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Diversity analyses: genetic data analysis of priority CWR

Wy should we consider genetic diversity of priority CWR in conservation planning?

The ultimate aim of plant genetic resources conservation is to ensure that its diversity is efficiently conserved over time. Diversity includes both species diversity and the genetic diversity within species. The intrinsic value of CWR, being a source of adaptive genes for crop improvement, emphasizes the importance of conserving their genetic breadth so this is available for utilization in crop breeding programmes. Genetic diversity information can be used for: (i) assessing genetic diversity across the geographic breadth of the species, (ii) obtaining genetic baseline information against which to detect changes in diversity and identify genetic erosion  [1], (iii) targeting CWR populations for in situ and ex situ conservation and (iv) identifying traits of interest for crop improvement. Therefore, considering either existing CWR genetic diversity data or undertaking novel genetic diversity studies in order to aid conservation planning is of utmost importance. Note that often these data are not available or there are no (economic or human) resources to undertake novel genetic diversity studies. In these cases, ecogeographic diversity  [2] can be used as a proxy for genetic diversity, the premise being that conserving the widest possible ecogeographic range of populations of a species will maximize the overall genetic diversity of the species conserved.

1. Assess the genetic diversity within the target taxon. Typically, conservation biology aims to conserve the maximum number of species and numbers of individuals within a species. However, the conservation of intrinsic genetic diversity within a taxon has been identified as equally important. The genetic diversity within a species represents not only a potential exploitable resource for human utilization but also encompasses the species’ evolutionary potential to evolve and adapt within a changing environment. Therefore, when assessing genetic diversity, it is important to identify the allelic richness (relative number of different alleles) and evenness (frequency of different alleles) across the geographic breadth of the species. Nevertheless, it should be emphasized that what we should really consider in CWR conservation planning is the adaptive diversity rather than neutral genetic diversity  [3]. However, adaptive studies are time consuming and expensive and the alternative approaches are either to use neutral genetic diversity or ecogeographic diversity as a proxy for adaptive genetic diversity.

Wild barley (Hordeum spontaneum K. Koch) collected in Jordan and germinated for leaf tissue collection needed for DNA extraction and genetic diversity analysis. (Photo: Imke Thormann)
Leaf tissue obtained from germinated seeds of wild barley (Hordeum spontaneum K. Koch) collected in Jordan, needed for DNA extraction and genetic diversity analysis. (Photo: Imke Thormann)

2. Establish genetic baseline information. An understanding of the pattern of allelic richness and evenness across the geographic breadth of the species establishes a relative baseline against which change can be measured. Just as population ecologists measure demographic changes in population number, so population geneticists measure changes in allelic richness and evenness over time. Like demographic changes in population number, changes in allelic richness and evenness over time can occur naturally. By monitoring genetic change, natural changes can be distinguished from those associated with adverse population management that result in genetic erosion genetic erosion  [1] and would ultimately lead to population extinction. Establishing the genetic baseline and assessing genetic diversity regularly over time assessing genetic diversity regularly over time  [6] enables these deleterious changes to be detected early and population management changes to be implemented before there is significant genetic erosion.

3. Target CWR populations for conservation. The selection of priority populations for in situ and ex situ conservation based on genetic criteria has been debated for a long time (e.g. Marshall and Brown 1975, Lynch 1996, Petit et al. 1998, Delgado et al. 2008). The amount and patterns of genetic diversity both within and among populations of a species, genetic population structure, and common and localized alleles  [7] are some of the data that can be useful when prioritizing populations for conservation. For instance, if a particular CWR is genetically homogenous or if the partitioning of genetic diversity is considerably higher within, rather than among, populations, then a limited number or even a single genetic reserve may be enough to efficiently conserve the species (for example, the population with higher genetic diversity and with the highest number of common and localized alleles). However, if different populations of the same CWR are genetically distinct, or if the partitioning of genetic diversity among populations is high—indicating significant differentiation among populations—multiple genetic reserves would probably be needed to ensure that all genetic diversity within that particular CWR is conserved. It is also important to take into account that, even in cases where there is only a small fraction of genetic differentiation among populations, this diversity can be very important as it may contain adaptive traits which are critical for the species’ ability to inhabit different environmental conditions. This is particularly important when considering the conservation of populations in the margins of a species’ range, due to the need for species to adapt to changing environmental conditions brought about by climate change.

4. Identify traits of interest for crop improvement. CWR adaptive traits form the functional component of genetic variation relevant for crop improvement. Ultimately, the identification and utilization of these traits justifies the conservation of CWR.

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