PHYLOGENETIC SYSTEMATICS AND BIOGEOGRAPHY OF THE GIGARTINACEAE (GIGARTINALES, RHODOPHYTA) BASED ON SEQUENCE-ANALYSIS OF RBCL

Recently Hommersand, Guiry, Fredericq and Leister (1993, Hydrobiologia 260/261 : 105-120) proposed a revised classification of the marine red algal family Gigartinaceae in which sixty-nine species were classified into four extant (Chondrus Stackhouse, Gigartina Stackhouse, Iridaea Bory, nom. cons., Rhodoglossum J. Agardh) and three reinstated (Chondracanthus Kutzing, Sarcothalia Kutzing, Mazzaella G. de Toni f.) genera based on developmental and morphological criteria. We have undertaken a preliminary study of the phylogenetic systematics and biogeography of the Gigartinaceae based on an independent data set derived from sequence analysis of rbcL, the gene that codes for the large subunit of RuBisCO. The topology of the rbcL tree, which contains 43 species, generally supports our recent systematic revision, while highlighting some taxonomic problems. 'Gigartina' alveata occupies a basal position isolated from all other taxa. Chondracanthus forms a distinct clade with centers of speciation in East Asia and Pacific North America. The Gigartina/Rhodoglossum clade is primarily austral and appears to have originated in the southwestern Pacific Ocean. Sarcothalia is antiboreal. Iridaea, 'Gigartina' skottsbergii, three Mazzaella clades, and Chondrus form a cluster partly localized in antiboreal waters but extending along Pacific South and North America to East Asia, with one species, Chondrus crispus, in the North Atlantic Ocean. We propose that ancestral taxa belonging to the Gigartinaceae originated along the eastern edge of Gondwanaland in the Mesozoic and spread around the perimeter of the supercontinent Pangea giving rise to present-day genera in more or less linear sequence, followed by secondary dispersal of some species.