Crop gene pool methodology
Step 2: Selection of target taxa
In general, it is not practical to conserve all the taxa within the crop gene pool due to resource limitations; therefore, we need to prioritize and select taxa from the list that will be proposed for active conservation. Factors that can be used to ascribe ‘value’ and establish conservation priorities include (Maxted et al., 1997):
- Current conservation status
- Socio-economic use
- Threat of genetic erosion
- Genetic distinctiveness
- Ecogeographic distribution
- Biological importance
- Cultural importance
- Cost, feasibility and sustainability
- Ethical and aesthetic considerations
- Priorities of the conservation agency
For CWR, an initial, simple prioritization on the basis of socio-economic use of the associated crop and relative threat has been proposed (e.g., Ford-Lloyd et al., 2008a; Magos Brehm et al., 2008). In addition to these two factors, Maxted and Kell (2009) proposed that within each crop gene pool, the closest wild relatives should be afforded higher conservation priority over the more distantly related species because these are the taxa that can more easily be used in crop improvement using conventional breeding methods. However, the literature on the taxa within the target crop gene pool should be thoroughly searched to check for cases where a more distantly related taxon has been highlighted as a gene donor (or potential gene donor), and these taxa should also be afforded conservation priority. Of these prioritized taxa, those in most urgent need of conservation action (i.e., those with a very limited geographic range (often rare or endemic taxa) and/or known to be under threat), are given precedence.
This methodology therefore primarily targets the taxa that are most closely related to the crop species (or that have shown promise in crop improvement programs) and that are threatened or have restricted distribution ranges. However, ideally, national and regional in situ networks of CWR genetic reserves should in the long term be expanded to ensure that all taxa of potential importance for crop improvement are actively conserved. In particular, selected populations of the primary and secondary wild relatives that are widespread and common should be actively conserved throughout their range, ensuring that populations representing the extremes of the range (both geographically and topographically) are conserved. Individual populations of these taxa may harbour important genes adapted to particular environmental conditions—genes that may confer important traits to improve crops in the future. Populations of these taxa that already occur within protected areas should also be monitored. In some cases, it may be possible to establish a reserve that conserves multiple CWR taxa, which, when possible, has obvious advantages.
There are two stages to the selection of target taxa: a) creation of a level 1 prioritized list based on actual or potential use as gene donors, and b) creation of a level 2 prioritized list based on threat and/or distribution. In this methodology, the two steps are presented sequentially (i.e., the level 2 prioritized list is based on the level 1 prioritized list). The advantage of this approach is that in cases where there is limited information on the distribution of the taxa and/or for gene pools containing a very large number of taxa, the level 1 prioritization narrows the list of taxa down to those that are likely to be most important as gene donors for crop improvement and further information is only sought for that list of taxa. The disadvantage of this approach is that some of the more distantly related taxa in the gene pool that are threatened or have restricted distributions may be missed in the conservation planning process. Therefore, in cases where a gene pool contains a relatively small number of taxa or where distribution data is readily available for all the taxa, it is desirable to undertake the prioritization in the reverse order by collating threat and distribution data on all taxa in the gene pool first, then applying the second level of prioritization based on potential use as gene donors. Using this approach, more distantly related taxa that are threatened or have restricted distributions can be highlighted as a conservation priority on that one criterion, and even though they may still not be given the highest level of priority for immediate conservation action, they may be promoted as candidates for conservation at a later date. Furthermore, if it is not immediately possible to put in place in situ conservation measures for these taxa, they can be earmarked for collection and storage in ex situ collections.
(i.e., those with a very limited geographic range (often rare or endemic taxa) and/or known to be under threat), are given precedence.
- Gene Pool concept (Harlan and de Wet, 1971)
Step 2 actions – a) create a level 1 prioritized taxon list
At this stage, the aim is to prioritize the taxon list on the basis of value or potential value as gene donors for crop improvement, as follows:
- Organize the list of all taxa within the crop gene pool according to their degree of relationship to the crop. To achieve this, search the available literature on the crop complex. Taxa should be organized into a table showing primary, secondary or tertiary wild relatives using one of the following three methods:
Select the taxa from the categorized list that occur within the geographical area of the CWR conservation strategy by matching against the geographically defined list of taxa created under Step 1. You will now have two tables, both categorized according to their degree of relationship to the crop—one showing all taxa in the crop genus, the other showing the taxa that occur within the geographical area defined by the CWR conservation strategy.In both tables, flag the taxa that have been identified as gene donors or potential gene donors. To achieve this, consult online information sources, such as GRIN Taxonomy and/or carry out literature searches using online library databases. Specialist libraries can also be consulted, such as those housed in gene banks, botanic gardens and other research institutes. Consult the data sources page for further information. Provide references for any taxa flagged as gene donors or potential gene donors in the table. These may be in the tertiary list, as well and the primary and secondary lists. Make a separate list of the primary and secondary wild relatives and any tertiary wild relatives that have been identified as gene donors or potential gene donors from the list of taxa within the geographical area defined by the CWR conservation strategy. This list (level 1 priority list) now forms the basis for the second level of prioritization under Step 2b.
- Where genetic information is available and taxa have been classified using the Gene Pool concept (Harlan and de Wet, 1971), organize the taxa into the table listing those in GP1B as primary wild relatives, those in GP2 as secondary wild relatives and those in GP3 as tertiary wild relatives. The Gene Pool concept works like this:
- GP1A: cultivated forms of the crop
- GP1B: wild or weedy forms of the crop
- GP2: the coenospecies (less closely related species) from which gene transfer to the crop is possible but difficult using conventional breeding techniques
- GP3: the species from which gene transfer to the crop is impossible, or if possible, requires sophisticated techniques, such as embryo rescue, somatic fusion or genetic engineering.
- Where genetic information is not available, if possible, substitute the Gene Pool concept with the Taxon Group concept (Maxted et al., 2006), which provides a proxy for taxon genetic relatedness. Organize the taxa into the table listing those in TG1b as primary wild relatives, those in TG2 as secondary wild relatives, and those in TG3 and TG4 as tertiary wild relatives. The Taxon Group concept works like this:
- TG1a: crop
- TG1b: same species as crop
- TG2: same series or section as crop
- TG3: same subgenus as crop
- TG4: same genus.
- For crop genera that have not been classified using the Gene Pool concept and not sub-classified into sections and subgenera, the available information on genetic and/or taxonomic distance must be analysed to make reasoned assumptions about the most closely related taxa.
- Whichever system is used, ensure that references are provided to substantiate the assumptions made about taxon relatedness.
Step 2 actions – b) create a level 2 prioritized taxon list
At this stage, the aim is to prioritize the level 1 priority taxon list on the basis of threat status. However, as already noted, this level of prioritization can be applied to all taxa listed under Step 1, followed by prioritization on the basis of actual or potential value for crop improvement, depending on the size of the gene pool and on the availability of distribution and/or threat data. Therefore, it is also possible to carry out the prioritization process in reverse order (i.e., by carrying out Step 2b followed by Step 2a).
There are two steps to this selection process.
- Select the taxa in the level 1 priority list (or Step 1 list) that are known to be under threat. Where existing evidence is available to show that a taxon in the list is under threat, these taxa should automatically be prioritized for conservation action. To find out whether a taxon is known to be under threat:
Select the taxa in the level 1 priority list (or Step 1 list) that are not known to be under immediate threat, but are known to have limited distribution ranges. At this stage, a degree of objectivity is required, since there is no clear dividing line between a taxon with a limited distribution range and one with a distribution range that is deemed to enable ‘classification’ of the taxon as one not in immediate need of conservation action, unless very detailed information is already available about the taxa. However, where the range of a taxon is known, the methodology proposed by Ford-Lloyd et al., (2008a,b) can be used as a guide when establishing taxon conservation priorities at regional level (e.g., across Europe). Generally speaking, if a taxon is recorded as occurring in several countries or subnational units throughout the region, it is less likely to be under threat. Taxa that are known to be endemic to a country or subnational unit or those that occur in only a few countries or subnational units are more likely to be under threat at regional level. Similarly, at national or subnational level, available information must be gathered on the range of the taxa in order to establish which are most likely to be threatened by their limited distribution range.
- Search the IUCN Red List of Threatened Species to find out if any of the taxa are listed.
- Where possible, consult national or regional Red Lists to see whether a taxon in the list is considered threatened at national or regional level.
- If a taxon is not included in the IUCN Red List, or in a regional or national Red List (i.e., it has not been formally assessed using a set of Red List criteria), it does not mean that it is not threatened. A literature search may reveal important information about the threat status of a taxon. To achieve this, carry out literature searches using online library databases. Specialist libraries can also be consulted, such as those housed in gene banks, botanic gardens and other research institutes.
- Consult the data sources page for further information and advice on how to carry out these searches.
To find the distribution range of the taxa:
- At European level, the Crop Wild Relative Catalogue for Europe and the Mediterranean (Kell et al., 2005)—which can be accessed via the Crop Wild Relative Information System (CWRIS)—provides a list of country or subnational occurrences for taxa throughout Europe.
- At national or subnational level, a good starting point is GBIF, which provides known taxon occurrences through data provided by organizations managing biodiversity data around the world (e.g., ex situ germplasm collections and herbarium data). For a finer level of detail, search available national databases and literature sources. Taxon specialists, conservation agencies and protected area managers are further important sources of information.
- Consult the data sources page for further information and advice on how to carry out these searches.
After carrying out Step 2, you now have a reduced list of taxa that have been selected on the basis of their value as gene donors and relative threat. This list of target taxa now forms the basis for immediate conservation planning for the crop gene pool.
Crop gene pool methodology introduction > Step 1 > Step 2 > Step 3 > Step 4