Long-range and targeted ectopic recombination between the two homeologous chromosomes 11 and 12 in Oryza species

Whole genome duplication (WGD) and subsequent evolution of gene pairs have been shown to have shaped the present day genomes of most, if not all, plants and to have played an essential role in the evolution of many eukaryotic genomes. Analysis of the rice (Oryza sativa ssp. japonica) genome sequence suggested an ancestral WGD ～50-70 Ma common to all cereals and a segmental duplication between chromosomes 11 and 12 as recently as 5 Ma. More recent studies based on coding sequences have demonstrated that gene conversion is responsible for the high sequence conservation which suggested such a recent duplication. We previously showed that gene conversion has been a recurrent process throughout the Oryza genus and in closely related species and that orthologous duplicated regions are also highly conserved in other cereal genomes. We have extended these studies to compare megabase regions of genomic (coding and noncoding) sequences between two cultivated (O. sativa, Oryza glaberrima) and one wild (Oryza brachyantha) rice species using a novel approach of topological incongruency. The high levels of intraspecies conservation of both gene and nongene sequences, particularly in O. brachyantha, indicate long-range conversion events less than 4 Ma in all three species. These observations demonstrate megabase-scale conversion initiated within a highly rearranged region located at ～2.1 Mb from the chromosome termini and emphasize the importance of gene conversion in cereal genome evolution.