Patterns in grass genome evolution.

Increasingly comprehensive, species-rich, and large-scale comparisons of grass genome structure have uncovered an even higher level of genomic rearrangement than originally observed by recombinational mapping or orthologous clone sequence comparisons. Small rearrangements are exceedingly abundant, even in comparisons of closely related species. The mechanisms of these small rearrangements, mostly tiny deletions caused by illegitimate recombination, appear to be active in all of the plant species investigated, but their relative aggressiveness differs dramatically in different plant lineages. Transposable element amplification, including the acquisition and occasional fusion of gene fragments from multiple loci, is also common in all grasses studied, but has been a much more major contributor in some species than in others. The reasons for these quantitative differences are not known, but it is clear that they lead to species that have very different levels of genomic instability. Similarly, polyploidy and segmental duplication followed by gene loss are standard phenomena in the history of all flowering plants, including the grasses, but their frequency and final outcomes are very different in different lineages. Now that genomic instability has begun to be characterized in detail across an array of plant species, it is time for comprehensive studies to investigate the relationships between particular changes in genome structure and organismal function or fitness.