Hybridization has long been considered a source of taxonomic complexity in Rubus. This study uses molecular data to examine natural hybridization between R. caesius, a facultatively agamospermous tetraploid of subgenus Rubus (blackberries) and R. idaeus, a sexual diploid of subgenus Idaeobatus (raspberries). Both species are widespread in Europe, and hybridization between them is postulated to have given rise to numerous species, including R. maximiformis and R. picticaulis. We sequenced the nuclear ribosomal DNA internal transcribed spacer (ITS) region of R. caesius, R. idaeus, five putative hybrids from Sweden, and one individual each of R. maximiformis and R. picticaulis from Germany. Rubus caesius differs from R. idaeus at 20 nucleotide sites and one insertion/deletion event. The putative hybrids exhibit nucleotide polymorphism additivity at all or most of these sites indicating that gene flow does occur naturally between R. caesius and R. idaeus. For each of five putative hybrids we analyzed two to 21 ITS region clones by sequencing and using two restriction enzymes that distinguish the parental species at three sites. Individual 729 has only intact parental ITS sequences and may therefore be an F1, while the other four show chimeric (mixtures of parental) ITS sequences, suggesting that they may be later-generation hybrids.
Communicating Editor: James R. Manhart