Phylogeography of the cosmopolitan red alga Caulacanthus ustulatus (Caulacanthaceae, Gigartinales)

Abstract

SUMMARY Molecular analyses of mitochondrial (cox2–3 spacer) and plastidal (Rubisco spacer) genomic DNA sequence data show that specimens of Caulacanthus ustulatus (Turner) Kutzing form two distinct lineages: a Pacific lineage containing samples from China, Korea, Australia, the Philippines and the USA, plus a cryptic introduction to Roscoff, France; and an Atlantic Ocean lineage containing samples from Spain and Portugal. Although two well-supported ocean basin lineages are evident, the position of the tropical Pacific Ocean samples, within the Pacific Ocean lineage, remains unresolved owing to apparent incongruence between the two data sets. The partition homogeneity test indicates no incongruence in data sets, although some tree topology tests indicate significant differences in tree topology. Combined data sets produce trees with reduced resolution in some branches, indicating that even in uniparentally inherited organellar DNA, the appropriateness of combining data sets must be thoroughly tested. Caulacanthus okamurae Yamada has been proposed as distinct from C. ustulatus based on axis diameter. Literature descriptions of Caulacanthus species showing a wide range of axis diameters have been reported. Changes in culture conditions (increased light levels, water motion) stimulated thickening of vegetative shoots, reproduction, cuticle exfoliation and hair cell formation in isolates of C. ustulatus. This morphological plasticity, confusion in the literature and sequencing of a putative isolate of C. okamurae indicates that at present only one species of Caulacanthus should be recognized.