Molecular evidence for allopolyploid speciation and a single origin of the narrow endemic Draba ladina (Brassicaceae)

Abstract

Draba ladina (Brassicaceae) is a small alpine flower endemic to the Swiss Alps. It occurs exclusively at elevations between 2600 and 3000 m and is restricted to less than a dozen mountains in the Lower Engadin. Morphological characters and polyploidy suggest a hybrid origin. Potential diploid progenitor species are distributed widely and often occur sympatrically. To study the evolutionary history of D. ladina we assessed intra- and interspecific sequence variation at noncoding chloroplast DNA loci and nuclear rDNA ITS sequences in D. ladina and its presumed progenitor species D. aizoides, D. dubia, and D. tomentosa. A single ITS (Internal Transcribed Spacer) genotype was found in each of D. aizoides and D. dubia and two in D. tomentosa. Additivity of ITS sequences of D. aizoides and D. tomentosa was found in D. ladina, supporting the hypothesis of an allotetraploid origin. Intraspecific cpDNA variation was found in all diploid species, but not in D. ladina. The single chloroplast DNA haplotype found in the latter was closest to one cpDNA haplotype found in D. tomentosa, suggesting that D. tomentosa was the maternal parent. These results suggest that D. ladina is a relatively young, presumably postglacial, taxon with a single allopolyploid origin.