Evolution of four Southern Hemisphere Bostrychia (Rhodomelaceae, Rhodophyta) species: phylogeny, species delimitation and divergence times

摘要

Defining species boundaries is important to address evolutionary questions and understand true biodiversity in a region. Determining the actual number of species is not trivial, and numerous species-delimitation algorithms have been developed and extensively applied to a wide range of organisms. The cosmopolitan red-algal genus Bostrychia has been used as a model system to study evolutionary processes, yet phylogeny within the genus and its true species composition are still unresolved. The four species B. arbuscula, B. gracilis, B. intricata and B. vaga represented a distinctive morphotype that was once the basis for segregation into the separate genus Stictosiphonia, and all appeared to be restricted to the Southern Hemisphere. We sequenced genes from all three genomes [plastid: rbcL; mitochondrial: cytochrome c oxidase subunit I(COI); and nuclear: large-subunit ribosomal RNA] to examine the phylogenetic relationships of the four species and to establish their diacritical features. Our phylogenetic analyses from combined data sets strongly supported the monophyly of these species, with B. vaga as a sister species to the other three. Results from phylogenetic analyses of a combined data set and species-delimitation methods based on COI data demonstrated a congruent pattern of species boundaries, indicating cryptic species diversity within presently constituted B. intricata and B. vaga. We also estimated the divergence time of these species using substitution rates of combined rbcL and COI data sets calibrated from B. calliptera collected around the Isthmus of Panama. Results indicated that these four species formed in the middle Oligocene epoch (c. 30 million years ago), suggesting a post-Gondwana origin and relatively ancient divergence. We speculate that evolution of these species may have been due to transoceanic dispersal that was facilitated by the circulation of the Antarctic Circumpolar Current and subsequent isolation.