Further evidence for two progenitors in the origin of maize
The most popular hypothesis for the origin of maize (Zea mays L.) today is that teosinte (Zea mays ssp. mexicana) is the wild progenitor of maize (Doebley, Econ. Bot. 44 (Suppl.):6-27, 1990 Iltis;, Science 222:186-188, 1983; Galinat, Econ. Bot. 49:3-12, 1995). Fertile plants derived from crossing diploid perennial teosinte (Zea diploperennis Iltis, Doebley and Guzmán), hereafter referred to as diploperennis, and Eastern gamagrass (Tripsacum dactyloides L.), however, revived the possibility that the earlier hypothesis of Mangelsdorf and Reeves (TX Agr. Sta. Bull. 574,1939) proposing maize arose from intergeneric hybridization involving Tripsacum is the more likely scenario (Eubanks, Econ. Bot. 49:172-182, 1995). Successful reciprocal crosses have been obtained using a tetraploid T. dactyloides and Z. diploperennis (MNL 67:39, 1993), and more recently with a diploid T. dactyloides and Z. diploperennis. Restriction fragment length polymorphism (RFLP) analysis of Southern blots hybridized with maize probes showed Tripsacum X diploperennis plants inherit unique restriction sites from both parents that are stably inherited in F2 progeny (Eubanks, Theor. Applied Genet. 1997, in press).
Findings reported here address two propositions that further test the hypothesis of a role for intergeneric hybridization in the origin of maize: (1) If Tripsacum was involved in maize evolution, unique restriction sites from Tripsacum should occur in maize but not in teosinte; (2) if hybrids between Tripsacum and diploperennis approximate a reconstructed prototype of early maize, restriction sites in the hybrids should also be present in maize.
A series of three molecular assays in which genomic DNA of all Tripsacum and diploperennis parent plants; F1, F2, and F3 Tripsacum X diploperennis progeny; maize lines B73 and Funk's G4522, and F1 and F2 progeny from crosses between B73 and G4522 with Tripsacum-diploperennis hybrids, was isolated, digested with EcoRI, BamHI, EcoRV, and HindIII, transferred to Southern blots, then probed with 15 maize molecular markers (Eubanks, Econ. Bot. 49:172-182, 1995, Theor. Applied Genet. 1997, in press). Pairwise analysis of autoradiograph restriction sites was conducted to determine if any unique Tripsacum sites are found in maize but not in teosinte, and if new recombinant bands in Tripsacum-diploperennis hybrids are present in maize. pBR> Eleven restriction sites revealed by nine of the 15 molecular markers are found in Tripsacum and maize but not in teosinte. Three of those sites are also present in Tripsacum-diploperennis hybrids.
Fourteen restriction sites revealed by ten of the 15 molecular markers appear in Tripsacum-diploperennis hybrids that are not present in either parent and evidently arose de novo through recombination as a result of intergeneric crossing. Molecular markers revealing these mutable sites correlate with phenotypic traits linked to developmental changes involved in the evolution of the maize ear. Four of the fourteen de novo recombinant sites are also observed in both maize lines.
Since some restriction sites are present in Tripsacum and maize but not teosinte, and since some recombinant sites in Tripsacum-diploperennis hybrids not present in either parent are found in maize, results of these small-scale molecular assays lend support to the hypothesis of a hybrid origin for maize involving Tripsacum. DNA fingerprinting with molecular markers spanning a greater segment of the maize genome and including other teosinte species will provide additional data for further elucidation of how maize originated.
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