Speciation-induced heritable cytosine methylation changes in polyploid wheat

To study possible epigenetic changes accompanying polyploid speciation, gen omic DNA from natural polyploid wheats and their putative diploid progenito rs were digested with a pair of isoschizomers Hpa II /Msp I and hybridized to 21 different types of low-copy DNA sequences. It was found that cytosine methylation changes were abundant in natural polyploid wheats after their speciation. The hybridization of the same set of sequences to a synthetic h exaploid wheat along with its parental lines indicated that the extensive D NA methylation changes already existed in the early generations (S-5, S-6 a nd S-7) of this plant. Moreover, the high !;similarity of the changed restr iction fragment length polymorphism (RFLP) patterns among three randomly ch osen individual plants suggested that the methylation changes occurred even earlier, and/or were of a nonrandom nature. The changed patterns were stab ly inherited in the three successive selfed generations. Though methylation changes are probably a genome-wide occurence, they appeared to be confined to the specific types of DNA sequences. The possible implications of the r apid and extensive cytosine methylation changes for several attributes of a llopolyploid genome evolution, such as genetic diploidization and gene dive rsification, are discussed.