Paleogenomic Analysis of the Short Arm of Chromosome 3 Reveals the History of the African and Asian Progenitors of Cultivated Rices

Rice is one of the most important crops, feeding more than half of the world population. There are two cultivated species, the African rice Oryza glaberrima and the Asian rice O. sativa. Although the African species is gradually replaced by O. sativa in most of African rice agrosystems, this species represents an important reservoir of genes of agronomical interest. Their exploitation for the development of modern African rice varieties requires a good understanding of the genetic relationships between the two cultivated species. We took advantage of the recent availability of the sequence of the chromosome 3 short arm of O. glaberrima to estimate the date of radiation between O. glaberrima and O. sativa lineages, using all the long terminal repeat (LTR)-retrotransposons as paleogenomic markers. We first demonstrated that in two distinct lineages, LTR-retrotransposons mutate at the same rate. Based on LTR-retrotransposons shared by both species in orthologous position, we then estimated that O. glaberrima and O. sativa progenitors diverged 1.2 Ma. This constitutes one of the first studies using such a large sample of transposable elements to reconstruct the phylogeny of species. Given the number of genome sequencing projects, there is no doubt that such approach will allow to resolve phylogenetic incongruities. The application of this method to other plant genomes will also facilitate further understanding of evolution of LTR-retrotransposons and eventually of the whole genome in divergent plant lineages.