Chloroplast DNA restriction site variation and RAPD-analyses in Cochlearia (Brassicaceae): Biosystematics and speciation

Abstract

Chloroplast DNA (cpDNA) restriction site variation (RFLP) has been analysed in the genus Cochlearia L. sections Cochlearia O.E. Schulz and Glaucocochlearia O.E. Schulz. Members of section Glaucocochlearia are clearly separated from those of section Cochlearia by 28 sire mutations, thus supporting taxonomic arrangements into the two sections mentioned. RAPD studies provide further strong evidence for closer relationships among taxa within each section than between sections. CpDNA sequence divergence values in section Cochlearia are extremely low. Although 91 accessions representing 11 species have been analysed by 25 restriction enzymes, only four restriction site mutations were detected characterising six different cp genome types. Low levels of chloroplast DNA divergence would suggest that taxa of section Cochlearia are closely related and most likely have diversified recently. Polyploid Cochlearia officinalis (2n = 24), C. anglica (2n = 48) and C. danica (2n = 42) displayed infraspecific variation for cp genomes, which could be partly explained by multiple origin and by reticulate evolution. RAPD analyses (22 primers, 140 informative characters, ten taxa from 17 accessions) do not only strongly support these arguments but also helped to clarify speciation processes and biogeographic aspects in more detail. It is argued that South West Europe was the primary center of origin of the polyploids, second centers may be the British Isles, Iceland and locally restricted parts of Middle Europe.