A Window on Maize Evolution

Abstract

Evolution is iffy business. Between one generation and the next, the millions or billions of base pairs that make up an organism's genetic material go through subtle but sometimes significant changes, with bits added, deleted, and moved around in the off chance that the new combination may serve its owner better than the previous one did its parents. Clearly some do-we are here, and so are nematodes, giraffes, jellyfish, and dandelions , all cut from the same cloth and shaped by such random alterations. But far many more do not. The road to biodiversity is littered with the fleeting memory of genetic remixes that rendered their short-lived owners incapable of survival. But what if an organism had two sets of chromosome pairs-one to provision it with the basic functions it needs to stay alive, and the other to play around with, evolutionarily? If a new combination in the doubled assemblage provided an adaptive advantage, so much the better. If not, the process of trying it out would likely not be lethal, thanks to the backup provided by the second chromosome set. It turns out that's just the game that many lineages of eukaryotes, especially plants, have up their genome sleeves. Somewhere in the course of their evolutionary history, maize, wheat, and a number of other species underwent a process that left them polyploid, or in possession of multiple pairs of chromosomes , rather than just the single pair most animals own. These extra pairs tend to make them bigger and, from the human perspective, more productive. They also provide their owners with more raw material for evolution, allowing them to undergo major alterations in genetic composition without the risk of losing fundamental function. Just how plants let evolution play with this extra set of genes has been the subject of study in Arabidopsis, a little dicot with the big distinction of being the white rat, research-wise, of the plant world. Arabi-dopsis is also multiply tetraploid, but has evolved to act like a diploid. Four years ago, researchers made a mechanism-mystifying discovery: this species' two sets of chromosomes were differentially altered, or fractionated, with more of the modifications accumulating on one set than the other. This selectivity is both adaptive-it keeps the original pair intact, providing a safety net during experimentation as the other homeolog (matched chromosome) gradually sheds and recombines genetic material, ridding excess and creating new combinations-and mystifying. How can change be directed toward one set, but not Selected PLoS Biology research articles are accompanied by a synopsis written for a general audience to provide non-experts with insight into the significance of the published work. The genome of maize is the result of a whole-genome duplication that created two duplicate genomes-each orthologous to the entire sorghum genome-that have been reduced by fractionation. This process of fractionation can be seen as analogous to the evolution of inflorescence between the two species, with the perfect flowers of sorghum's single inflorescence fractionating into imperfect male and female flowers on separate inflorescences in maize. Left panel: maize male flower; middle panel: sorghum flower; right panel: ear of maize topped with female tassels.