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a USDA-ARS Plant Genetic Resources Conservation Unit, Univ. of Georgia, Griffin, GA 30223-1797
b USDA-ARS National Temperate Forage Legume Germplasm Resources Unit, Washington State Univ., Prosser, WA 99350-9687
* Corresponding author (bmorris{at}gaes.griffin.peachnet.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONDefining the Scope of...Defining a Broadbase Collection...CONCLUSIONREFERENCES |
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Abbreviations: CGC, Crop Germplasm Committee • NPGS, National Plant Germplasm System • USFWS, U.S. Fish and Wildlife Service
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONDefining the Scope of...Defining a Broadbase Collection...CONCLUSIONREFERENCES |
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200 species collected or donated from more than 90 countries (GRIN, 2000). The current NPGS Trifolium collection reflects the historic objectives of plant introduction, crop improvement, and preservation of cultivated germplasm (Greene, 1998). Sixty percent of the collection consists of accessions representing cultivated species. Of the cultivated species, 35% of the accessions represent Trifolium pratense L., 21% represent T. repens L., and 18% represent other minor cultivated species. Representation of species considered to be gene sources for the cultivated species is limited. Only 4.5% of the collection represents species that are closely related to red and white clover. The NPGS Trifolium collection stands out in its representation of wild species. About 88% of the species in the genus have been acquired and added to the collection in the last 20 yr, reflecting priorities established in the 1970s (Taylor et al., 1977). The collection also contains 12 accessions of T. stoloniferum Muhl. Ex Eaton, and one accession each of T. amoenum Greene and T. trichocalyx A. Heller. These species have been listed as endangered by the U.S. Fish and Wildlife Service (USFWS). The 1997 IUCN Red List of Threatened Plants (Walter and Gillett, 1998) cites 39 Trifolium species as having a world status of threatened. Fourteen of these species are represented in the NPGS collection. Forage species in general have received a high priority for in situ conservation (IBPGR, 1984). Although 33 species of Trifolium are endemic to the USA, no in situ efforts are being supported by the NPGS. Limited representation of gene-source species, minor-use species, and threatened species suggest the collection could be diversified to more effectively serve the needs of forage improvement, the development of new uses for traditional forage species (value-added products), the diverse needs of botanical research, as well as serve a stronger conservation role.
Greene and Morris (2001) outlined a strategy for diversifying germplasm collections so they more effectively meet multiple-use objectives, yet remain manageable in size. This strategy will be used to develop a plan for diversifying the NPGS Trifolium collection. The term multiple-use describes a germplasm collection that has value to a diverse array of users and contributes to the conservation of plant biodiversity. Although clover is considered relatively minor, it has characteristics that are common to many crops, which makes it a relevant example for illustrating the strategies outlined in Greene and Morris (2001). This article will also serve the Trifolium community as a reference that outlines the gene pools present in this genus, explores current and future uses for the crop, and documents current and future threats to genetic erosion.
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Defining the Scope of the Trifolium Collection |
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TOPABSTRACTINTRODUCTIONDefining the Scope of...Defining a Broadbase Collection...CONCLUSIONREFERENCES |
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60% of the Trifolium species are native to Eurasia. Zohary and Heller (1984) list 7% of the species native to Euro-Siberian–Irano-Turanian. Generally, the Eurasian species tend to be centered in the Mediterranean region. Most countries bordering the Mediterranean possess some endemic species, the greatest numbers being found in Turkey (100) and Bulgaria (67) (Pederson et al., 1999). Sub-Saharan Africa serves as a native habitat for about 15% of the species. Most of the North African species are in the Mediterranean region. The majority of the non-Mediterranean species are native to the Eritreo-Arabian province, primarily in the subalpine and alpine highlands of Ethiopia, Sudan, Eritrea, Kenya, Tanzania, and Uganda. Approximately nine species are native to South America, and their range of distribution extends from southern Peru along the Andes Mountains to Bolivia and Chile.
About 21% of the species originated in the North American continent. Distribution extends from British Columbia and Vancouver Island, southward through the western USA and central Mexico. Some species have spread east toward the delta of the Missouri and the Appalachian mountains.
Less than half of all species (48%) have limited global distribution. Thirty-one percent have a narrow global distribution, with the greatest number (33 species) restricted to the western USA (USDA/NPGS, 1995).
The basic chromosome number in 157 of the examined species is n = 8. Other basic numbers are n = 7 (17 species), n = 6 (1 species), and n = 5 (4 species). While a basic number of n = 8 has been recorded for 47% of the annuals, species with n = 7 are mostly annuals and counts of n = 6 and n = 5 were noted for European and Mediterranean annuals. Of the 118 perennial species investigated, 31 are either polyploid or display polyploid and diploid strains, while only 7 annuals out of 180 species are polyploid (Zohary and Heller, 1984). Taylor et al. (1979) studied Trifolium genetic system relationships. Of the species investigated, they found all species with 2n = 10 chromosomes were annuals, and species with 48 to 130 (2n) chromosomes were perennials. In addition, they reported that species with 10 and 12 (2n) chromosomes were self-pollinated and that self-pollinated species were primarily from the Mediterranean climate. Species with 2n 
32 were cross-pollinated, and rhizomatous species were mostly cross-pollinated.
Considering (i) the high levels of genetic diversity expected among and within species due to wide geographic distribution, (ii) the various modes of reproduction and ploidy levels, and (iii) the strong representation of wild species in the existing collection, a reasonable scope for the NPGS Trifolium collection would be to include all species representing the genus. This is consistent with Taylor et al.'s (1977) suggestion to give high priority to obtaining a complete collection of all nonrelated, noncultivated species of the genus.
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Defining a Broadbase Collection for Trifolium |
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TOPABSTRACTINTRODUCTIONDefining the Scope of...Defining a Broadbase Collection...CONCLUSIONREFERENCES |
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Primary Gene Pool of the Two Major Cultivated Species
To determine the size and relative importance of the various components of the primary gene pool, a review of the domestication and agronomic history of the cultivated species is essential. We list the important literature below for those interested in further information on the history of the cultivated clover species. Although 16 species are cultivated, in this article we will focus primarily on defining the primary gene pools for red and white clover.
Taylor (1985) discussed the agronomic history of T. pratense, T. repens, T. hybridum L., T. incarnatum L., T. alexandrinum L., and T. subterraneum L. More detailed discussions for T. pratense can be found in Piper (1924), Smith et al. (1985), Taylor and Smith (1995), and Taylor and Quesenberry (1996). Discussions for T. repens can be found in Gibson and Cope (1985) and Pederson (1995). Annual clover species are discussed in Piper (1924), Knight (1985a)(1985b), Love (1985), McGuire (1985), Miller and Wells (1985), Hoveland and Evers (1995), Steiner et al. (1998). The minor perennial species are discussed in Piper (1924), Townsend (1985), and Pederson (1995).
Unlike many domesticated crop species, red and white clover are relatively recent domesticates. Since they are used primarily as forage, they remain similar to their wild counterparts (Harlan, 1983). Considering this, we can classify the primary gene pools of red and white clover into five subpools, ranging from modern cultivars to undomesticated wild forms of the same species. Current and obsolete cultivars developed by plant breeders for intensely managed pastures and hay fields can be considered the first component in the primary gene pool. Selection-based populations such as regional strains and breeding populations are a second component of the primary gene pool. Landraces originating from wild populations in natural pastures managed by farmers make up the third component. The fourth and fifth components consist of germplasm from either naturalized populations that escaped from cultivation, or naturally occurring endemic wild populations. Considering that cultivated forms in most parts of the temperate zone have probably contaminated wild forms, it may be difficult to actually separate the fourth and fifth subpools.
The relative size of the subpools within the primary gene pool can also be inferred from crop history. Taylor (1985) suggested that red and white clover were domesticated in the 18th century, a recent occurrence compared with other crop species. Therefore, we would expect the first and second components, which include cultivars and selection-based populations, to be relatively larger compared with the third component, which represents landrace material. We would expect the fourth component to contain a relatively large amount of germplasm since the cultivated species have escaped to form naturalized populations across a large geographic range. Populations representing wild forms of the cultivated species are probably rare because of their limited geographic distribution and the probable hybridization between wild, cultivated, and naturalized populations.
The relative value of each subpool within the primary gene pool can also be inferred from the literature. For example, Rumbaugh (1990)( 1991) reported that the majority of forage-legume cultivars developed in the USA have been derived from cultivars, selected strains, and landraces. From this we would consider the first, second, and third subpools to be particularly valuable for improving clover in the traditional manner (e.g., as forage and pasture). The fourth and fifth components contain a tremendous amount of diversity, which may yield new adaptive traits of interest to plant breeders (Pederson and Brink, 1998; Brink et al., 1999). Although the literature provides needed information, the task of ultimately determining the relative value of each subcomponent of the primary gene pool (a necessary step in determining the number of accessions needed to represent each component) needs to be carried out by a diverse group of stakeholders to ensure collection priorities remain consistent with the objectives of multiple-use.
Defining the Remaining Gene Pools
Generally, plant breeders use the secondary gene pool if they cannot find desirable alleles in the primary gene pool, and then turn to the tertiary gene pool only after exhausting the secondary pool (Fehr, 1987). For each of the cultivated species, Table 1 lists the related species in the secondary and tertiary gene pools on the basis of their ability to hybridize with the cultivated Trifolium species as reported in the literature. For T. pratense, T. diffusum Ehrh., and T. pallidum Waldst. and Kit. qualify as members of the secondary gene pool, while T. alpestre L., T. heldreichianum (Gibelli and Belli) Hausskn., T. medium, T. noricum Wulfen, and T. rubens L., appear to qualify as members of the tertiary gene pool. For T. repens, T. argutum Banks and Sol., T. nigrescens, and T. uniflorum L. qualify as members of the secondary gene pool, and T. ambiguum and T. isthmocarpum Brot. qualify as members of the tertiary gene pool.
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200 species. In terms of biological diversity, the quaternary pool would contain the greatest level of genetic variation. Although we have suggested that the scope of the Trifolium collection could realistically contain representatives of each species within the genus, the actual number of accessions needed to represent each species would need to be determined through careful dialogue between collection managers, a diverse range of collection users, and others interested in the conservation of the genus. The dialogue would need to center on determining a "relative value" for each species, based on perceived usefulness or need for protection. Greene and Morris (2001) discussed the approach of assigning a value to each species using a set of weighting factors that are developed by a diverse group of stakeholders. The number of accessions representing each species would then be proportional to the aggregate of weighting factors. These factors need to focus on criteria that indicate the relative value of species in terms of usefulness and vulnerability.
Defining Areas in the Collection that Justify Greater Representation of Diversity
Information on species use, and current and future trends in the use of cultivated and wild species, help identify areas in the collection where a greater number of accessions is justified by high user interest. Additionally, information on the vulnerability of Trifolium species helps identify areas in the collection where a greater number of accessions will ensure the collection makes a significant contribution to conservation efforts. We briefly review the literature to illustrate how it can help us define areas within the Trifolium collection that justify greater representation in terms of number of accessions.
Trifolium Species Having Agronomic Importance
Currently, 16 species are cultivated worldwide (Taylor and Quesenberry, 1996). Clovers are used for soil enrichment and livestock herbage (Taylor et al., 1977), as bee plants and ornamentals (Duke, 1981), for erosion control, as cover crops (Hanson, 1974), and as gene sources for pest resistance (Cope and Taylor, 1985). In the past, clovers have also been used for human food (Glob, 1970). Ranked in general descending order of agricultural importance, the major taxa used as forage are T. pratense L. and T. repens L. The taxa T. incarnatum L., T. vesiculosum Savi., T. hybridum L., T. subterraneum L., T. hirtum All., T. alexandrinum L., T. medium L., T. lappaceum L., T. nigrescens Viv., T. fragiferum L., T. ambiguum M. Bieb., T. resupinatum L., T. glomeratum L. , and T. dubium Sibth. have importance as forage (Taylor and Quesenberry, 1996). Table 2 lists the somatic chromosome number, life form, geographic distribution, habitat, and use of the major and minor cultivated species in Trifolium.
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A second emerging area of interest is the use of clovers in remediating soils contaminated with nitrogen, phosphorus, or even heavy metals. While the current techniques of removing and disposing of contaminated soil have been estimated at costing two million dollars ha-1, using crops to remove or degrade soil contaminants has been estimated to cost 2 to 4 orders of magnitude less (Cunningham and Ow, 1996). The effective use of clovers in reclaiming land such as mine spoils has been recognized for decades. The wide adaptability of clovers suggests that genetic diversity may be available and should be exploited to develop cultivars that are particularly effective for phytoremediation.
Perhaps the most significant new use of clovers has been in the pharmaceutical area. Studies have indicated that dietary phytoestrogens play an important role in preventing menopausal symptoms, osteoporosis, cancer, and heart disease (Kurzer and Xu, 1997). Phytoestrogens have been identified in several common clover species including T. montanum L., T. fragiferum, T. incarntum, T. repens, T. alpestre (Vetter, 1995), T. pratense (Beckstrom-Sternberg and Duke, 1994; Vetter, 1995), and T. subterraneum (Vetter, 1995). Novogen, an Australian pharmaceutical company, is marketing a phytoestrogen product directly derived from T. pratense.
Although the literature suggests that current trends in use would justify the addition of accessions that were particularly effective at taking up nitrogen or produced high levels of phytoestrogens, the suitability of adding these types of accessions needs to be carefully weighed against other collection needs to ensure that the collection can be maintained under available funding. Again, a diverse interest group needs to be engaged to determine priorities that support collection objectives of multiple-use.
Genetic Vulnerability in Trifolium
The key to efficient and effective conservation is to identify germplasm that is vulnerable to genetic erosion, obtain population samples that adequately represent local variation, and maintain the samples using methods that minimize change in the genetic structure of the populations. Once again, diverse stakeholders need to work together to establish priorities to ensure that the collection remains balanced in serving the needs of users and conservation with a manageable number of accessions. A review of the literature and databases reveals species and genetic vulnerability in the genus.
Currently, 10 species endemic to the USA have been identified as being threatened. These include T. amoenum Greene, T. stoloniferum Muhl. ex Eaton, T. trichocalyx Heller, T. andinum var. podocephalum Nutt., T. calcaricum J. L. Collins and Wieboldt, T. haydenii var. barnebyi Porter, T. leibergii A. Nelson and J. F. Macbr., T. polyodon Greene, T. kingii subsp. rollinsii J., and T. thompsonii (Walter and Gillett, 1998). Sixteen Old World taxa are known or suspected to be endangered, vulnerable, or rare worldwide (Table 3). These species are endemic to Turkey, Greece, Italy, Ethiopia, and Yugoslavia. The species known as T. acutiflorum is thought to be extinct in the wild (Walter and Gillett, 1998).
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The genetic diversity in native and naturalized ecotypes of the major cultivated Trifolium species is also threatened by changing land-use patterns. These populations are vulnerable since they are commonly found in areas that are likely to be disturbed. The substitution and abandonment of Trifolium landraces and primitive varieties, brought about by the use of highly productive forage crops such as alfalfa (Medicago sativa L.) and increased use of elite Trifolium cultivars, may also contribute to the loss of variation representing ecogeographic adaptation to less optimal environments.
Although we have identified conservation efforts needed to protect species and intraspecific diversity within the genus Trifolium, we are not suggesting that the NPGS collection focus on conservation efforts at the expense of providing germplasm useful to plant breeders and other collection users. A rational approach would be to establish priorities that ensure that the collection serves diverse users and plays a conservation role. For example, dialogue between collection managers, users, and those interested in conservation may determine that the NPGS collection can most effectively serve to conserve wild species that are endemic to the USA by preserving seed ex situ and assisting with in situ efforts. Alternatively, lack of conservation efforts outside the USA may suggest that the NPGS collection should play a stronger global conservation role. A key point is that dialogue needs to occur among the diverse groups having an interest in using and conserving Trifolium genetic resources. And importantly, any conservation efforts need to complement efforts being carried out by other agencies and institutes.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONDefining the Scope of...Defining a Broadbase Collection...CONCLUSIONREFERENCES |
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Greene and Morris (2001) discussed the importance of prioritizing the changes needed to diversify germplasm collections. Prioritization is essential to develop collections that effectively and efficiently serve multiple uses, yet remain a manageable size. Throughout this article we have stressed the need to have a divergent group of collection stakeholders engaged in the prioritization process. Because the current NPGS budget for forage-legume germplasm is barely sufficient to maintain the collections at their current size, and because resources are no longer available to support collection evaluation, prioritization is essential. The challenge faced by those having an interest in the NPGS forage-legume collections is to assign priorities, recognizing that funding is currently not adequate to support collection growth.
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ACKNOWLEDGMENTS |
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Received for publication October 12, 1999.
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TOPABSTRACTINTRODUCTIONDefining the Scope of...Defining a Broadbase Collection...CONCLUSIONREFERENCES |
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