Ribosomal DNA revealed an extensive role of allopolyploidy in the radiation of Ulex L.

Abstract

We studied the phylogeny of Ulex L., a genus of spiny legumes, which its center of diversity in the Iberian Peninsula, using ribosomal DNA markers (rDNA), namely ETS, 5.8S and ITS (45S), and 5S intergenic spacer regions. One of the main findings was the presence of very different haplotypes in 5S-IGS genes and, to a less extent, in ETS and ITS, in seven polyploid taxa. We interpreted these results as an indication of hybrid origins and proposed allopolyploidy for U. argenteus ssp. subsericeus, U. australis ssp. australis, U. australis ssp. welwitschianus, U. densus, U. europaeus ssp. europaeus, U. europaeus ssp. latebracteatus, and U. jussiaei. These results reinforce an early hypothesis which stated that the radiation of Ulex occurred mainly by polyplodization. Phylograms showed two main clades, one grouping the hydrophilic U. gallii, U. breoganii and U. minor, and the other grouping the southern, xerophytic, taxa. The putative allopolyploids showed haplotypes, which grouped in both clades, indicating that allopolyploidy, occurred through hybridization from these hydrophilic and xerophytic lineages. The phylogenetic position of U. micranthus is not certain and it is discussed. The 5S-IGS showed to retain more polymorphisms than ETS gene or ITS markers. This result is compatible with the hypothesis that 5S rDNA region is less vulnerable to inter-loci concerted evolution than 45S, providing a more suitable marker for reconstructing histories of allopolyploid species. We discuss the taxonomic consequences of these results.