Phylogenetic analyses of Caulerpa taxifolia (Chlorophyta) and of its associated bacterial microflora provide clues to the origin of the Mediterraneanintroduction

The accidental introduction of Caulerpa taxifolia into the Mediterranean is no longer under dispute. What has eluded researchers until now is definiti ve evidence for the original, biogeographical source population. Here we pr esent two independent lines of evidence that support an Australian origin f or the Mediterranean populations of C. taxifolia. First, we reanalysed alga l rDNA-internal transcribed spacer (rDNA-ITS) sequences, combining previous ly published sequences from different studies with 22 new sequences. The IT S sequence comparison showed that the Australian sample is the sister group of the Mediterranean-aquarium clade. Second, cloned bacterial 16S rDNA gen e sequences were analysed from the associated microflora of C. taxifolia co llected from Australia, Tahiti, the Philippines and the Mediterranean. Five bacterial lineages were identified, of which three were dominant. alpha Pr oteobacteria were the most abundant and were found in all samples. In contr ast, members of the beta Proteobacterial line and Cytophaga-Flexibacter-Bac teroides line (CFB) were mainly associated with Mediterranean and Australia n samples. Frequency distributions of the five bacterial lineages were sign ificantly different among biogeographical locations. Phylogenetic analyses of the 54 bacterial sequences derived from the four C. taxifolia individual s resulted in a well-resolved tree with high bootstrap support. The topolog ies of the beta Proteobacteria and CFB mirror the geographical sources of t heir algal hosts. Bacterial-algal associations provide an identification to ol that may have wide application for the detection of marine invasions.