American Journal of Botany 91 10 1494 1507 2004 ASSESSING RED ALGAL SUPRAORDINAL DIVERSITY AND TAXONOMY IN THE CONTEXT OF CONTEMPORARY SYSTEMATIC DATA1 GARY W SAUNDERS2 4 AND MAX H HOMMERSAND3 Centre for Environmental Molecular Algal Research Department of Biology University of New Brunswick Fredericton New Brunswick Canada E3B 6E1 and 3Department of Biology University of North Carolina Chapel Hill North Carolina 27599 3280 USA 2 The wondrously diverse eukaryotes that constitute the red algae have been the focus of numerous recent molecular surveys and remain a rich source of undescribed and little known species for the traditional taxonomist Molecular studies place the red algae in the kingdom Plantae however supraordinal classification has been largely confined to debate on subclass vs class level status for the two recognized subgroups one of which is widely acknowledged as paraphyletic This narrow focus has generally masked the extent to which red algal classification needs modification We provide a comprehensive review of the literature pertaining to the antiquity diversity and systematics of the red algae and propose a contemporary classification based on recent and traditional evidence Key words Bangiophyceae Compsopogonophyceae Cyanidiophyta Eurhodophytina Florideophyceae Metarhodophytina Rhodophyta Rhodoplantae The biological significance of red algae is only beginning to be appreciated Even among professional biologists knowledge of these organisms is often minimal and based on cursory information contained in general botany textbooks Woelkerling 1990 p 1 Macroalgal systematics traces its modern era to rather humble beginnings the multitude of diverse species assigned to a few genera in a subdivision Algae of the class Cryptogamia which also included the ferns mosses and fungi positioned among 23 classes of cone bearing and flowering plants in the plant kingdom Linnaeus 1753 This early classification clearly underrepresented macroalgal diversity and substantial taxonomic refinements inevitably followed Lamouroux 1813 was the first to use color to segregate algal assemblages when he removed certain red algae from their respective associations with species of like morphology to the Floride es Harvey 1836 took the biochemical marker further and established the Chlorospermae Melanospermae and Rhodospermae for green brown and red algae respectively in essence establishing the three major groups of macroalgae that are recognized today Numerous taxonomic changes were implemented in the ensuing decades but the relatively recent advent of ultrastructural and molecular systematic data in particular have uncovered the bewildering diversity as well as evolutionary affinities of the chlorophytic and chromophytic lineages Recent taxonomic treatments vary but schemes including as many as 10 classes in two phyla are now presented for the chlorophytic line cf Lewis and McCourt 2004 The brown algae are included in a larger chromophytic lineage Heterokontophyta Manuscript received 15 January 2004 revision accepted 17 June 2004 During preparation of this review GWS was supported by funds from the Natural Sciences and Engineering Research Council of Canada and the Canada Research Chair Program and MHH was supported by NSF PEET grant DEB 0328491 We are indebted to P Silva and M J Wynne for helpful discussions regarding the ICBN as well as P Silva for drawing the work of Doweld to our attention We thank R Moe for Latin translations Comments from J D Palmer and two anonymous reviewers were genuinely appreciated and served to improve the clarity of this manuscript 4 E mail gws unb ca 1 including a wide diversity of lineages distributed among some 15 classes cf Andersen 2004 The Rhodophyta have not experienced a similar explosion in taxonomic breadth with our current perspective on supraordinal relationships virtually unchanged since ca 1900 and confined to a continuing debate as to whether the two constituent lineages Bangiophyceae and Florideophyceae should be recognized as distinct classes or subsumed as subclasses within a single class Rhodophyceae cf Dixon 1973 The current system of red algal classification creates the illusion that this lineage is relatively limited in its diversity when compared to chlorophytes and chromophytes this despite the wide range of morphology observed among red algae and a wealth of contemporary ultrastructural and molecular data that speak to the antiquity and diversity within Rhodophyta This review sets as its aim to amass the available information on red algal phylogeny diversity and antiquity and to use this to reform red algal taxonomy It is not our intent to deal with the broader issue of red algal affinities relative to the other major eukaryotic lines The reader is directed to an overview of that topic by Keeling 2004 in this issue FOSSILS AND ANTIQUITY OF THE RED ALGAE The red algae are a study in extremes Morphologically more diverse than any other group of algae they range from single cells to large ornate multicellular plants Uniquely among nonfungal eukaryotes they lack both flagella and centrioles and exhibit a remarkable often bizarre range of reproductive strategies Butterfield 2000 p 386 The earliest putative red algal fossils date to ca 2 billion years before present and are superficially similar to the extant taxa of the Porphyridiales Bangiophyceae Tappan 1976 Although the evolutionary scenario presented by Tappan in which the red algae represent a direct link between the prokaryotic cyanophytes and other eukaryotes is widely rejected in light of recent phylogenetic hypotheses e g Ragan and Gutell 1995 the possibility that unicellular red algae did exist at such an early stage Fig 1 is compatible with the fossils 1494 October 2004 SAUNDERS AND HOMMERSAND RED ALGAL SYSTEMATICS 1495 Fig 1 Geological time scale from a general biology text Campbell and Reece 2002 modified to place red algal fossils in context with other major evolutionary events observed by Tappan and other scientists Most notable in this regard is the old 1 2 billion years and relatively advanced fossil taxon Bangiomorpha Butterfield 2000 Bangiomorpha pubescens Butterfield 2000 was described from the 1200million year old Hunting Formation in the Canadian Arctic and represents the earliest putative record for sex and taxonomically resolvable complex multicellularity among eukaryotes Thus its likeness to taxa currently included in the red algal order Bangiales is truly
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