Banoth Madhu: Case Study – Maize Origins, Domestication, Evolution and Selection. Origin of maize from South central Mexico , Archeological remains of the earliest maize cob found at Guila Naquitz in the Oaxaca Valley of Mexico (6250 years ago). Microfossil evidence suggesting dispersal by 7,000 – 5,000 BP . Estimation of domestication of maize: 12,000-6,000 BP. According to Vavilov maize was originated in Mexico

1. Case Study – Maize Origins, Domestication,
Evolution and Selection
Banoth Madhu
Ph.D Research Scholar
Genetics and Plant Breeding
Tamil Nadu Agricultural University, Coimbatore

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 Origin of maize from South central Mexico , Archeological remains of the earliest maize cob found at
Guila Naquitz in the Oaxaca Valley of Mexico (6250 years ago)(Fig.1).
 Microfossil evidence suggesting dispersal by 7,000 – 5,000 BP . Estimation of domestication of
maize: 12,000-6,000 BP. According to Vavilov maize was originated in Mexico (Buckler et.al., 2001)
Fig.1 Maize cob found at Guila Naquitz in the Oaxaca Valley
A Over View of Maize Origins, Domestication, Evolution and Selection
Edward S. Buckler and Natalie M. Stevens

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 Origin of Maize includes three Hypothesis -Tripartite hypothesis, Teosinte hypothesis
and Recombination hypothesis.

5. Cytogenetics evidence to evolution
 Chromosome number of cultivated maize 2n=2x=20. Chromosome number of teosinte (Z. diploperennis)
2n=2x=20.
 Z. luxurians, has chromosomes that are cytologically distinct from those of maize, and that maize-Z.
luxurians hybrids exhibit two or more unpaired chromosomes during metaphase and partially sterile.
 Z. mays ssp. mexicana, has chromosomes that are cytologically similar to those of maize, and its hybrids
with maize exhibit complete chromosomal pairing and full fertility.
Molecular Evidence
 Isozyme allele frequencies of teosintes such as Z. luxurians, Z. diploperennis, and Z. perennis (the latter
two perennials) are strongly differentiated from those of maize.
 Allele frequencies of one Mexican annual teosinte, Z. mays ssp. mexicana, are more maize-like, although
still distinct from maize.
 Allele frequencies of another Mexican annual teosinte, Z. mays ssp. parviglumis or Balsas teosinte, are
essentially indistinguishable from those of maize.

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 One to four Tripsacum chromosomes sometimes associate with Zea chromosomes in hybrids between
Zea mays (2n=20) and T. dactyloides (2n=72).
 These hybrids with10 Zea and 36 Tripsacum chromasomes frequently produce functional female
gametes with 36 Tripsacum chromosomes only.
 In these individuals, intergenome pairing is the rule, and when they are pollinated with maize, their
offspring have 36 Tripsacum and 10, 12, 14, 16, 18 or 20 Zea chromosomes.
 Successive backcrosses with maize selectively eliminate Tripsacum chromosomes, and eventually plants
with 2n = 20 Zea chromosomes are recovered (Barghooran et al ., 1954).

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Conclusion:
 Attempts by Mangelsdorf and Reeves (1939) to transfer Tripsacum genes to modern maize met with little
success.
 The hybrids between tetraploid Tripsacum 2n= 36 and maize 2n=20 were female sterile and when
backcrossed with maize, the cytologically unreduced female gamete functioned sexually to produce
offspring with 20 maize and 18 Tripsacum chromosomes.
 Hybrids between octaploid T. dactyloides (2n = 72) and maize, produced by were completely sterile.
 All hybrids resemble Tripsacum more than Zea with respect to gross inflorescence morphology, no matter
how many haploid Zea and Tripsacum genomes are involved.
 The Tripsacum chromosomes in most hybrids with 36 Td and 10 Zm chromosomes associate into
bivalents during meiotic prophase, with the Zea mays chromosomes present as univalents (Dewet et al.
1972a, Dewet et al. 1972b).

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References:
1. VAVILOV, N. I. 1951. The origin, variation, immunity, and breeding of cultivated plants (Trans. K. S. Chester).
Chron. Bot. 13: 1-364.
2. WILKES, H. G. 1967. Teosinte: the closest relative in maize. Ph.D. thesis, Harvard Univ. Bussey Institute.