Phylogenetic positions of Lithophyllum incrustans and Titanoderma pustulatum (Corallinaceae, Rhodophyta) based on 18S rRNA gene sequence analyses, with a revised classification of the Lithophylloideae

This study was initiated to reexamine phylogenetic relationships among coralline algae having secondary pit connections and lacking cell fusions. Nuclear small-subunit ribosomal RNA (18S rRNA) gene sequences were determined for the nongeniculate species Lithophyllum incrustans Philippi and Titanoderma pustulatum (Lamouroux) Nägeli and aligned with 18S rRNA sequences for 35 other coralline species. Parsimony and maximum likelihood analyses of these data yielded congruent trees in which four major lineages were resolved. In these trees, geniculate and nongeniculate species having secondary pit connections and lacking cell fusions formed a well-supported clade within the Corallinaceae. Within this clade, the geniculate genera Amphiroa and Lithothrix do not form a monophyletic group. Results suggest that the genicula of Amphiroa and Lithothrix are probably not homologous; these genera are more closely related to nongeniculate species than they are to one another. A close relationship inferred between Amphiroa and Titanoderma was robustly supported in all analyses performed. The molecular data imply that Titanoderma and Lithophyllum are phylogenetically distinct and should be maintained as separate genera. Based on these and other data, a revised classification of the Lithophylloideae is presented and the subfamily is emended. Possible phylogenetic relationships among the Lithophylloideae and other coralline subfamilies are discussed and an overview of genicular evolution within the Corallinales is presented.