DESCRIPTION OF THE PHYLUM BICOSOECIDA (GRASSE 1926)
| EUKARYA> CHROMALVEOLATA> HETEROKONTAE> BICOSOECIDA |
BICOSOECIDA LINKS
| Bicosoecida (bi-ko-SE-si-da) comes from two Latin words meaning two (bi) and blind (caecus). The name likely stems from the common genus, Bicosoeca (also given as Bicoeca). As presented here, Bicosoecida has a much broader meaning and is at a higher rank than was described by Grasse (1926). |
| INTRODUCTION TO THE BICOSOECIDA Members of this group are flagellated and usually sessile and attach to the substrate by a posteriorly-directed or recurrent flagellum (text with tooltip) Recurrent flagella bend to the posterior end of the cell. Typically, they are identified as recurrent when anteriorly-directed flagella are present. . The anterior flagellum may or may not have tripartite tubular hairs and may or may not have a lorica (text with tooltip) A lorica is a covering that occurs outside of the cell membrane. It is secreted by the cell and usually is organic. Loricas do not completely enclose the cell. Periplasts, structures similar to loricas do enclose the cell. Lorica stands for armour. . Cells feed by directing food particles into a cytostome near the base of the anterior flagellum. They are rarely noticed but occur in marine and freshwater environments. There are six types (orders?) of bicosoecids (sensu stricta), but the most common form are the loricate bicosoecids (Figures 1 and 2), which superficially resemble choanoflagellates in the way that they feed. Other members of the phylum are variable in the types of flagella. Caecitellus has two smooth but unequal flagella, and the pseudodendromonads have two smooth but equal flagella. Symbiomonas, a member of the picoplankton, has a single anteriorly-directed tinsel flagellum. None of these are loricate. In a broader sense (Bicosoecida sensu latu), the bicosoecida may also include two unusual flagellates: Placididea (Moriya et al. 2002) and Rictus (Yubuki et al. 2010). The placedids are benthic gliding flagellates with two unequal flagella. The anterior flagellum is tinsel with tripartite tubular hairs, but the terminal ends of the hairs are of different lengths. Like typical bicosoecids, the posterior flagellum is used for attachment. Rictus, an unusual cell that was described only in 2010 (Yubuki et al. 2010) has heterodynamic flagella of the typical heterokont type. The authors identified key ultrastructural characters in common with bicosoecids. Their molecular phylogenetic analysis (SSU rDNA) put them together with the other bicosoecids in a monophyletic group. Curiously, their analysis did not unite the typical bicosoecids with the placidids. Margulis and Schwartz (1988) and Dyer (1990b) consider this group to be part of a large, heterogeneous collection of flagellated organisms called the “Zoomastigina” (Pr-8). Later, Margulis and Schwartz (1998) removed the amitochondriate taxa and renamed the new phylum Zoomastigota (Pr-30). Sleigh et al. (1984) consider the bicosoecids to be a group of uncertain status but (perhaps) allied with the chrysophytes. Margulis and Schwartz (1988), Kudo (1966) and Grell (1973) all consider them to be closely related to the choanoflagellates and show affinities with the Chrysophyta. Dyer (1990) dimisses the relationship with the choanoflagellates because of the differences in their flagella, and he suggests a stronger relationship with the Chrysophyta. Cavalier-Smith (1989) believes that the bicosoecids are very primitive in form and may represent the group which first entered into a symbiosis with a chlorophyll c-containing symbiont to form the chromophytes. Since Cavalier-Smith (1989) believes that the chromophytes occupy a different kingdom (called Chromista), the bicosoecids are very important in his systematic scheme. We follow this view in placing them together with other heterokont groups as do Sogin and Patterson (Tree of Life Project), Patterson (1999), and Baldauf (2003). Figure 4 illustrates the relationships between the Bicosoecida (sensu latu) and other heterotrophic heterokonts. We used this as a basis to define higher taxa for the phylum. |
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| FIGURE 1. A solitary bicosoecid that shows the lorica, cell body, and emergent or anteriorly-directed flagellum. Image from http://microscope.mbl.edu/baypaul/microscope/images/t_imgAZ/ | FIGURE 2. Bicosoecid cells in a loose colony. Image from http://www.biol.tsukuba.ac.jp/~inouye/ino/st/mis/bico01.gif | FIGURE 3. SEM micrographs of Rictus lutensis. The posterior flagellum (F1) is longer than the anterior flagellum (F2). The arrow in A points to the cytostome. Image from: Yubuki et al. (2010). |
 | FIGURE 4. Bicosoecida (taxa in shaded box) are in a clade with the Labyrithomorpha among the heterotrophic heterokont taxa (taxa in bold). The topology of this cladogram is informed by Andersen (2004) and Yubuki et al (2010). |
| LITERATURE CITED Adl, S. M., A. G. B. Simpson, M. A. Farmer, R. A. Andersen, O. R. Anderson, J. R. Barta, S. S. Bowser, G. Brugerolle, R. A. Fensome, S. Fredericq, T. Y. James, S. Karpov, P. Kugrens, J. Krug, C. E. Lane, L. A. Lewis, J. Lodge, D. H. Lynn, D. G. Mann, R. M. McCourt, L. Mendoza, O. Moestrup, S. E. Mozley-Standridge, T. A. Nerad, C. A. Shearer, A. V. Smirnov, F. W. Spiegel, and M. F. J. R. Taylor. 2005. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology. 52(5):399-451. [3,L] Baldauf, S. L. 2003a. The deep roots of eukaryotes. Science. 300 (5626): 1701-1703. Cavalier-Smith, T. 1989. The Kingdom Chromista. In: Green, J.C., B.S.C. Leadbeater, and W.L. Diver, eds. The Chromophyte Algae: Problems and Perspectives. Systematics Association Special Volume No. 38. Clarendon Press. Oxford. pp. 381-407. Dyer, B. D. 1990b. Bicoecids. In: Margulis, L., J.O. Corliss, M. Melkonian, and D.J. Chapman, eds. 1990. Handbook of the Protoctista; the Structure, Cultivation, Habits and Life Histories of the Eukaryotic Microorganisms and Their Descendants Exclusive of Animals, Plants and Fungi. Jones and Bartlett Publishers. Boston. pp. 191-193. Grell, K. G. 1973. Protozoology. Springer-Verlag. New York. Grassé, P.-P. 1926. Contribution à l’étude des Flagellés parasites. Arch. Zool., Expér. Générales. 65: 345–602. Karpov, S. A. 2000. Ultrastructure of the aloricate bicosoecid Pseudobodo tremulans with revision of the order Bicosoecida. Protistology. 1:100-108. Kudo, R.R. 1966. Protozoology. 5th ed. Charles C. Thomas Publisher. Springfield. Margulis, L. and K. Schwartz. 1988. Five kingdoms, an illustrated guide to the phyla of life on earth. 2nd Edition. W.H. Freeman and Co. New York. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Moriya, M., T. Nakayama, and I. Inouye. 2002. A new class of the Stramenopiles, Placididea classis nova: description of Placidia cafeteriopsis gen. et sp. nov. Protist. 153: 143-156. Patterson, D. J. 1999. The diversity of eukaryotes. American Naturalist. 154 (Suppl.): S96–S124. Sleigh, M.A., J.D. Dodge and D.J. Patterson. 1984. Kingdom Protista. In: Barnes, R.K.S., ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc. Sunderland, Mass. Sogin, M. L. and D. J. Patterson. 1995. Stramenopiles. Version 01 January 1995 (under construction). http://tolweb.org/Stramenopiles/2380/1995.01.01 In: The Tree of Life Web Project, http://tolweb.org/ Yubuki, N., B. S. Leander, and J. D. Silberman. 2010. Ultrastructural and molecular phylogenetic position of a novel phagotrophic stramenopile from low oxygen environments: Rictus lutensis gen. et sp. nov. (Bicosoecida, incertae sedis). Protist. 161: 264-278. [C] |
| By Jack R. Holt. Last revised: 02/19/2013 |
