European Crop Wild Relative Diversity Assessment and Conservation Forum
The Taxon Group Concept
What is the Taxon Group Concept?
The taxon group concept is introduced and explained by Maxted et al. (2006). It provides a classification system for the relationship between crops and their wild relatives.
Harlan and de Wet (1971) were the first to classify wild plants in relation to their associated crops using the gene pool concept. Within each crop complex there is a potential pool of genetic diversity available for utilisation and a gradation of that diversity dependent on the relative crossing ability between the crop itself and the primarily non-domesticated species in the primary, secondary or tertiary gene pool of the crop.
However, applications of this genetically based concept have been limited because more often than not, crossing ability and patterns of genetic diversity between crops and their wild related taxa are only known for the major crop complexes. The taxon group concept makes it possible to classify taxa even for species for which we have little or no genetic diversity data. The taxon group categories are as follows:
Taxon Group 1a - crop
Taxon Group 1b - same species as crop
Taxon Group 2 - same series or section as crop
Taxon Group 3 - same subgenus as crop
Taxon Group 4 - same genus as crop
Taxon Group 5 - different genus to the crop
An example of the taxon group concept compared to the gene pool concept is given in Table 1 for narbon bean (Vicia narbonensis var. narbonensis).
Table 1. Gene pool and taxon group concepts applied to the narbon bean (Vicia narbonesisis var. narbonensis)
Thus, in the absence of genetic information, it is possible to apply the taxon group concept as a proxy indicator of the degree of relationship between crop species and their wild relatives.
Application of the taxon group concept to Trifolium batmanicum Katzn.
Trifolium batmanicum Katzn. is an endemic Turkish annual clover species for which a CWR case study is provided in CWRIS. The purpose of this text is to explain the relationship between the wild T. batmanicum and its related crop, Trifolium repens L.
The genus Trifolium L. includes 237 species divided into eight sections with 49 subsections and series (Zohary and Heller, 1984). White clover, T. repens is placed in the largest section, Lotoidea. Species of this section have a basic chromosome number of 8 and many are polyploids. The section is considered the most primitive of the eight Trifolium sections and the other seven sections are thought to have evolved from Lotoidea.
Ideally the relationship between the crop and related taxa would be defined by applying the Harlan and de Wet (1971) gene pool concept, which states that those species that most easily hybridise with the crop are the crop’s closest wild relatives and those less easily hybridised with the crop are more remote wild relatives. Application of the gene pool concept implies that the necessary hybridisation experiments have been performed.
Attempts to produce inter-specific hybrids in Trifolium have met few successes and hybridisation has only been successful between species of the same section (Zohary and Heller 1984). T. repens is a tetraploid (2n=4x=32) and hybridises easily with T. uniflorum L., T. nigrescens Viv. and T. occidentale Coombe, and with greater difficulty with T. ambiguum M.B., T. isthmocarpum Brot. and T. hybridum L., all of which are in the same section Lotoidea (Caradus 1995). The first three species, T. uniflorum, T. nigrescens and T. occidentale can therefore be considered as belonging to the primary gene pool of T. repens, while the other three T. ambiguum, T. isthmocarpum and T. hybridum belong to the secondary gene pool.
However, no such hybridisation experiments have been undertaken for Trifolium batmanicum which has a chromosome number 2n=16. As such it is not possible to define the relationship between T. repens and T. batmanicum in terms of the gene pool concept. However, the taxonomic relationship between the two species is well known, each species falling into remote sections of the genus. Therefore, as T. batmanicum is placed in a different section to T. repens, it belongs in taxon group 3, as shown in Table 2. Being placed in taxon group 3 implies that T. batmanicum is a relatively distant relative of the crop T. repens.
Table 2. The taxon group concept applied to Trifolium batmanicum as a wild relative of Trifolium repens
Application of the taxon group concept means that it is now possible to establish the degree of relatedness for all crop species and their wild relatives. It should be noted that where hybridisation results are available, application of the gene pool concept has precedence over the taxon group concept, because the former is based on intrinsic genetic relationships and not the more subjective interpretation inherent in taxonomic classifications.
Abberton, M. (2005). Progress in breeding perennial clovers for temperate agriculture. Journal of Agricultural Science 143: 117-135.
Caradus, J.R. (1995). White Clover Trifolium repens L. In: Smartt, J. and Simmonds, N.W. (Eds.) Evolution of crop plants. Second Edition. Longman Group UK
Harlan, J. and J. de Wet (1971). Towards a rational classification of cultivated plants. Taxon 20: 509-517.
Maxted, N., B.V. Ford-Lloyd, S.L. Jury, S.P. Kell and M.A. Scholten (2006). Towards a definition of a crop wild relative. Biodiversity and Conservation 14: 1-13.
Zohary, M. and D. Heller (1984). The genus Trifolium. The Israel Academy of Sciences and Humanities, Jerusalem.
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