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Title: What limits the use of crop wild relatives for crop improvement? Contrasting case studies (Zea mays and Helianthus annuus L.) provide clues to identify and overcome limiting factors

Author
item Warburton, Marilyn
item RAUF, SAEED - University Of Sargodha
item MAREK, LAURA - Iowa State University
item HUSSAIN, MUBASHAR - University Of Sargodha
item OGUNOLA, OLUWASEUN - Mississippi State University
item GONZALEZ, JOSE DE JESUS - University Of Guanajuato

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2017
Publication Date: 4/20/2017
Citation: Warburton, M.L., Rauf, S., Marek, L., Hussain, M., Ogunola, O., Gonzalez, J. 2017. The Use of Crop Wild Relatives in Maize and Sunflower Breeding. Crop Science. 57:1-14. doi:10.2135/cropsci2016.10.0855.

Interpretive Summary: Large collections of cultivated and wild related species of plants are kept in gene banks around the world. The banks are expensive to maintain, but the loss of the material in the bank would be catastrophic, because they represent future improvement of crop species by plant breeding. For all new traits that will be needed, including resistance to new diseases, insects, drought, waterlogging, lack of fertilizers, or new uses for old crops, the gene banks are where the most user friendly genes are probably already waiting for breeders to use. In some cases, these genes are accessed regularly and incorporated into new varieties; this is the case, for example, with sunflowers. In other cases, the genes in the gene banks have not been accessed by breeders. This is generally the case with corn (maize). Most corn in the US are now highly uniform and related to each other, which leaves US farmers in a very vulnerable position, as a new disease or insect epidemic can reduce or eliminate corn yields over most of the US. Expanding the genetic base of US corn varieties using wild relatives would be a step towards reducing this risk. This review discusses why sunflowers are often bred with the genes available from crop wild relatives in the gene banks, but corn is not, and how this could be improved for corn.

Technical Abstract: The vast genetic potential present in crop wild relatives (CWR) is often difficult to tap, as identification and transfer of superior alleles into breeding pools to create new crop cultivars is challenging. Conservation of 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. Some crop species have been more amenable to introgression of traits from wild relatives than others. These species tend to be less diverged from their wild ancestors, which become a good source of qualitative (mono to oligogenic) and sometimes quantitative traits. Sunflower (Helianthus annuus) 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. In Zea, this is due to the division of tropical and temperate maize in their adaptation, and the fact that most landraces and all wild relatives are tropical; the carefully balanced heterotic patterns into which most elite maize is assigned, and which introgression of exotic germplasm would disturb; and the very high yield demanded by growers, which is generally suppressed, if only for a few generations, by genetic drag during introgression. Here, we review the 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.