Maize as a model for the evolution of plant nuclear genomes

The maize genome is replete with chromosomal duplications and repetitive DN A, The duplications resulted from an ancient polyploid event that occurred over 11 million years ago. Based on DNA sequence data, the polyploid event occurred after the divergence between sorghum and maize, and hence the poly ploid event explains some of the difference in DNA content between these tw o species. Genomic rearrangement and diploidization followed the polyploid event. Most of the repetitive DNA in the maize genome is retrotransposable elements, and they comprise 50% of the genome. Retrotransposon multiplicati on has been relatively recent-within the last 5-6 million years-suggesting that the proliferation of retrotransposons has also contributed to differen ces in DNA content between sorghum and maize. There are still unanswered qu estions about repetitive DNA, including the distribution of repetitive DNA throughout the genome, the relative impacts of retrotransposons and chromos omal duplication in plant genome evolution, and the hypothesized correlatio n of duplication events with transposition. Population genetic processes al so affect the evolution of genomes. We discuss how centromeric genes should , in theory, contain less genetic diversity than noncentromeric genes. In a ddition, studies of diversity in the wild relatives of maize indicate that different genes have different histories and also show that domestication a nd intensive breeding have had heterogeneous effects on genetic diversity a cross genes.