RESUMO
Our understanding of polyploid genome evolution is constrained because we cannot know the exact founders of a particular polyploid. To differentiate between founder effects and post polyploidization evolution, we use a pan-genomic approach to study the allotetraploid Brachypodium hybridum and its diploid progenitors. Comparative analysis suggests that most B. hybridum whole gene presence/absence variation is part of the standing variation in its diploid progenitors. Analysis of nuclear single nucleotide variants, plastomes and k-mers associated with retrotransposons reveals two independent origins for B. hybridum, ~1.4 and ~0.14 million years ago. Examination of gene expression in the younger B. hybridum lineage reveals no bias in overall subgenome expression. Our results are consistent with a gradual accumulation of genomic changes after polyploidization and a lack of subgenome expression dominance. Significantly, if we did not use a pan-genomic approach, we would grossly overestimate the number of genomic changes attributable to post polyploidization evolution.
Assuntos
Brachypodium/genética , Diploide , Evolução Molecular , Genoma de Planta , Poliploidia , Cromossomos de Plantas/genética , Genoma de Cloroplastos , Genômica , Hibridização Genética , Filogenia , Polimorfismo de Nucleotídeo Único , Retroelementos/genética , Especificidade da EspécieRESUMO
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) that differ in chromosome evolution and number. We 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. No hybrids were obtained from reciprocal crosses or when autotetraploids of the parental species were crossed. Colchicine treatment was used to double the genome of the F1 amphihaploid lines leading to allotetraploids. The genome-doubled F1 plants produced a few S1 (first selfed generation) seeds after self-pollination. S1 plants from one parental combination (Bd3-1×Bsta5) were fertile and gave rise to further generations whereas those of another parental combination (Bd21×ABR114) were sterile, illustrating the importance of the parental lineages crossed. The synthetic allotetraploids were stable and resembled the natural B. hybridum at the phenotypic, cytogenetic and genomic levels. The successful creation of synthetic B. hybridum 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.
