Molecular and Historical Aspects of Corn Belt Dent Diversity

Troyer, 2000). The Southern Dents, dominant during colonial times in the southeastern USA, were introTens-of-thousands of open-pollinated cultivars of corn (Zea mays duced from Mexican sources via Cuba by Spanish ConL.) are being maintained in germplasm banks. Knowledge of the amount and distribution of genetic variation within and among accesquistadors during the 1500s (Doebley et al., 1988; Goodsions can aid end users in choosing among them. We estimated molecuman and Brown, 1988; Hudson, 1994). Dent corn spread lar genetic variation and looked for influences of pedigree, adaptation, northward from Cuba to Florida in 1539, to South Caroand migration in the genetic makeup of conserved Corn-Belt Dentlina in 1560, and to the Chesapeake Bay area in 1570 related germplasm. Plants sampled from 57 accessions representing (Troyer, 2000). Corn-Belt Dents, Northern Flints, Southern Dents, plus 12 public Because flint corn arrived in the USA 2500 yr before inbreds, were genotyped at 20 simple sequence repeat (SSR) loci. For dent, and the two types were isolated for an additional 47 of the accessions, between 5 and 23 plants per accession were 500 yr by flowering time, they became highly genetically genotyped (mean 9.3). Mean number of alleles per locus was 6.5 differentiated from each other. Today, Northern Flint overall, 3.17 within accessions, and 3.20 within pooled inbreds. Mean and Southern Dent races are considered to be so differgene diversity was 0.53 within accessions and 0.61 within pooled inbreds. Open-pollinated accessions showed a tendency toward inbreedent that, relative to the variation found within the wild ing (FIS 0.09), and 85% of genetic variation was shared among grasses, they would be considered different species and them. A Fitch-Margoliash tree strongly supported the distinctiveness possibly members of different genera (Anderson and of flint from dent germplasm but did not otherwise reveal evidence Brown, 1952). Flint corn carries certain traits for adaptaof genetic structure. Mantel tests revealed significant correlations tion to the cool, moist northeastern climate. The typical between genetic distance and geographical (r 0.54, P 0.04) or hard, smooth, and shallow kernels are spaced widely on maturity zone (r 0.33, P 0.03) distance only if flint germplasm the ears, allowing them to ripen earlier, be resistant to was included in the analyses. A significant correlation (r 0.76, P molds, dry down quickly, and withstand early fall frosts 0.01) was found between days to pollen shed and maturity zone of without injury so they germinate well the following accession origin. Pedigree, rather than migration or selection, has spring. This is in contrast to dent types that have kernels most influenced the genetic structure of the extant representatives of the open-pollinated cultivars at these SSR loci. arranged compactly on the ear, with deep, rough (dented) grain containing softer starch. On average, flint corn matures earlier than dent corn, but dent corn yields more because of its longer growing season (Jones et al., T racial complexes of corn have been described 1924). in the USA, the economically most important one After the American Revolution, the U.S. government being the Corn-Belt Dents (Goodman and Brown, began giving away unsettled land or selling it very 1988). Many Corn-Belt Dents originated in the 1800s cheaply, and settlers gradually began moving westward from the hybridization of two distinct races of corn, into what is now the U.S. Corn Belt (Hudson, 1994). Northern Flints and Southern Dents (Wallace and Corn was not highly valued as a commodity at the time, Brown, 1956). Northern Flint can be traced back to ca so it was not the focus of much improvement. In 1814, 1000 BC in the southwestern USA (Smith, 1995). It five varieties (now known as cultivars) of corn were spread throughout the Great Plains, moved east of the commonly known: ‘Big Yellow’, ‘Big White’, ‘Little YelMississippi ca AD 600, and was found throughout the low’, ‘Little White’ (all flint types) and ‘Gourdseed’ eastern USA ca AD 1000 (Brown and Anderson, 1947; (white or yellow, the first popular nonflint) (Atkinson and Wilson, 1915). Ten Eyck (1913) claimed that ≈40 J.A. Labate, USDA-ARS, Plant Genetic Resources Unit, Cornell corn cultivars were recognized by 1840, and that they Univ., Geneva, NY 14456-0462; K.R. Lamkey, USDA-ARS, Corn were not very distinct from one another. Montgomery Insects and Crop Genetics Res. Unit, Dep. of Agronomy, Iowa State (1916) estimated that as many as 250 cultivars were in Univ., Ames, IA 50011-1010; S.E. Mitchell and S. Kresovich, Institute for Genomic Diversity, Biotechnology Bldg., Cornell Univ., Ithaca, existence by 1840. The cultivars that were most successNY 14853-2703; H. Sullivan and J.S.C. Smith, Pioneer Hi-Bred Int., ful in spreading through the Corn Belt traced back to Johnston, IA 50131-1004. Joint contribution from the Corn Insects early crosses between flint and dent types. The most and Crop Genetics Res. Unit, USDA-ARS and Dep. of Agronomy, famous example of this is illustrated by the story of Iowa State Univ. Journal Paper No. J-19149 of the Iowa Agric. and Robert Reid, who serendipitously crossed a semiHome Economics Exp. Stn., Ames, IA. Project No. 3755, and supported by Hatch Act and State of Iowa. J.A. Labate and K.R. Lamkey gourdseed dent with Little Yellow in Illinois in 1847 to gratefully acknowledge funding from Pioneer Hi-Bred International, create ‘Reid Yellow Dent’ (Wallace and Brown, 1956; Cooperative Agreement No. 58-3625-7-407. J.A. Labate, K.R. LamTroyer, 1999). The hybrid origin of Corn-Belt Dent key, and S. Kresovich acknowledge funding from the USDA-NRI plant genome program (CSREES Award No. 99-35300-7757). ReAbbreviations: CI, confidence interval; QTL, quantitative trait loci; ceived 11 Feb. 2002. *Corresponding author ([email protected]). RFLP, restriction fragment length polymorphism; SSR, simple sequence repeat. Published in Crop Sci. 43:80–91 (2003).