Nuclear small-subunit ribosomal DNA sequences were determined for 65 members of the Gigartinales and related orders. With representatives of 15 families of the Gigartinales sensu Kraft and Robins included for the first time, our alignment now includes members of all but two of the ca. 40 families. Our data continue to support ordinal status for the Plocamiales, to which we provisionally transfer the Pseudoanemoniaceae and Sarcodiaceae. The Halymeniales is retained at the ordinal level and consists of the Halymeniaceae (including the Corynomorphaceae), Sebdeniaceae, and Tsengiaceae. In the Halymeniaceae, Grateloupia intestinalis is only distantly related to the type species, Grateloupia filicina, but is closely affiliated with the genus Polyopes. The Nemastomatales is composed of the Nemastomataceae and Schizymeniaceae. The Acrosymphytaceae (now including Schimmelmannia, formerly of the Gloiosiphoniaceae) and the Calosipho niaceae (represented by Schmitzia) have unresolved affinities and are considered as incertae sedis among lineage 4 orders. We consider the Gigartinales sensu stricto to include 29 families, although many contain only one or a few genera and mergers will probably result following further investigation. Although the small-subunit ribosomal DNA was generally too conservative to resolve family relationships within the Gigartinales sensu stricto, a few key conclusions are supported. The Hypneaceae, questionably distinct from the Cystocloniaceae on anatomical grounds, is now subsumed into the latter family. As recently suggested, the Wurdemanniaceae should be incorporated into the Solieriaceae, but the latter should not be merged with the Areschougiaceae. The Corynocystaceae Kraft, fam. nov., is described and added to the Gigartinales sensu stricto.Key words: Corynocystaceae, Cryptonemiales, Florideophyceae, Gigartinales, Rhodymeniales, systematics.
Phylogenetic relationships of land plants using mitochondrial small-subunit rDNA sequences
