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Abstract
Small-subunit ribosomal DNA (SSU rDNA) sequences have been newly determined for 22 species of Rhodymeniales, bringing to 32 the total number included in our data set. Analyses of these sequences constitute a molecular-systematic survey of some 60% of the genera in the order, 60% of the treated genera represented by type species. As a result, several aspects of the currently accepted infraordinal groupings of taxa are challenged because the three component families (Champiaceae, Lomentariaceae, and Rhodymeniaceae) and several of their tribal subdivisions are not resolved as monophyletic. We therefore emend the defining characters of the families, producing a more restricted Rhodymeniaceae (with removal of Gelidiopsis and Kylin's Fauchea Group) and an expanded Champiaceae (by inclusion of the heretofore unplaced Dictyothamnion) and Lomentariaceae (by inclusion of the genus Gelidiopsis). A new family, Faucheaceae I.M.Strachan, G.W. Saunders et Kraft, is established for Kylin's Fauchea Group. Two new genera, Irvinea Guiry in G.W. Saunders, I.M. Strachan et Kraft and Sparlingia G.W. Saunders, I.M. Strachan et Kraft, are proposed for Botryocladia ardreana Brodie et Guiry and Rhodymenia pertusa (Postels et Ruprecht) J. Agardh, respectively—two species that differ significantly in both morphology and SSU rDNA sequences from other species of the genera to which they are currently assigned. Based on evidence from our molecular data, morphological features of the medulla, on which familial classification of the Rhodymeniales now largely rests, are viewed as being of minor importance. We feel that greater taxonomic value should be placed on reproductive features, in particular, carpogonial-branch cell number and tetrasporangial position and cleavage pattern, and we suggest hypothetical evolutionary scenarios that might have led to the present-day distribution of these features within the order.