Genomic relationships between maize and its wild relatives

Recent molecular studies confirm the long-held theory that maize is a tetraploid, but the identity of the ancestral diploid species remains an enigma. The various hypotheses were investigated using genomic in situ hybridization (GISH). Total genomic DNA from 10 wild relatives of maize were used as probes onto maize chromosomes to see if this could identify the ancestral genome donors in maize. While none of the taxa hybridized to a subset of chromosomes, genomic DNA from Zea mays ssp. mexicana, Z. mays ssp. parviglumis, Z. diploperennis, Tripsacum dactyloides and Coix lacryma-jobi all showed a similar hybridization pattern consisting of a dispersed signal over all maize chromosomes. Moreover, the first four species also showed highly localized subtelomeric signal on the long arms of maize chromosomes 5, 6, 7, and 8. In contrast, three Sorghum species tested (S. bicolor, S. halapense, and S. versicolor) only showed hybridization at the nucleolar organizer region. In light of recent data on retrotransposon occurrence in maize, the results may provide insights into the timing of speciation of Zea, Tripsacum, and Coix. Data obtained from the tetraploid Z. perennis strongly supported its taxonomic separation from the diploid Z. diploperennis.