During the last centuries, species introductions outside their native range have dramatically increased and have accelerated ecological changes, native species threat, but also biological diversification through rapid expansion of introduced plants, interspecific hybridization with natives and allopolyploid speciation. In evolutionary ecology, the polyploid grass genus Spartina provides one of the most fascinating examples with this respect, and it well-illustrates the multifaceted challenges of invasive population management. Spartina contains several species that have become highly successful invaders of intertidal mudflats and saltmarshes where they play an important ecological role as “ecosystem engineers”. In Europe, such examples include (i) the cryptic introduction of the East-American tetraploid S. patens (which has until recently been considered as a Mediterranean endemic under the name of S. versicolor), (ii) introduction of the South-American heptaploid S. densiflora to the south-west coasts of the Iberian Peninsula and its subsequent hybridization with the native hexaploid S. maritima and (iii) introduction of the East-American hexaploid S. alterniflora to western Europe (UK, France, Spain) and its hybridization with S. maritima in England and in southwest France. In England, hybrid genome duplication resulted in a highly successful allododecaploid species, S. anglica that has now colonized European saltmarshes and is introduced in several continents. Spartina represents then an excellent model system to explore at various evolutionary time scales the genomic determinants of species expansion. Hybrid and polyploid genome dynamics, gene expression evolution in stressful conditions will be discussed in the light of their ecological implications