance of genetic control and environmental factors. The documentation that accompanies well-managed germplasm may help you view your findings in an evolutionary and/or ecological context. It also provides a system whereby these same populations can be used again in future studies.
Its integral role in plant breeding
Plant breeding is the intentional manipulation of genetic diversity to improve agricultural production. Without a sufficient base of diverse genotypes, plant breeders may be unable to achieve the particular goals of their breeding programs. Many plant breeders work primarily with elite germplasm, using landrace and wild or weedy germplasm only as sources of particular traits absent from elite lines. It is particularly common for breeders and other specialists to evaluate less-highly bred germplasm of major crops for new sources of insect and disease resistance. Breeders of forest trees, ornamentals, and certain forages rely more heavily on landrace, or wild and weedy germplasm. For most plant species, elite germplasm has yet to be bred, or is just now being developed. As agriculture changes through changes in production practices and inputs, the evolution of new pathogens and other pests, and the development of new crops and products, genetic diversity becomes increasingly important. And, as well-managed germplasm collections grow, information documenting and evaluating those collections will increase. With modern data management tools, the ability of plant breeders to gain access to germplasm, and to identify useful characteristics in these collections, should improve, making germplasm increasingly well-integrated with breeding projects.
It is assumed that, in general, advances in biotechnology will make extant germplasm more valuable because, as discussed above under "gene pools," new methods of gene transfer (e.g., genetic transformation via electroporation or particle gun) can circumvent barriers to crossing, or hybrid inviability, making a broader range of germplasm available for crop improvement. Biotechnology also assists genetic resource management by providing (i) new disease detection methods via monoclonal antibodies (ELISA) and DNA probes; and (ii) tissue culture and cryopreservation methods that significantly enhance genetic resource managers' ability to conserve clonally propagated plants, and those with seeds that are difficult to store. On the other hand, biotechnologists rely on genetic resource managers for information regarding the available germplasm, its characteristics and problems, where collections are held, and how to gain access to them. Biotechnology, particularly the sub-field of genetic engineering, will probably not render managed germplasm collections obsolete in the near future because the collections provide genes that may ultimately serve as models for targeted molecular genetic synthesis. At present, biotechnologists are copying, transferring, and otherwise manipulating extant genes from nature, and will probably be limited to doing so for at least the short-term.
Recall that our concept of plant germplasm--from nucleic acids to populations/species--is very broad; consequently, the scope and context of "genetic erosion" and "germplasm vulnerability" are also quite extensive. In general, plant germplasm can be considered vulnerable for either the short- or long-term because it is (i) dangerously homogeneous genetically; or (ii) simply, imperiled by extinction.
Contemporary, elite germplasm has been considered particularly vulnerable to novel biotic (pests, diseases) and/or abiotic (weather or soil) stresses because of its relative genetic homogeneity. The southern corn leaf blight in the United States during 1970-71 serves as a classic example. Most commercial hybrid maize varieties in the U. S. then were produced with seed parents having a mitochondrial gene that not only conferred pollen sterility, but also susceptibility to a fungal phytotoxin, so all this elite maize germplasm was vulnerable to the disease. Consequently, this fungal epidemic reduced the U. S. national maize crop by ca. 15% in 1971; local or regional losses, especially in the South, were often much higher. During the next year, 1972, a severe winter in Ukraine reduced that region's winter wheat crop dramatically. During the preceding years of relatively mild winters, more than 15 million hectares had been planted in a single, relatively high-yielding variety, Bezostaja, which was not very winter hardy. When the cold winters returned in 1972, much of the winter wheat crop in Ukraine perished (Fischbeck 1981, cited by Ford-Lloyd and Jackson 1986).
The preceding cases, and many others, exemplify that elite germplasm with too narrow a genetic base has been vulnerable to epidemics. Perhaps more importantly, a narrow genetic base reduces the plants' long-term "fitness." For example, wild/weedy plants may lack the genetic variability required to evolve and adapt to changing abiotic and biotic conditions.
For elite germplasm with a reduced "genetic base," little genetic variation may be available for increasing yield via genetic gain. Germplasm that, when crossed with elite germplasm, displays exceptional hybrid vigor may become extinct before potentially novel heterotic patterns are even identified. Plant breeders may have no choice but to use genotypes with undesirable genetic linkages between agronomically favorable and deleterious genes. Without a diverse germplasm base, the efficiency with which breeders develop new varieties or hybrids tailored for new societal needs may be severely restricted.
For obsolete cultivars, landraces, weeds, or wild plants, genetic vulnerability generally refers to: (i) destruction of their "native" habitats, (ii) destruction of traditional agroecosystems and traditional human cultures, (iii) loss of ex situ collections, and/or (iv) reductions in their population sizes sufficiently severe that the germplasm is endangered with extinction either imminently, or in the long-term.
Finally, for germplasm conserved as nucleic acid fragments, genes, genetic stocks, and the like, genetic vulnerability may be equated to "genetic erosion" via the loss of the genes, etc., from the gene pool(s) via extinction of those individuals carrying them.
This diagram vividly illustrates the relative genetic diversity of wild and domesticated plants, stressing the narrowing genetic base of domesticates as compared to their wild relatives/progenitors. Note that the categories of germplasm employed are slightly different than those we presented above: weeds are not mentioned here, nor are wild plants that are used by humans, but have not been domesticated. Note this phrase: "Wild relatives and crops share common ancestors". The robust hybrid maize maturing in local fields may not have originated directly from its relatively anemic-looking contemporary wild relatives, the teosintes- rather it probably passed through some transitional stages which are now extinct. Note the qualifier "probably." The non-existence of clearly identifiable transitional forms, i.e., "missing links" between some domesticates and their wild relatives has led to many theories--some elegant, some rather bizarre--for the precise evolutionary paths that terminate in our major crops. We will discuss some of these theories later in the course.
The second box, labeled "land-races and primitive cultivars", has some deficiencies. First, women probably were instrumental in the domestication of most plants, yet they are not depicted here. Secondly, traditional or peasant farming, generally should not be considered primitive. As we shall see, in some ways it may be as complicated and sophisticated as highly mechanized farming with high yielding, improved seed. Elements of traditional farming are now being incorporated into the "low input sustainable agriculture" that is becoming increasingly popular in the Midwest. Third, one wonders why "selection over many generations" isn't classified as deliberate breeding? Yes, traditional farmers may not know genetics or statistics but, through their judicious selection programs, they domesticated plants in the first place!
Obsolete cultivars are not currently used commercially, but some reside in germplasm collections. Incidentally, the average commercial life-span of a maize hybrid in the U. S. is ca. 5 years, so hybrids and their constituent inbred lines become "obsolete", in the strictest sense, very quickly in U. S. agriculture.
Many would consider domesticated crop plants to be humanity's most valuable plant germplasm. This germplasm could be ranked according to its (i) total monetary value world wide, (ii) the monetary value per acre or hectare, (iii) the total number of acres/hectares planted to the crop or, as in this example, (iv) estimated annual world production as measured by weight in metric tons. Review this list, and view some web sites illustrating some of these species' morphology and habit.
1) Wheat (primarily Triticum aestivum, also Triticum turgidum, Poaceae): flour in bread, cakes, cereals, pasta. Cool temperate countries--U.S., Canada, the Former Soviet Union, China, northern Europe, southern South America. More on wheat from Valmont industries, and an image from the National Small Grains Collection. Genetic information about wheat can be found at GrainGenes, a genome database for the small grains and sugar cane.
2) Maize (Zea mays, Poaceae): flour in lesser developed countries, animal feed in developed countries. Tropics and temperate zones world-wide. Purdue University has articles (with pictures) on field corn, sweet corn and corn oil. Also try visiting the Corn Collection at the North Central Regional Plant Introduction Station and MaizeDB, the corn genome database.
3) Rice (Oryza sativa primarily, and Oryza glaberrima in Africa, Poaceae): human consumption primarily in lesser developed tropical countries, especially China, India, and other Asian countries. An image of rice can be found at the National Small Grains Collection and information on rice production and uses can be accessed at the University of Oxford. The RiceGenes database can provide genetic information on rice.
4) Potatoes (Solanum tuberosum ssp. tuberosum primarily, also Solanum tuberosum ssp. andigena in Andean South America, Solanaceae): human consumption, especially in cooler tropical and temperate zones. Facts about potato distribution, yield and use (with images) can be found at Max-Planck-Institut Fur Zuchtungsforschung. CGIAR provides a large set of images of potatoes, sweet potatoes, and cassava. Genetic information about potato (also tomato and pepper) can be found at SolGenes, the Solanaceae genome database.
5) Barley (Hordeum vulgare, Poaceae): animal feed, brewing malt, human consumption, cool temperate zones world-wide, most of it is consumed locally. A Hordeum image can be viewed at the National Small Grains Collection site. GrainGenes has genetic information on barley and other small grains.
6) Sweet potatoes (Ipomoea batatas, Convolvulaceae): tuber as vegetable and animal feed. Most important in lesser-developed countries of the tropics. More on the production and use of sweet potato roots at the University of Oxford. CGIAR provides a large set of images of potatoes, sweet potatoes, and cassava.
7) Cassava or manioc (Manihot esculenta, Euphorbiaceae): human consumption of rhizome, nearly exclusively in lesser-developed nations of South America, Africa, and southeast Asia, some industrial uses for starch. The University of Witwatersrand has an interesting site for information on cassava production and uses. CGIAR provides a large set of images of potatoes, sweet potatoes, and cassava.
8) Soybeans (Glycine max, Fabaceae): human consumption, industrial oil source, warm temperate and tropical nations, especially in eastern Asia and the Americas. Max-Planck-Institut Fur Zuchtungsforschung has a summary of soybean production with images. Also try Purdue University for information on Glycine max. Genetic information for soybeans can be sought at the SoyBase genome database.
9) Grapes (Vitis vinifera, Vitaceae): wine, table grapes, raisins, regions with Mediterranean climate; some in cool temperate zones. More on grape production and use at the University of Oxford. Find out more about grape germplasm at the Apple and Grape Germplasm Repository.
10) Oats (Avena sativa, Poaceae): human consumption, grain and foliage animal feed. Cool temperate, especially maritime, regions world-wide. Oat production and use is summarized at Max-Planck-Institut Fur Zuchtungsforschung and an image can be viewed at the National Small Grains Collection. The oat genome database is part of GrainGenes.
11) Sorghum (Sorghum bicolor, Poaceae): human and animal feed as a grain and as forage. Most important in semi-arid Africa, the Indian subcontinent, China. More about Sorghum via the Agricultural Research Service of the South Atlantic Area. SorghumDB is the genome database for sorghum genetic information.
12) Sugar cane (Saccharum officinarum, Poaceae): sugar and molasses, forage, ethanol production. Nearly exclusively tropical or sub-tropical, world-wide. More about sugar cane production at Purdue University. Genome information for sugar cane is contained in GrainGenes.
13) Oranges (Citrus sinensis, Rutaceae): fruit and juice for human consumption. Tropical and subtropical regions. Find information on sweet oranges at Purdue University and an image of an orange tree at the Vascular Plant Image Gallery.
14) Millet (Setaria, Echinochloa, Eleusine, Panicum, Pennisetum spp., Poaceae): human consumption as grain, flour, animal feed as grain and fodder. Primarily tropical and subtropical Africa, China, Indian subcontinent.
15) Bananas (Musa spp., especially Musa acuminata, Musaceae): human consumption of fruit, pantropical. More on bananas (with images) at Lychee woods. Also, links to other banana sites at the U.C. Fruit and Nut Research and Information Center Banana Index.
16) Tomatoes (Lycopersicon esculentum, Solanaceae): fruit and juice for human consumption. World-wide. Try the tomato page at Max-Planck-Institut Fur Zuchtungsforschung. More tomato production images can be found at the USDA Vegetable Laboratory. Genetic information can be found at the SolGenes genome database.
17) Sugar beets (Beta vulgaris, Chenopodiaceae): root provides sugar for human consumption and animal fodder. Cool-cold regions especially of Northern Hemisphere. Max-Planck-Institut Fur Zuchtungsforschung has a page on sugar beets, and one on beetroot.
18) Rye (Secale cereale, Poaceae): flour, foliage for forage, grain for alcohol. Primarily cool temperate regions of Northern Hemisphere. There is a Secale image at the National Small Grains Collection and information on Rye grain at Purdue. GrainGenes contains genome information for rye and other small grains.
19) Apples (Malus pumila, Rosaceae): fruit and juice for human consumption. Cool temperate regions world-wide. Try the multi-page apple site at 212.net. You can find some information about apple germplasm at the Apple and Grape Germplasm Repository.
20) Coconuts (Cocos nucifera, Arecaceae): fruit for human consumption as flesh or oil, fiber, timber, multi-purpose, strictly lowland tropical. Try the University of Oxford coconut page for more information. The following image of a coconut tree was obtained from the Agricultural Research Service Photo Library.
21) Cotton (Gossypium hirsutum primarily, also Gossypium barbadense, Malvaceae): fiber, oil from seed for human consumption, seed meal with various industrial uses, warm temperate regions, arid/semi-arid sub-tropics and tropics. An article can be found at APHIS providing cotton information, and Max-Planck-Institut Fur Zuchtungsforschung has a page of cotton facts, including images. CottonDB is the genome database for cotton this is also a good starting place for links to more cotton sites.
22) Peanuts (Arachis hypogaea, Fabaceae): human and animal feed as grain, cooking oil, some industrial uses. Tropical and warm-temperate regions, especially India, Africa, China. Peanut pages can be found at Purdue, and Max-Planck-Institut Fur Zuchtungsforschung
23) Yams (Dioscorea spp., Dioscoreaceae): root consumed as human and animal feed, source of drug precursors, tropical, especially West Africa. University of Oxford has a series of pages on the Dioscorea species D.alata, D. esculenta, D. cayenensis, and D. rotunda.
24) Watermelons (Citrullus lanatus, Cucurbitaceae): fruits and seeds consumed by humans, warm temperate, dry tropical and subtropical regions. Cornell University's Cucurbitaceae page has some images of watermelons and you can find information about watermelons at Max-Planck-Institut Fur Zuchtungsforschung and also at the Malaysia Ministry of Agriculture under the name tembikai.
25) Cole crops (Brassica oleracea, Brassicaceae): leaves, swollen buds, and inflorescenses consumed by humans, cool temperate zones world wide. Images of the Brassicaceae are held at the TAMU Image Gallery.
26) Onions (Allium cepa and other spp., Liliaceae): spice and vegetable for human consumption. Mediterranean and warm and cool temperate regions world-wide. Max-Planck-Institut Fur Zuchtungsforschung has an onion page, and Purdue has a set of pages on different onion types.
27) "Dry beans" (Phaseolus spp., Fabaceae): grain for human consumption, world-wide, especially important in lesser developed tropical nations especially in the Americas and Africa. North Dakota State houses BeanGenes, a genome database for the Phaseolus and Vigna species. BeanRef is a collection of external links to aspects of bean research.
28) Peas (Pisum sativum, Fabaceae): grain for human consumption world-wide, cool countries in Northern Hemisphere tropical countries, such as India, as a winter crop. For more on peas and pea production, including images, try Max-Planck-Institut Fur Zuchtungsforschung. CoolGenes (the cool season food legume genome database) contains genetic information about peas, lentils and chickpeas.
29) Sunflower (Helianthus annuus, Asteraceae): fruit for human and animal feed, oilseed, especially in Europe, Russia, and North America. Find out about the sunflower collection at the North Central Regional Plant Introduction Station, and find links there to other sunflower sites.
30) Mangoes (Mangifera indica, Anacardiaceae): fruit and juice for human consumption, exclusively in tropical regions, especially India. More on mangos (with images) at Lychee Woods.
There are many more World Wide Web sites containing information on production, uses, marketing, biology and genetics of these and other crop species. More documents can be found by spending a little time at one of the many WWW search sites such as Altavista, HotBot, Excite, Infoseek, Lycos, Magellan, WebCrawler, Galaxy, Nerdworld, Yahoo. (Note: The above sites are listed in probable order of usefulness in a search for scientific subject matter, but the same search run at different sites will not always return an identical list of documents).
Nine of the species on the preceding list are grasses, four are legumes, two belong to the Solanacaeae, and the other 15 each belong to a different angiosperm family. Plotkin (1988) notes that fewer than 20 of these species produce most of humanity's food. The top four crops--wheat, maize, rice, and potatoes--feed more than the next 26 crops combined. Plotkin notes further that most of the foods consumed world-wide is derived from plants that originated in the tropics.
This table from Harlan's book, Crops and Man, contains a fairly comprehensive list of major and minor domesticated plants, and covers especially well crops of the Near East and Africa, where Harlan traveled extensively. The coverage for the New World is less complete, so a table from Crop genetic resources in Central America, written by several experts at CATIE, the Centro Agronómico Tropical de Investigación y Enseñanza, in Costa Rica, is also included, as are several tables from Jorge León's 1987 book, Botánica de los cultivos tropicales, which contains information regarding many tropical cultivars.
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