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a USDA-ARS, Plant Genetic Resources Unit, Cornell Univ., Geneva, NY 14456-0462
b USDA-ARS, Corn Insects and Crop Genetics Res. Unit, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1010
c Institute for Genomic Diversity, Biotechnology Bldg., Cornell Univ., Ithaca, NY 14853-2703
d Pioneer Hi-Bred Int., Johnston, IA 50131-1004
* Corresponding author (jl265{at}cornell.edu)
Tens-of-thousands of open-pollinated cultivars of corn (Zea mays L.) are being maintained in germplasm banks. Knowledge of the amount and distribution of genetic variation within and among accessions can aid end users in choosing among them. We estimated molecular genetic variation and looked for influences of pedigree, adaptation, and migration in the genetic makeup of conserved Corn-Belt Dent-related germplasm. Plants sampled from 57 accessions representing Corn-Belt Dents, Northern Flints, Southern Dents, plus 12 public inbreds, were genotyped at 20 simple sequence repeat (SSR) loci. For 47 of the accessions, between 5 and 23 plants per accession were genotyped (mean = 9.3). Mean number of alleles per locus was 6.5 overall, 3.17 within accessions, and 3.20 within pooled inbreds. Mean gene diversity was 0.53 within accessions and 0.61 within pooled inbreds. Open-pollinated accessions showed a tendency toward inbreeding (FIS = 0.09), and 85% of genetic variation was shared among them. A Fitch-Margoliash tree strongly supported the distinctiveness of flint from dent germplasm but did not otherwise reveal evidence of genetic structure. Mantel tests revealed significant correlations between genetic distance and geographical (r = 0.54, P = 0.04) or maturity zone (r = 0.33, P = 0.03) distance only if flint germplasm was included in the analyses. A significant correlation (r = 0.76, P < 0.01) was found between days to pollen shed and maturity zone of accession origin. Pedigree, rather than migration or selection, has most influenced the genetic structure of the extant representatives of the open-pollinated cultivars at these SSR loci.
Abbreviations: CI, confidence interval • QTL, quantitative trait loci • RFLP, restriction fragment length polymorphism • SSR, simple sequence repeat
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