title: Genetic erosion in maize's center of origin.
abstract: Crop genetic diversity is an indispensable resource for farmers and professional breeders responding to changing climate, pests, and diseases. Anecdotal appraisals in centers of crop origin have suggested serious threats to this diversity for over half a century. However, a nationwide inventory recently found all maize races previously described for Mexico, including some formerly considered nearly extinct. A flurry of social studies seems to confirm that farmers maintain considerable diversity. Here, we compare estimates of maize diversity from case studies over the past 15 y with nationally and regionally representative matched longitudinal data from farmers across rural Mexico. Our findings reveal an increasing bias in inferences based on case study results and widespread loss of diversity. Cross-sectional, case study data suggest that farm-level richness has increased by 0.04 y(-1) nationwide; however, direct estimates using matched longitudinal data reveal that richness dropped -0.04 y(-1) between 2002 and 2007, from 1.43 to 1.22 varieties per farm. Varietal losses occurred across regions and altitudinal zones, and regardless of farm turnover within the sector. Extinction of local maize populations may not have resulted in an immediate loss of alleles, but low varietal richness and changes in maize's metapopulation dynamics may prevent farmers from accessing germplasm suitable to a rapidly changing climate. Declining yields could then lead farmers to leave the sector and result in a further loss of diversity. Similarities in research approaches across crops suggest that methodological biases could conceal a loss of diversity at other centers of crop origin.
authors.affiliation: Abt Associates, Inc., Mexico 11560 D.F., Mexico; Centro de Estudios Económicos, El Colegio de México, Mexico 10740 D.F., Mexico; georgie.dyer@gmail.com.
authors.forename: George A
authors.initials: GA
authors.lastname: Dyer
authors.affiliation: División de Economía, Centro de Investigación y Docencia Económicas, Mexico 01210 D.F., Mexico;
authors.forename: Alejandro
authors.initials: A
authors.lastname: López-Feldman
authors.affiliation: Centro de Estudios Económicos, El Colegio de México, Mexico 10740 D.F., Mexico; Desarrollo y Alimentación Sustentable, A.C., Mexico 04318 D.F., Mexico; and.
authors.forename: Antonio
authors.initials: A
authors.lastname: Yúnez-Naude
authors.affiliation: Department of Agricultural and Resource Economics and Giannini Foundation of Agricultural Economics, University of California, Davis, CA 95616.
authors.forename: J Edward
authors.initials: JE
authors.lastname: Taylor
mesh_terms: D000383:Agriculture; D057231:Climate Change; D003247:Conservation of Natural Resources; D018556:Crops, Agricultural / Q000145:classification / Q000235:genetics*; D014644:Genetic Variation; D008800:Mexico; D013997:Time Factors; D003313:Zea mays / Q000145:classification / Q000235:genetics*
keywords: climate change; corn; crop evolution; in situ conservation