Origin of Spergularia ×kurkae, a hybrid between the rare endemic S. echinosperma and its widespread congener S. rubra.

The origin of Spergularia ×kurkae, a presumed tetraploid hybrid between the diploid central- European endemic S. echinosperma and its widespread tetraploid congener S. rubra, was investigated by sequencing the nrDNA ITS region and cpDNA rpoC1 intron. Spergularia echinosperma and S. rubra differed markedly in their ITS sequences. The presence of both sequences within the genome of S. ×kurkae confirmed its hybrid origin and parentage; cpDNA sequences identified S. echinosperma as the sole maternal parent. Because both parental ITS homeologs were clearly visible in the sequences of almost all of the S. ×kurkae individuals, we conclude that this taxon is of a relatively young age. We hypothesize that S. ×kurkae might have evolved as a result of human-mediated introduction of S. rubra into fishponds. Cross-amplification of species-specific ITS primers revealed high levels of intra-individual ITS polymorphisms in S. echinosperma and S. rubra. Our results suggest ongoing gene flow from S. ×kurkae to S. rubra. In contrast, no evidence of gene flow from S. ×kurkae or S. rubra to S. echinosperma was found, providing, despite concerns, no support for the threat of the genetic assimilation of S. echinosperma. Our current data also support the view of S. kurkae as a stabilized, separate allopolyploid species.