The use of crop wild relatives in maize and sunflower breeding

Conservation of crop wild relatives (CWR) has always been predicated on the promise of new and useful traits, and thus modern genetics and genomics tools must help fulfill the promise and continue to secure the conservation of these resources. However, the vast genetic potential present in CWR is often difficult to tap, as identification of superior alleles can be hampered by the effects of the environment on expression of these alleles and masked in different genetic backgrounds; transfer of superior alleles into breeding pools to create new crop varieties can be slow and expensive. Some crop species have been more amenable to introgression of traits from wild relatives than others. In some cases, these species may be less diverged from their wild ancestors, which become a good source of mono- to oligogenic traits, many of which are more qualitative in nature, and sometimes of quantitative traits. Sunflower (Helianthus annuus L.) is an introgression success story, and many traits, including cytoplasmic male sterility, herbicide tolerance, drought and biotic stress resistance, and modified fatty acid profiles, have been introgressed into the cultivated gene pool from wild relatives without depression of oil yield and quality. Others, including maize (Zea mays L.), have shown little progress in widening the cultivated gene pool using exotic sources due to temporary yield depression, potential for loss of quality, and disturbance of current logistical habits. Here, we review the breeding history of sunflower and maize and explore variables that have limited the use of CWR in some species and allowed success in others. Surprisingly, in both sunflower and maize, biological limitations are similar and smaller than expected and appear to be surmountable with sufficient determination. Possible new technologies and policies to allow a deeper mining of these genetic resources in all crop species are discussed.