Most Spartina species originate from the New World, and among the four species which are native to the Old world, three are of hybrid origin, being the result of the introduction of the East American Spartina alterniflora in western-Europe, and its subsequent hybridization with the indigenous Spartina maritima. In England, hybridization resulted in a sterile hybrid, S. x townsendii, which gave rise around 1890, to the new fertile allopolyploid Spartina anglica. This species has since spread into several continents, causing dramatic ecological changes. Another sterile hybrid between S. alterniflora and S. maritima has been reported at the end of the 19th century in southwest France, which has been named S. x neyrautii. In order to analyze early molecular changes affecting natural hybrid and allopolyploid species, and their role in adaptation and stabilization of a new species, molecular investigations have been undertaken on populations of European spartines. All the species involved in the speciation process are analyzed: the parents S. alterniflora and S. maritima, the F1 sterile hybrids, and the allopolyploid S. anglica. The latter species has been analyzed in both its native range and in more recently colonized areas (Australia). Different portions of the genome have been investigated using sequencing, RFLPs, and retrotransposon based fingerprinting methods, and genetic divergence between the parental species has been estimated. Our results show that the parental species display consistent genetic differentiation, and that both genomes remain unchanged when reunited and duplicated in the same nucleus. Most populations of Spartina anglica contain the same major genotype, which has been colonizing different continents. This young species then represents a system where genetic diversity is restricted to the intra-individual (intergenomic) level, as a result of both genetic botteleneck at the time of the species formation, and structural stasis of the homeologue subgenomes.

Key words: Allopolyploid, genome evolution, hybridization, Spartina