rbcL sequences reveal multiple cryptic introductions of the Japanese red alga Polysiphonia harveyi

In Europe, the last 20 years have seen a spectacular increase in accidental introductions of marine species, but it has recently been suggested that b oth the actual number of invaders and their impacts have been seriously und erestimated because of the prevalence of sibling species in marine habitats . The red alga Polysiphonia harveyi is regarded as an alien in the British Isles and Atlantic Europe, having appeared in various locations there durin g the past 170 years. Similar or conspecific populations are known from Atl antic North America and Japan, To choose between three competing hypotheses concerning the origin of P. harveyi in Europe, we employed rbcL sequence a nalysis in conjunction with karyological and interbreeding data for samples and isolates of P. harveyi and various congeners from the Pacific and Nort h Atlantic Oceans. all cultured isolates of P. harveyi were completely inte rfertile, and there was no evidence of polyploidy or aneuploidy. Thus, this biological species is both morphologically and genetically variable: intra specific rbcL divergences of up to 2.1% are high even for red algae. Seven rbcL haplotypes were identified. The four most divergent haplotypes were ob served in Japanese samples from Hokkaido and south-central Honshu, which ar e linked by hypothetical 'missing' haplotypes that may be located in northe rn Honshu. These data are consistent with Japan being the centre of diversi ty and origin for P. harveyi. Two non-Japanese lineages were linked to Hokk aido and Honshu, respectively. A single haplotype was found in all North At lantic and Mediterranean accessions, except for North Carolina, where the h aplotype found was the same as that invading in New Zealand and California. The introduction of P. harveyi into New Zealand has gone unnoticed because P. strictissima is a morphologically indistinguishable native sibling spec ies. The sequence divergence between them is 4-5%, greater than between sam e morphologically distinct red algal species. Two different types of crypti c invasions of P. harveyi have therefore occurred. In addition to its intro duction as a cryptic sibling species in New Zealand, P. harveyi has been in troduced at least twice into the North Atlantic from presumed different sou rce populations. These two introductions are genetically and probably also physiologically divergent but completely interfertile.