Maize (Zea mays or corn), both a major food source and an important cytogenetic model, evolved from a tetraploidthat arose about 4.8 million years ago (Mya). As a result, maize has extensive duplicated regions within its genome.We have sequenced the two copies of one such region, generating 7.8 Mb of sequence spanning 17.4 cM of the shortarm of chromosome 1 and 6.6 Mb (25.6 cM) from the long arm of chromosome 9. Rice, which did not undergo asimilar whole genome duplication event, has only one orthologous region (4.9 Mb) on the short arm of chromosome3, and can be used as reference for the maize homoeologous regions. Alignment of the three regions allowedidentification of syntenic blocks, and indicated that the maize regions have undergone differential contraction ingenic and intergenic regions and expansion by the insertion of retrotransposable elements. Approximately 9% of thepredicted genes in each duplicated region are completely missing in the rice genome, and almost 20% have movedto other genomic locations. Predicted genes within these regions tend to be larger in maize than in rice, primarilybecause of the presence of predicted genes in maize with larger introns. Interestingly, the general gene methylationpatterns in the maize homoeologous regions do not appear to have changed with contraction or expansion of theirchromosomes. In addition, no differences in methylation of single genes and tandemly repeated gene copies havebeen detected. These results, therefore, provide new insights into the diploidization of polyploid species.

Data from cytological and genetic mapping studies suggest that maize arose asa tetraploid. Two previous studies investigating the most likely mode of maizeorigin arrived at different conclusions. Gaut and Doebley [7] proposed a segmentalallotetraploid origin of the maize genome and estimated that the two maizeprogenitors diverged at 20.5 million years ago (mya). In a similar study, using largerdata set, Brendel and colleagues (quoted in [8]) suggested a single genome duplicationat 16 mya. One of the key components of such analyses is to examine sequencedivergence among strictly orthologous genes. In order to identify such genes, Laiand colleagues [10] sequenced five duplicated chromosomal regions from the maizegenome and the orthologous counterparts from the sorghum genome. They alsoidentified the orthologous regions in rice. Using positional information of geneticcomponents, they identified 11 orthologous genes across the two duplicated regionsof maize, and the sorghum and rice regions. Swigonova et al. [12] analyzed the 11orthologues, and showed that all five maize chromosomal regions duplicated at thesame time, supporting a tetraploid origin of maize, and that the two maize progenitorsdiverged from each other at about the same time as each of them diverged fromsorghum, about 11.9 mya. Copyright  2004 John Wiley & Sons, Ltd.

The amino acid methionine is a common protein building block that is also important in other cellularprocesses. Plants, unlike animals, synthesize methionine de novo and are thus a dietary source of thisnutrient. A new approach for using maize as a source of nutrient methionine is described. Maize seeds, amajor component of animal feeds, have variable levels of protein-bound methionine. This variability is aresult of post-transcriptional regulation of the Dzs10 gene, which encodes a seed-speci®c highmethioninestorage protein. Here we eliminate methionine variability by identifying and replacing thecis-acting site for Dzs10 regulation using transgenic seeds. Interestingly, two different mechanisms affectmRNA accumulation, one dependent on and the other independent of the untranslated regions (UTRs) ofDzs10 RNA. Accumulation of chimeric Dzs10 mRNA was not reduced in hybrid crosses and wasuncoupled from genomic imprinting by Dzr1, a regulator of Dzs10. Uniform high levels of Dzs10 proteinwere maintained over ®ve backcross generations of the transgene. The increased level of methionine inthese transgenic seeds allowed the formulation of a useful animal feed ration without the addition ofsynthetic methionine.

Maize (Zea mays L. ssp. mays), one of the most important agricultural crops in the world, originated by hybridizationof two closely related progenitors. To investigate the fate of its genes after tetraploidization, we analyzed thesequence of five duplicated regions from different chromosomal locations. We also compared corresponding regionsfrom sorghum and rice, two important crops that have largely collinear maps with maize. The split of sorghum andmaize progenitors was recently estimated to be 11.9 Mya, whereas rice diverged from the common ancestor of maizeand sorghum ∼50 Mya. A data set of roughly 4 Mb yielded 206 predicted genes from the three species, excludingany transposon-related genes, but including eight gene remnants. On average, 14% of the genes within the alignedregions are noncollinear between any two species. However, scoring each maize region separately, the set ofnoncollinear genes between all four regions jumps to 68%. This is largely because at least 50% of the duplicatedgenes from the two progenitors of maize have been lost over a very short period of time, possibly as short as 5million years. Using the nearly completed rice sequence, we found noncollinear genes in other chromosomalpositions, frequently in more than one. This demonstrates that many genes in these species have moved to newchromosomal locations in the last 50 million years or less, most as single gene events that did not dramatically altergene structure.