Patterns of Sequence Divergence and Evolution of the S1 Orthologous Regions between Asian and African Cultivated Rice Species

A strong postzygotic reproductive barrier separates the recently diverged Asian and African cultivated rice species, Oryza sativa and O. glaberrima . Recently a model of genetic incompatibilities between three adjacent loci: S 1 A , S 1 and S 1 B (called together the S 1 regions) interacting epistatically, was postulated to cause the allelic elimination of female gametes in interspecific hybrids. Two candidate factors for the S 1 locus (including a putative F-box gene) were proposed, but candidates for S 1 A and S 1 B remained undetermined. Here, to better understand the basis of the evolution of regions involved in reproductive isolation, we studied the genic and structural changes accumulated in the S 1 regions between orthologous sequences. First, we established an 813 kb genomic sequence in O. glaberrima , covering completely the S 1 A , S 1 and the majority of the S 1 B regions, and compared it with the orthologous regions of O. sativa . An overall strong structural conservation was observed, with the exception of three isolated regions of disturbed collinearity: (1) a local invasion of transposable elements around a putative F-box gene within S 1 , (2) the multiple duplication and subsequent divergence of the same F-box gene within S 1 A , (3) an interspecific chromosomal inversion in S 1 B , which restricts recombination in our O. sativa × O. glaberrima crosses. Beside these few structural variations, a uniform conservative pattern of coding sequence divergence was found all along the S 1 regions. Hence, the S 1 regions have undergone no drastic variation in their recent divergence and evolution between O. sativa and O. glaberrima , suggesting that a small accumulation of genic changes, following a Bateson-Dobzhansky-Muller (BDM) model, might be involved in the establishment of the sterility barrier. In this context, genetic incompatibilities involving the duplicated F-box genes as putative candidates, and a possible strengthening step involving the chromosomal inversion might participate to the reproductive barrier between Asian and African rice species.