Characterization of two CENH3 genes and their roles in wheat evolution

Wheat evolution is complex as a result of successive rounds of allopolyploidization and continuous selection during domestication. Diploid and tetraploid wheat species (Triticum spp.) were used as model systems in which to study the role of centromere-specific histone H3 variant (CENH3) in wheat evolution. We characterized two types of CENH3 genes, named αCENH3 and βCENH3, each of which has three slightly different copies derived from the AA, BB and DD genomes. Specific antibodies were raised against the two CENH3 proteins and were co-localized to centromeres with subtle differences. In most tetraploid wheat species, CENH3 genes are more highly expressed from the AA genome. In wild tetraploids, βCENH3 has a much lower expression level than αCENH3, while in cultivated tetraploids βCENH3 transcripts are enhanced to near αCENH3 levels. Comparison of the CENH3 proteins in wild and cultivated tetraploids revealed that the histone folding domain (HFD) of only βCENH3 is under positive selection, especially in the region responsible for targeting of CENH3 to the centromere. Taken together, positive selection of βCENH3 and its increased expression in tetraploid cultivars are indicative of adaptive evolution. Furthermore, the differences in localization between αCENH3 and βCENH3 observed using fiber fluorescence in situ hybridization (FISH) and immunodetection and in developmental phenotypes resulting from virus-reduced gene silencing imply their functional diversification during wheat evolution.