Speciation in Callitriche (Plantaginaceae): the allopolyploid origin of C. platycarpa

Genomic in situ hybridisation (GISH) experiments involving hybridising labelled genomic DNA from the diploid species C. stagnalis, C. obtusangula and C. cophocarpa to chromosome spreads of the tetraploid C. platycarpa (2n = 4× = 20) show that C. stagnalis and C. cophocarpa DNA each hybridise with a different ten chromosomes of the C. platycarpa complement, whereas C. obtusangula DNA does not differentiate between the two genomes. We conclude on the basis of this and other evidence that C. platycarpa is likely to be an allotetraploid derivative of C. cophocarpa and C. stagnalis. This work demonstrates the successful application of GISH to the study of polyploid evolution in the genus Callitriche.