CROP WILD RELATIVES
Crop wild relatives (CWR) are taxa closely related to crops and are defined by their potential ability to contribute beneficial traits for crop improvement; for example, to confer resistance to pests and diseases, improve tolerance to environmental conditions such as extreme temperatures, drought and flooding, and to improve nutrition, flavour, colour, texture and handling qualities . A working definition of a CWR based on the Gene Pool concept or, in the absence of crossing and genetic diversity information, the Taxon Group concept1, has been proposed:
‘‘A crop wild relative is a wild plant taxon that has an indirect use derived from its relatively close genetic relationship to a crop; this relationship is defined in terms of the CWR belonging to gene pools 1 or 2, or taxon groups 1 to 4 of the crop’’.
Genetic erosion is a key problem for CWR. What is genetic erosion?
Genetic erosion is a fundamental problem for CWR and has been referred to in the literature as the permanent reduction in richness (total number of alleles) or evenness (i.e. spread of allelic diversity)3 of common local alleles, or the loss of combinations of alleles over time in a defined area4. Genetic erosion can affect wild populations conserved in situ and ex situ collections (i.e. when the ex situ collection goes through the regeneration process and are inadvertently selected to suit the regeneration site). It is important to distinguish genetic changes that are
detrimental to populations from the ‘normal’ background levels of change4. Any loss of genetic erosion means the individual is less likely to be able to adapt to their changing environment and means potentially useful traits are unavailable to the breeder.
Why are CWR threatened?
There are numerous factors that negatively impact wild plant populations resulting in genetic erosion, and potentially eventual loss (extinction) of taxa (varieties, subspecies, and species).
The main factors that contribute to the genetic erosion of CWR diversity include: Expansion of the human population (which leads to the unequal and
unsustainable use of natural resources, and is the basis of all other threats);
Climate change which is expected to directly affect the cropping patterns and extinction of wild plant species, particularly in drier regions where certain CWR
may already be at the edge of their distribution;
Habitat destruction, degradation, homogenisation and fragmentation;
Changes in agricultural practices, soil and land use;