Phylogeny and systematics of the marine algal family Gracilariaceae (Gracilariales, Rhodophyta) based on small subunit rDNA and its sequences of Atlantic and Pacific species

We sequenced the small subunit rDNA and internal transcribed spacer region of Gracilariaceae from the tropical Atlantic and Pacific, with emphasis on flattened or compressed species. Sequence comparisons confirmed three main lineages of Gracilariaceae: Curdiea /Melanthalia , Gracilariopsis /Gracilariophila , and Gracilaria. The Curdiea /Melanthalia diverged early in the family. Gracilariopsis was paraphyletic, because at least one Gracilariophila species evolved from it. The Atlantic Gracilariopsis were monophyletic and separated from the Pacific lineages. The Gracilaria included all species referable to its own species and to Hydropuntia , which was paraphyletic, formed by distantly related lineages. The new combination Gracilaria pauciramosa (N. Rodriguez Rios) Bellorin, M. C. Oliveira et E. C. Oliveira is proposed for Polycavernosa pauciramosa N. Rodriguez Rios. Recognition of subgenera within Gracilaria , based on spermatangial arrangement, was not supported. Instead, infrageneric groups were delineated by geographic origins and combinations of reproductive characters. Most Pacific species with either "textorii " or "verrucosa " type spermatangia were deeply separated from Atlantic species. Within the Atlantic Gracilaria , a lineage encompassing mostly tropical cylindrical species with "henriquesiana " type spermatangia and distinctive cystocarp anatomy was recognized. A lineage was also retrieved for cold water stringy species with verrucosa type spermatangia. Several species from the western Atlantic are closely related to Gracilaria tikvahiae McLachlan with nearly identical morphology. On the other hand, most flattened species from the tropical Atlantic were closely related despite their diverse morphologies. The interpretation of our data in addition to the literature indicates that more populations from the Indo-Pacific must be studied before a general picture of Gracilariaceae evolution can be framed.