Rice Genetics IV - IRRI books - International Rice Research Institute

Miniature inverted repeattransposable elements help creategenomic diversity in maize and riceS.R. Wessler, A. Nagel, and A. CasaMiniature inverted repeat transposable elements (MITEs) are the most prevalentelements associated with the genes of maize and rice, in which theyreside in introns and in 5’ and 3’ flanking regions. Several MITEs have recentlybeen amplified in the genomes of maize and rice, thus giving rise tostructurally distinct alleles. This MITE-mediated variation can be assayedand quantified using a modification of the amplified fragment length polymorphismmethod called transposon display. In this way, the genome-wide distributionof MITEs can be mapped and MITE markers isolated and added togeneticists’ toolboxes.Transposable elements (TEs) were discovered more than a half century ago as thegenetic agents responsible for the sectors of anthocyanin on mutant maize kernels(McClintock 1950). This seemingly benign discovery led to a revolution in our conceptof the content and stability of the genetic material in virtually all living things.This revolution continues to this day.The existence of TEs implied that genomes are not just collections of the genesnecessary to assemble and maintain living organisms. In fact, we now know that theTE component can account for the largest fraction of genomic DNA in a variety oforganisms including many plant species. The differential amplification of TEs hasrecently been shown to be responsible for the C value paradox in the grasses. The Cvalue paradox is the observed lack of correlation between DNA content and complexityof organisms (Thomas 1971). It has been documented for both animal and plantspecies but, to date, only appears to be “solved” for members of the grass clade. Thatis, the increase in genome size from rice to sorghum to maize to barley (from 0.4 to 5Mb) reflects, in large part, TE amplification in the larger genomes (Kumar andBennetzen 1999).Miniature inverted repeat transposable elements . . . 107