by Emma J Devereux
Citation: Devereux, E.J.,(2021), “Sustainable Agri Genetics: Crop Wild Relatives (CWRs) & Crop-to-Wild Gene Flow”, EcoFoodDev, https://www.ecofooddev.com/sustainable-agri-genetics-crop-wild-relatives-cwrs%e2%80%af-crop-to-wild-gene-flow/
Crop Wild Relatives are wild plant species that evolved with, co-occur with, and are genetically related to, domesticated plants. They could also be regarded as the ancestors of modern crops. These wild species have not been the focus of human management or modification, have not passed through the genetic bottleneck of domestication, and thus continued to evolve and develop beneficial traits, such as salt tolerance, drought tolerance or pathogen resistance. These traits can be of benefit to agriculture as Crop Wild Relatives can be cross-bred with economic crops (such as wheat and barley) to develop crop varieties with these favourable characteristics. The ultimate goal is that these species will not be heavily dependent upon inputs and will be more environmentally sustainable. It is therefore within this field that archaeobotany/paleoecology can be of immense relevance to modern agricultural and plant research, as ancient plant community composition and behaviour, the origins of agriculture, and ancient ecosystems are the subject matter of environmental archaeologists/paleoecologists.
It is a fact that crop modification occurs. Most modern crops are hybrids that have either been produced by breeding programmes or domesticated from naturally occurring polyploids (Bevan et al, 2017). Genetic methods of modification were first applied to crop improvement in the early twentieth century and involved selection for plants with preferred phenotypes (observable physical properties of an organism) that were also highly inheritable. DNA technologies are today widely applied to improve the efficiency of crop breeding. Large numbers of informative markers for DNA sequence polymorphisms are available for use in breeding programmes. These include single-nucleotide polymorphisms (SNPs, called “snips”) that can be assayed via high-throughput methods (Bevan et al, 2017; Monteiro et al., 2018). Each SNP represents one difference in a DNA building block, called a nucleotide. A SNP identifies differences in an organism’s sensitivity to certain diseases or conditions, as well as responses to diseases and conditions. Therefore, SNPs affect how species develop and respond to diseases, hence their importance.
In addition to this modern method of modification, non-genetic crop modification has existed for thousands of years, with domestication of cereal crops emerging with the move to increased sedentism ~12,000 years ago (the beginning the Holocene) (see for example http://www.ecofooddev.com/the-ancient-origins-of-modern-barley/). Centres of domestication for the economic cereal crops of barley and wheat have been identified in an area known as the ‘Fertile Crescent’, in modern southwest Asia (Vavilov, 1926). This type of modification involved the identification of species of plants with desirable traits, and deliberate cultivation and continuing selection (over thousands of years) of a limited range of plant types for improved adaptation to the environmental conditions required- in principle the same as the modern genetic method, one might argue. This recurrent selection increased the frequency of desirable traits while reducing the frequency of undesirable traits such as low palatability (Bevan et al, 2017).
Crop to wild relative research (CWR) represents a way to potentially modify crops using a mixture of these two approaches – modern methods to confer more sustainable, “natural” traits to modern cereal crops. Maxted et al., (2006) propose a definition of CWRs:
“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.”
Maxted, N., Amri, A., Castañeda‐Álvarez, N.P., Dias, S., Dulloo, M.E., Fielder, H., Ford-Lloyd, B.V., Iriondo, J.M., Brehm, J.M., Nilsen, L.B. and Thormann, I., 2016. 10. Joining up the dots: A systematic perspective of crop wild relative conservation and use. Enhancing crop genepool use: capturing wild relative and landrace diversity for crop improvement, p.87.
CWRs are distributed across all continents (except Antarctica) and have been identified as providers of beneficial genetic diversity essential for crop improvement. Crops such as wheat and sugar beet are some of the important economic crops to which CWR research has been applied to confer salt tolerance, pathogen resistance and yield improvements.
How Does it Work?
To identify the degree of relatedness between CWRs and crops, several schemes have been proposed:
Comparative genomics has shown that gene content and gene order are conserved across related species and genera, particularly amongst grasses (Devos and Gale, 1997). Methods based on genetically structured populations, genetic-association analyses, and targeted sequencing can be used to screen progenitors and wild relatives of crops to identify new haplotypes (group of inherited genes) with greater variation, fewer deleterious (harmful) alleles, and improved phenotypes for incorporation into breeding programmes (Bevan et al, 2017).
Gene flow is a natural process that occurs among sexually compatible individuals in which cross-pollination can lead to the production of viable seeds. In order to obtain the desired, beneficial traits from CWRs, researchers screen for wild relatives in habitats where the selective pressures of interest occur (e.g., salt marshes for salinity tolerance) (Monteiro et al., 2018).
Steps in Crop to Wild Gene Flow
- Crop to wild gene flow begins with interspecific hybridization. Interspecific hybridization is the crossing of two species from the same genus. This allows the exploitation of useful genes from wild, unimproved species for the benefit of the cultivated species.
- Next, chromatid exchange between crop and wild chromosomes occurs. A chromatid is one of two identical halves of a replicated chromosome. A chromosome is a long DNA molecule with part or all of the genetic material of an organism.
- Next, functional gametes are produced and fertilization occurs. Gametes are an organism’s reproductive cells. They are also referred to as sex cells.
- Finally, the transmission of crop-wild recombinant chromosomes occurs during karyotype evolution. Recombination is a process by which pieces of DNA are broken and recombined to produce new combinations of alleles. Karyotyping is the process of pairing and ordering all the chromosomes of an organism. The below image summarises the gene transfer path in crop to wild gene flow:
Following hybridization (the process of interbreeding individuals from genetically different populations to produce a hybrid), gene flow may result in the transfer of chromosome segments from one species to another by direct and recurrent backcrosses (i.e. crossing a donor parent to a line, and crossing the offspring with the ‘desired gene(s)’ back to the parent) (Benavente et al., 2007). However, crop-CWR hybrids are often sterile, but techniques exist to restore fertility in hybrids. These resultant forms establish important pathways and gene reservoirs for consequent gene flow back to their diploid progenitors (Benavente et al., 2007; Monteiro et al., 2018; Zhang et al, 2016).
CWR Example: CWR research to improve salt tolerance in wheat
Improvement of wheat crops is of vital importance as wheat (Triticum spp.) is one of the world’s major food and fodder cereals. Wheat is grown in irrigated and rain-fed conditions, which are both threatened by salinization (Colmer et al., 2006). Hence, significant increases in the salt tolerance of this crop are needed. Wild relatives may be a source of this improved salt tolerance in wheat. The below image summarizes the effect of increased salinity on plant growth.
Members of the Triticeae tribe (wheat grasses) display a spectrum of variability in salt tolerance (Colmer et al., 2006). Several characteristics contribute to salt tolerance in wheat- chiefly the ability to limit the concentration of sodium (Na+) that enters the water transport system of the plant i.e. the xylem (termed “Na+ exclusion”). Quantification of Na+ concentration in the leaves can be used as an indicator of the relative ability of the plant to ‘exclude’ Na+ . Of importance to salt tolerance in many grasses is an ability to accumulate Na+ in the older leaves and the leaf sheaths (i.e., compartmentalization at the whole plant level). Maintaining potassium (K+ ) uptake and transport to growing tissues is also crucial for salt tolerance (Colmer et al., 2006). The traits described above can be attributed to wild species of the Triticeae, i.e. potential CWRs of wheat.
Example of CWRs of durum wheat & bread wheat
In wheat, new polyploids (polyploid organisms contain more than two sets of chromosomes) made by crossing different goat grass variations with tetraploid wheats, have acquired a wide range of beneficial genetic traits. These “synthetic” wheats are then crossed into elite wheat lines and then backcrossed to fix preferred phenotypes. A durum wheat (Triticum durum) line that sustains high yields when grown in saline soil was synthesized by introgressing (transferring) a region from the einkorn wheat (T. monococcum) genome (a diploid progenitor) that contains a sodium exclusion pump (Bevan et al, 2017). Durum and bread wheat possess an ability to maintain low Na+ and high K+ concentrations in leaves, and bread wheat is, in general, a better Na+ ‘excluder’ than durum wheat.
To transfer genes for salt tolerance for example, from durum wheat to bread wheat:
Risks faced by CWR and limitations of CWR research
As with other plant species, CWR genetic diversity is endangered by common, global threats such as climate change, changing land use & land scarcity, encroaching urbanization, disease, competition, changing farming methods, soil chemical change, aridification, overstocking, changing agricultural policy, and overpopulation leading to changing food demands and more.
From a Natural Heritage Conservation perspective, the IUCN Red List of Threatened Species is instrumental in assessing threats to CWR species, as well as national frameworks in individual states for plant protection. The IUCN’s Plants for People project assesses threats to CWRs, for example. Reduction of CWRs due to the above-mentioned threats will lead to genetic erosion of those species that survive, i.e. a loss of genetic diversity due to increased selection pressures This can lead to a reduction in the ability of CWRs to adapt to changing conditions- the very characteristic that makes them critical in future agri sustainability (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7463933/).
CWR research has also thus far focused on a small set of crop species, as summarised in the table below taken from Zhang et al, (2016). The focusing on only a few species means that CWR research is not as ubiquitous as GMO:
“Linkage drag”
Linkage drag refers to the effects of genes linked to the quantitative trait loci (QTL) that are being introgressed/transferred. Linkage drag is responsible for limiting the use of alien genes in breeding programs, and hence effort must be made to reduce it. If a desirable QTL for a particular trait lies close to an undesirable gene affecting another trait, you need to “break” the linkage drag, i.e., separate the good QTL from the bad (https://www.integratedbreeding.net/courses/marker-assisted-breeding/index7567.html?id=137). Molecular markers can be used to reduce linkage drag (Colmer, et al., 2006).
Conclusions
Crop Wild Relatives are important resources of genetic diversity that can be used in crop improvement in agriculture and in other sectors such as pharmaceuticals. Due to global concerns such as climate change and geopolitical instability, CWR genetic biodiversity is under threat. Therefore, the conservation of Crop Wild Relatives, both in situ and ex-situ is of vital importance. This is recognised by the Conference of the Parties (COP) to the CBD 2010 Biodiversity Target (www.biodiv.org/2010target), as well as a number of other strategies and treaties, such as the Global Strategy for Plant Conservation, the International Treaty on Plant Genetic Resources for Food and Agriculture, and the European Plant Conservation Strategy (Maxted et al, 2006).
Initiatives to protect and conserve CWR genetic information are underway, such as the Millenium Seed Bank at Kew Botanical Gardens. Global inventories are being collated to identify CWR species of vital importance in order to facilitate targeted conservation action (Vincent et al., 2013). Vincent el al. (2013) state that the next step will be to collate georeferenced data points for priority CWR and compare their distributions to identify priority areas for further conservation and study. In any case, the uncertain and urgent food security issues we are facing require such conservation action be taken sooner, rather than later.
References cited
- Benavente, E., Cifuentes, M., Dusautoir, J.C. and David, J., 2008. The use of cytogenetic tools for studies in the crop-to-wild gene transfer scenario. Cytogenetic and genome research, 120(3-4), pp.384-395.
- Bevan, M.W., Uauy, C., Wulff, B.B., Zhou, J., Krasileva, K. and Clark, M.D., 2017. Genomic innovation for crop improvement. Nature, 543(7645), pp.346-354.
- Colmer, T.D., Flowers, T.J. and Munns, R., 2006. Use of wild relatives to improve salt tolerance in wheat. Journal of Experimental Botany, 57(5), pp.1059-1078.
- Devos, K.M. and Gale, M.D., 1997. Comparative genetics in the grasses. In Oryza: From Molecule to Plant (pp. 3-15). Springer, Dordrecht.
- Fuller, D.Q., Willcox, G. and Allaby, R.G., 2012. Early agricultural pathways: moving outside the ‘core area’ hypothesis in Southwest Asia. Journal of experimental botany, 63(2), pp.617-633.
- Monteiro, F., Frese, L., Castro, S., Duarte, M.C., Paulo, O.S., Loureiro, J. and Romeiras, M.M., 2018. Genetic and genomic tools to asssist sugar beet improvement: the value of the crop wild relatives. Frontiers in plant science, 9, p.74.
- Monteiro, F., Romeiras, M.M., Batista, D. and Duarte, M.C., 2013. Biodiversity assessment of sugar beet species and its wild relatives: linking ecological data with new genetic approaches. American Journal of Plant Sciences, 2013.
- Rao, M.P.N., Dong, Z.Y., Xiao, M. and Li, W.J., 2019. Effect of Salt Stress on Plants and Role of Microbes in Promoting Plant Growth Under Salt Stress. In Microorganisms in Saline Environments: Strategies and Functions (pp. 423-435). Springer, Cham.
- Vavilov, N.I., 1926. The origin of cultivated plants. News in Agronomy, pp.76-85.
- Vincent, H., Wiersema, J., Kell, S., Fielder, H., Dobbie, S., Castañeda-Álvarez, N.P., Guarino, L., Eastwood, R., Leόn, B. and Maxted, N., 2013. A prioritized crop wild relative inventory to help underpin global food security. Biological conservation, 167, pp.265-275.
- Zhang, H., Mittal, N., Leamy, L.J., Barazani, O. and Song, B.H., 2017. Back into the wild—Apply untapped genetic diversity of wild relatives for crop improvement. Evolutionary Applications, 10(1), pp.5-24.
- Cover image: https://www.yara.co.uk/crop-nutrition/wheat/wheat-historical-development/
