Nuclear small subunit ribosomal RNA (18S rRNA) gene sequences were determined for six species representing three genera classified within the Mastophoroideae and also for two species of Amphiroa (Lithophylloideae). These data were combined with previously published 18S rRNA sequences for 37 other coralline species and analyzed to 1) determine the phylogenetic position of the Mastophoroideae within the Corallinales, and 2) examine relationships among Hydrolithon, Metamastophora, Neogoniolithon, and Spongites. Trees derived from parsimony and maximum likelihood analyses of these data indicate that the Mastophoroideae is not monophyletic. Instead, our data suggest that group is polyphyletic and includes species belonging to three (or more) distinct evolutionary lineages. The nongeniculate genus Neogoniolithon is resolved as sister to the geniculate subfamily Corallinoideae. Hydrolithon is resolved as sister to the geniculate genus Metagoniolithon, an Australian endemic. In turn, Spongites is positioned at the base of a clade including Hydrolithon/Metagoniolithon as well as species classified in the Lithophylloideae. Finally, Metamastophora is placed in a well supported, but isolated, position as sister to all other taxa bearing uniporate tetrasporangial conceptacles. Our results imply that some reproductive and vegetative characters among the suite of features used to delimit the Mastophoroideae were acquired independently and are not homologous. Significantly, our results suggest that genicula are nonhomologous structures that evolved independently in four separate coralline taxa and from four nongeniculate ancestors belonging to distinct evolutionary lineages. On the basis of our findings Porolithon pachydermum is transferred to Hydrolithon as H. pachydermum (Foslie) Gabel, Bailey et Freshwater.

Key words: 18S rRNA, algae, Corallinales, Mastophoroideae, phylogeny, taxonomy