Systematics and biogeography of the red algal order Hildenbrandiales (Rhodophyta)
The genetic variability of the genus 'Hildenbrandia' throughout its distributional range and the taxonomic implications of this variation were examined using a combination of DNA sequence analyses ('rbc'L and 18S rRNA genes, and ITS regions), other molecular marker techniques (ISSR analyses) and morphometric analyses. The phylogenetic relationship of ' Hildenbrandia' to the second genus of the Hildenbrandiales, ' Apophlaea', was also examined using these techniques. Analyses of North American 'Hildenbrandia' demonstrated high sequence divergence values within and among marine and freshwater forms (<=25.8% ['rbc 'L]; <=9.7% [18S rRNA]); although these levels were comparable in marine European members of the genus (<=24.9% ['rbc'L]; <=5.8% [18S rRNA]), they were much lower for freshwater European members (<=1.9% ['rbc'L]; <=3.6% [18S rRNA]). The 'rbc'L gene was transitionally saturated within the North American and global level comparisons, but the phylogenetic signal for the 18S rRNA gene was strong even among global collections. Biogeographic comparisons of marine and freshwater 'Hildenbrandia ' collections from a small geographical region (southern Sweden) revealed different relationships among the samples, but comparisons to other samples from Europe indicated a close phylogenetic relationship among all European freshwater 'Hildenbrandia'. Some trends revealed by morphometric analysis correlated with those yielded by phylogenetic analyses, especially at a global level. Except for the common marine species, 'H. rubra ', samples with similar morphologies formed clades in phylogenetic trees. Based on these analyses, as well as morphometric examinations of the type specimens and global representatives, taxonomic revision is recommended to reduce the number of currently accepted marine 'Hildenbrandia' taxa from 12 to seven, with no synonymies recommended for the freshwater taxa. Inclusion of 'Apophlaea' in the analyses indicated that although ' Apophlaea' is a monophyletic genus, 'Hildenbrandia' is not. A microscopic examination of the freshwater species, 'H. angolensis ', and its unique form of asexual reproduction demonstrated that gemmae develop within the algal thallus and are released from the thallus surface. Histochemical comparisons between gemma and thallus cells revealed large amounts of starch in the gemmae, but not in thallus cells.