Tetraploid European Salicornia species are best interpreted as ecotypes of multiple origin

Salicornia procumbens and S. stricta are two tetraploid European salt marsh species of locally adjacent but ecologically differentiated distribution. Whereas S. procumbens grows in the lowest part of the salt marsh, it is replaced by S. stricta in the middle part (and diploid Salicornias in the upper part). Using AFLPs and a reciprocal transplantation experiment, we investigated whether the two species represent distinct evolutionary lineages. The analysis of AFLP variation clearly showed that both species are not monophyletic. Also, accessions do not cluster according to geographical origin. The transplantation experiment revealed that S. procumbens shows significantly reduced fitness when transplanted into the habitat of S. stricta. Reduction of fitness of S. stricta in the habitat of S. procumbens is less obvious. In both habitats S. procumbens performs better than S. stricta in terms of seed production and dry weight. On the background of these results, it is hypothesized that S. procumbens and S. stricta represent intraspecific ecotypes which originated repeatedly in adaptation to their specific environments and which dispersed widely. We hypothesize that the ecologically vicariant distribution of S. procumbens and S. stricta along the inundation gradient in the saltmarsh in spite of their good performance in the respective non-native habitats results from strong selection during seed germination and seedling establishment. Reproductive isolation between locally adjacent or even intermingled ecotypes is suggested to be achieved by high levels of selfing.