Hybridization, polyploidy and invasion: lessons from Spartina (Poaceae)

In this paper, we examine how the Spartina system has helped our understanding of the genomic aspects of allopolyploid speciation in the context of biological invasion. More specifically the respective roles of hybridization and genome duplication in the success of newly formed allopolyploid species are explored. Hybridization appears to have triggered genetic and epigenetic changes in the two recently formed European homoploid hybrids S. x towsendii and S. x neyrautii. Deviation from parental structural additivity is observed in both hybrids, with different patterns when considering transposable element insertions or AFLP and methylation alteration. No important changes are observed in the invasive allopolyploid Spartina anglica that inherited the identical genome to S. x townsendii. The repeated rRNA genes are not homogenized in the allopolyploid, and both parental repeats are expressed in the populations examined. Transcriptomic changes suggest possible gene silencing in both hybrids and allopolyploid. In the long-term of evolutionary time, older hexaploid Spartina species (Spartina alterniflora, Spartina maritima and Spartina foliosa) appear to have selectively retained differential homeologous copies of nuclear genes. Waxy gene genealogies suggest a hybrid (allopolyploid) origin of this hexaploid lineage of Spartina. Finally, nuclear and chloroplast DNA data indicate a reticulate origin (alloheptaploid) of the invasive Spartina densiflora. All together these studies stress hybridization as a primary stimulus in the invasive success of polyploid Spartina species.