On the road to diploidization? Homoeolog loss in independently formed populations of the allopolyploid Tragopogon miscellus (Asteraceae)

BACKGROUND: Polyploidy (whole-genome duplication) is an important speciation mechanism, particularly in plants. Gene loss, silencing, and the formation of novel gene complexes are some of the consequences that the new polyploid genome may experience. Despite the recurrent nature of polyploidy, little is known about the genomic outcome of independent polyploidization events. Here, we analyze the fate of genes duplicated by polyploidy (homoeologs) in multiple individuals from ten natural populations of Tragopogon miscellus (Asteraceae), all of which formed independently from T. dubius and T. pratensis less than 80 years ago. RESULTS: Of the 13 loci analyzed in 84 T. miscellus individuals, 11 showed loss of at least one parental homoeolog in the young allopolyploids. Two loci were retained in duplicate for all polyploid individuals included in this study. Nearly half (48%) of the individuals examined lost a homoeolog of at least one locus, with several individuals showing loss at more than one locus. Patterns of loss were stochastic among individuals from the independently formed populations, except that the T. dubius copy was lost twice as often as T. pratensis. CONCLUSION: This study represents the most extensive survey of the fate of genes duplicated by allopolyploidy in individuals from natural populations. Our results indicate that the road to genome downsizing and ultimate genetic diploidization may occur quickly through homoeolog loss, but with some genes consistently maintained as duplicates. Other genes consistently show evidence of homoeolog loss, suggesting repetitive aspects to polyploid genome evolution.