European freshwater Hildenbrandia (Hildenbrandiales, Rhodophyta) has not been derived from multiple invasions from marine habitats

Abstract

Skuja [Botanical Review 4: 665–676 (193S)] proposed that some freshwater red algae might be derived through invasions by marine species into freshwater habitats. We examined the likelihood that this has occurred in Hildenbrandia by carrying out molecular analyses of 11 freshwater (H. rivularis) and five brackish (H. rubra) collections of the genus from a region of southern Sweden. DNA sequence analyses of the internal transcribed spacer (ITS) regions demonstrated a clear distinction between the freshwater and the brackish collections, in that the two groups of sequences were unalignable with each other. Network topologies based on DNA comparisons did not correspond to the ages of the freshwater bodies. Intersimple sequence repeat and ITS analyses yielded partially congruent groupings among the freshwater collections, with two main clusters evident based on both sets of data. Comparisons of five of the Swedish samples with 10 previous European collections, using the gene coding for the large subunit of the Rubisco enzyme (rbcL) and the ISS rRNA gene, demonstrated little or no corrected sequence divergence among the freshwater samples (0–2.0% for rbcL; 0–0.5% for ISS rRNA) but much higher divergence among marine samples (4.9–23.4% for rbcL; 0.6–3.4% for ISS rRNA). These data support the idea of a single origin for the European freshwater collections of Hildenbrandia, provided that there are no large differences in the rate of evolution between marine and freshwater Hildenbrandia species.