Development and Characterization of an Original Brachypodium Polyploid Model

Brachypodium hybridum (2n=30) is a natural allopolyploid with highly divergent sub-genomes derived from two extant diploid species, B. distachyon (2n=10) and B. stacei (2n=20) which have similar genome content but divergent chromosome evolution. We have generated and characterized synthetic autotetraploids and allopolyploids from this interesting polyploid model. Autopolyploids were generated, through colchicines treatment, from two inbred lines of B. distachyon (Bd21 and Bd3-1) and three ones of B. stacei (ABR114, Bsta5 and TE4.3) and were show to generally exceed their diploid progenitors in vegetative traits, but their fertility was reduced. We also created synthetic B. hybridum allotetraploids by hybridizing various lines of B. distachyon and B. stacei. The initial amphihaploid F1 interspecific hybrids were obtained at low frequencies when B. distachyon was used as the maternal parent (0.15% or 0.245% depending on the line used) and were sterile. The genome-doubled F1 plants produced few S1 (first selfed generation) seeds after self-pollination. The synthetic allotetraploids were stable and resembled the natural B. hybridum at the phenotypic, cytogenetic and genomic levels. The different autopolyploids and allopolyploids have now been characterized at the structural, gene expression and methylation levels. The successful creation of synthetic autopolyploids and allopolyploids of Brachypodium offers the possibility to study changes in genome structure and regulation at the earliest stages of allopolyploid formation in comparison with the parental species and natural B. hybridum.