Phylogeography of the Bostrychia calliptera-B-pinnata complex (Rhodomelaceae, Rhodophyta) and divergence rates based on nuclear, mitochondrial and plastid DNA markers

The species Bostrychia calliptera and B. pinnata, separated by cortication of main axis, were analysed by sequencing regions from the three genomes (nuclear: large-subunit ribosomal RNA; mitochondrial: cox2-3 spacer; and plastid: Rubisco spacer) to determine the phylogeographic relationships among pantropical isolates. All genetic regions produced congruent phyloenetic relationships, with three major lineages being revealed within this species complex. One lineage comprised isolates from the Americas, another comprised isolates from the Indo-Pacific and a third contained isolates from both the Indo-Pacific and the Americas. Analyses testing for molecular clock-like rates of base substitutions show that certain lineages within the species complex evolve in a clock-like manner. Phylogenies do not support separation of B. calliptera and B. pinnata based on cortication of the main axis, and samples found in close proximity can belong to different highly divergent lineages. Molecular clock calibrations of divergence times within this complex, based on the vicariant event of the last closure of the Isthmus of Panama, reveal that the estimated divergence rates in the different genetic elements are slower than most published rates. Furthermore, divergence times indicate that this species complex is very old, having remained morphologically identical at least since the Oligocene (23-40 Ma); it may possibly have been associated with mangrove vegetation since the establishment and worldwide spread of mangroves in the Tertiary. Biogeographical patterns are complex and may reflect ancient Tethyan distributions. Molecular analyses may be the only way that the biogeography and evolution of morphologically static and ancient red algal species can be resolved.