Wading Into the Gene Pool: Progress and Constraints Using Wild Species

Abstract

Harnessing beneficial traits from wild species to support the development of productive crops is a long tradition recently revitalized by new tools and needs. Crop wild relatives (CWR), wild species closely related to our domesticated crop species, are especially valuable to plant breeders as reservoirs of new and useful trait variation. But wild species present unique challenges. Wild germplasm availability remains a bottleneck, as does the characterization and evaluation of gene bank accessions to identify useful traits. Introgression is a lengthy process although an array of genomic approaches is providing innovative strategies to find and exploit useful wild alleles. In this special issue of Crop Science, we present a series of papers that cover the breadth of issues related to the effective conservation and utilization of wild species. Our starting point is prioritizing conservation efforts and acquiring germplasm. Kell et al. (2017), in “Broadening the Base, Narrowing the Task: Prioritizing Crop Wild Relative Taxa for Conservation Action” provide an overview of criteria used to prioritize the conservation of CWR and present a harmonized approach based on the relative value of individual crops, the potential value of wild relatives to improve crops, and their relative threat status. In “A Multi-Species Collecting Strategy for Crop Wild Relatives Based on Complementary Areas with a High Density of Ecogeographical Gaps,” García et al. (2017) present an effective strategy for identifying high priority collecting locations in Spain based on the presence of multiple species and ecogeographic gaps in the gene banks of the Spanish Network of Plant Genetic Resources for Food and Agriculture. Their strategy differs from a conventional gap analysis since it does not rely on species distribution modeling. The next aim of this special issue was to provide a general overview of the use of CWR. In “Past and Future Use of Wild Relatives in Crop Breeding,” Dempewolf et al. (2017) report broadly on the use of CWR, discussing the effective breeding methods and technologies that are allowing today's breeders to capitalize on CWR, and discuss the challenges that continue to limit CWR use. The next set of papers focus on specific crops. This includes Seiler et al. (2017), “Utilization of Sunflower Crop Wild Relatives for Cultivated Sunflower Improvement” and Stalker (2017), “Utilizing Wild Species for Peanut Improvement,” who provide an overview of how CWR have been used in sunflower and peanut, both crops with a breeding history reliant on diverse gene pools. Continuing with peanut, Sharma et al. (2017), in “Harnessing Genetic Diversity of Wild Arachis Species for Genetic Enhancement of Cultivated Peanut,” describes the strategies used by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) to develop useful introgression lines based on crossing amphidiploids and autotetraploids of diploid wild species with peanut cultivars. Sharma (2017) continues describing ICRISAT strategies to introgress wild species into chickpea, pigeonpea, groundnut, and pearl millet, in “Pre-breeding Utilizing Wild Species for Genetic Enhancement of Grain Legumes.” These two papers illustrate the breeding synergy that's possible through the close proximity of an extensive germplasm collection, curators, and plant breeders. The use of food legume CWR to improve abiotic stress tolerance is illustrated by Naim-Feil et al. (2017) in “Drought Response and Genetic Diversity in Pisum fulvum, a Wild Relative of Domesticated Pea” and Souter et al. (2017), in “Successful Introgression of Abiotic Stress Tolerance from Wild Tepary Bean to Common Bean.” Similar to sunflower and peanut, wheat is another crop that has benefited from a broad gene pool. Curwen-McAdams and Jones (2017), in “Breeding Perennial Grain Crops Based on Wheat,” discuss the use of wheat relatives to develop a perennial grain crop. Zhang et al. (2017), in “Meiotic Homoeologous Recombination-based Alien Gene Introgression in the Genomics Era of Wheat,” describe methodology that streamlines the identification and introgression of alien genes through homoeologous recombination and marker-assisted selection to improve the utilization of wild species in wheat. A wild barley advanced backcross-nested association mapping population, described by Nice et al. (2017) in “Mapping Agronomic Traits in a Wild Barley Advanced Backcross-Nested Association Mapping Population,” has been effective for identifying and mapping potentially useful traits for barley improvement. The use of CWR in cotton has been difficult due to whole genome incompatibility. Saha et al. (2017), in “Tri-species Shuffling of Chromosomes to Study the Effects on Fiber Traits Using Chromosome Substitution Lines,” report on the development of a set of chromosome substitution lines using two cotton relatives that provide an effective tool for identifying useful wild alleles. The previous papers reflect the innovative strategies and successful deployment of genomic tools employed by plant breeders to utilize CWR. However, challenges remain, and addressing them is the final aim of this special issue of Crop Science. Warburton et al. (2017), in “The Use of Crop Wild Relatives in Maize and Sunflower Breeding,” use contrasting case studies of CWR use to examine the factors that limit the use of CWR in crop improvement. Bethke et al. (2017) in “Are We Getting Better at Using Wild Potato Species in Light of New Tools?” explore factors that have limited the use of wild relatives in potato improvement. Both papers conclude that challenges may be overcome using new breeding approaches and technologies. A common thread among papers has been the need for comprehensive data at both whole plant and genomic levels. Shaw et al. (2017), in “Germinate 3: Development of a Common Platform to Support the Distribution of Experimental Data on Crop Wild Relatives,” report on the development of a common software platform to support the distribution and analysis of information relevant to the utilization of CWR. Our intention in this special issue of Crop Science was to explore the question, “are we getting better at using wild species in light of new tools?” Concluding this thread, the final paper in the issue reports on efforts to domesticate wild species in the genus Silphium. Van Tassel et al. (2017), in “Accelerating Silphium Domestication: An Opportunity to Develop New Crop Ideotypes and Breeding Strategies Informed by Multiple Disciplines,” report that broad collaborations that support innovation in field and laboratories are essential for the rapid achievement of domestication. Reflecting on all the papers in this special issue, we can conclude that new tools are indeed helping us better use wild species, and thus justify the expense and effort of conservation work. However, much work is still required to continue to conserve and make these genetic resources available to plant breeders and to overcome the challenges that limit their use.