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UW-Madison BOTANY 940 - DINOFLAGELLATES - A REMARKABLE EVOLUTIONARY EXPERIMENT

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1523American Journal of Botany 91(10): 1523–1534. 2004.DINOFLAGELLATES:A REMARKABLE EVOLUTIONARYEXPERIMENT1JEREMIAHD. HACKETT,2DONALDM. ANDERSON,3DEANAL. ERDNER,3ANDDEBASHISHBHATTACHARYA2,42Department of Biological Sciences and Center for Comparative Genomics, University of Iowa, Iowa City, Iowa 52242 USA; and3Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 USAIn this paper, we focus on dinoflagellate ecology, toxin production, fossil record, and a molecular phylogenetic analysis of hostsand plastids. Of ecological interest are the swimming and feeding behavior, bioluminescence, and symbioses of dinoflagellates withcorals. The many varieties of dinoflagellate toxins, their biological effects, and current knowledge of their origin are discussed.Knowledge of dinoflagellate evolution is aided by a rich fossil record that can be used to document their emergence and diversification.However, recent biogeochemical studies indicate that dinoflagellates may be much older than previously believed. A remarkable featureof dinoflagellates is their unique genome structure and gene regulation. The nuclear genomes of these algae are of enormous size, lacknucleosomes, and have permanently condensed chromosomes. This chapter reviews the current knowledge of gene regulation andtranscription in dinoflagellates with regard to the unique aspects of the nuclear genome. Previous work shows the plastid genome oftypical dinoflagellates to have been reduced to single-gene minicircles that encode only a small number of proteins. Recent studieshave demonstrated that the majority of the plastid genome has been transferred to the nucleus, which makes the dinoflagellates theonly eukaryotes to encode the majority of typical plastid genes in the nucleus. The evolution of the dinoflagellate plastid and theimplications of these results for understanding organellar genome evolution are discussed.Key words: dinoflagellate; endosymbiosis; evolution; harmful algal blooms.The dinoflagellates (division Pyrrhophyta, class Dinophy-ceae) are an important group of phytoplankton in marine andfresh waters. Their adaptation to a wide variety of environ-ments is reflected by a tremendous diversity in form and nu-trition and an extensive fossil record dating back several hun-dred million years (Graham and Wilcox, 2000). As swimmingcells, they can flourish under conditions that are unsuitable formany nonmotile phytoplankton, a success due in part to uniquebehavior patterns, including diel vertical migration (migrationthrough the water column on a 24-h cycle). Some dinoflagel-lates produce toxins that are dangerous to man, marine mam-mals, fish, seabirds, and other components of the marine foodchain (Van Dolah, 2000). Others are bioluminescent and emitlight; some function as parasites or symbionts that rely on hostorganisms for part of their nutrition. Many dinoflagellates arephotosynthetic and, through endosymbiosis, have acquired awide diversity of plastids from distant evolutionary lineages.The most common plastid in dinoflagellates has been subjectto drastic evolutionary changes that we are only beginning tounderstand. An equal number of dinoflagellates obtain theircarbon by ingesting other phytoplankton. Many are now beingshown to have both of these traits—i.e., to be mixotrophic. Itis thus no surprise that these organisms have been extensivelystudied and classified as plants by some workers and as ani-mals by others.General characteristics—Whether living as a swimming,solitary cell or a nonmotile symbiont within an invertebratehost, all living dinoflagellates have certain common character-istics (Steidinger, 1983). Most photosynthetic species containchlorophylls a and c2, the carotenoid beta-carotene, and agroup of xanthophylls that appears to be unique to dinofla-gellates, typically peridinin, dinoxanthin, and diadinoxanthin.1Manuscript received 20 January 2004; revision accepted 4 June 2004.4Author for reprint requests.These pigments give many dinoflagellates their typical golden-brown color. However, some dinoflagellates have acquired oth-er pigments through endosymbiosis, including fucoxanthin(see the following plastid discussion). Two different cell typescan be distinguished on the basis of the cell-wall covering ortheca. The ‘‘naked’’ or unarmored forms have an outer plas-malemma surrounding a single layer of flattened vesicles.These cells are fragile and distort easily. Armored dinoflagel-lates have cellulose or other polysaccharides within each ves-icle, giving the cells a more rigid, inflexible wall. These cel-lulose plates are arranged in distinct patterns (called ‘‘tabula-tion’’), which are extensively used as taxonomic ‘‘finger-prints.’’ For a detailed discussion of dinoflagellate taxonomy,see Fensome et al. (1993). The dinoflagellate nucleus is uniquein several ways, as elaborated in more detail later. The chro-mosomes, for example, are easily visible at all stages ofgrowth because they do not go through coiling and uncoiling,as is common in other phytoplankton, but instead remain per-manently condensed. Dinoflagellates also have few or no nu-cleosomes associated with their DNA and a unique pattern ofmitosis (Spector, 1984). Because these characteristics are sodifferent from both eukaryotic and prokaryotic cells, a newintermediate kingdom, Mesokaryota, was once proposed forthem (Dodge, 1965). Yet another distinguishing characteristicof dinoflagellates is that their motile cells have two unequalflagella. One is a flattened, ribbon-like flagellum, which en-circles the cell in a transverse groove, providing propulsiveand spinning force for the cell. The other flagellum is directedposteriorly along a longitudinal groove and presumably actslike a rudder for steering. Although all dinoflagellates sharecertain physiological and structural characteristics, they exhibita tremendous diversity in external morphology. Some cells aresmall and smoothly spherical, whereas others have elaboratestructures that resemble horns, wings, collars, or even armsand hands with fingers.1524 [Vol. 91AMERICANJOURNAL OFBOTANYFig. 1. An illustration of the dinoflagellate (Dn) Protoperidinium depres-sum feeding on a chain of diatoms (Dt) using a pallium, a retractile organellethat spreads over the long spines of diatoms so that the contents can be di-gested. Illustration by D. M. Jacobson (reproduced from Jacobson, 1987).Ecology—Several aspects of the behavior,


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UW-Madison BOTANY 940 - DINOFLAGELLATES - A REMARKABLE EVOLUTIONARY EXPERIMENT

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