DESCRIPTION OF THE PHYLUM PRASINOPHYTA (CHRISTIANSEN 1962)
| EUKARYA> ARCHAEPLASTIDA> VIRIDIPLANTAE> CHLOROBIONTA> PRASINOPHYTA |
PRASINOPHYTA LINKS
| Prasinophyta (pra-se-NA-fa-ta) is formed from two Greek roots that mean dark green (prasinos -πράσινος); and plant (phyto -φυτό). The reference is to the obvious color of the taxa in this phylum. Many of these taxa are grass green, but some are dark green. |
| INTRODUCTION TO THE PRASINOPHYTA The praesinophytes comprise a collection of organisms that generally occur as unicells, but some are attached as filaments (Figures 1-4). They occur in almost all aquatic environments. Indeed, there is a growing realization that these might be significant contributors to marine plankton primary production (Not et al. 2004). The motile cells usually are covered with cellulosic scales (text with tooltip) Protist scales are regular overlapping structures on the outside of the cell. They may be organic, silicaceous, or carbonaceous and may serve as an articulated cell wall. Scales may occur on the cells of a range of unrelated taxa. rather than a cell wall. In some the scales are quite complex. However, the fusion of scales in taxa like Pedinomonas might indicate how the more typical cell wall of the kingdom evolved. Although they are photosynthetic, most supplement their nutrition by ingesting bacteria and other organic matter, a process called mixotrophy (text with tooltip) A mixotrophic organism obtains food either by autotrophic or saprobic means. Also called mesotrophic. . Likely, the earliest members of this line were completely heterotrophic, a process by which they enslaved a chlorophyll a and b bearing Cyanobacterium to become the first photobionts. Until recently, the group was thought to be asexual. However, Suda et al. (1989) demonstrated that Nephroselmis went through a sexual life history that was isogamous (text with tooltip) Isogamous (adj) describes sexual reproduction in which the gametes are structurally identical. . No doubt, other sexual life histories will be reported. SYSTEMATICS OF THE PRASINOPHYTA The system that we follow is generally based on van den Hoek et al. (1995) but informed by the work of Melkonian (i.e. Melkonian 1989, 1990; Melkonian and Surek 1995; and Sym and Pienaar 1993). Molecular studies such as Melkonian and Surek (1995) indicate that the prasinophytes likely represent remnants of an early radiation of the “green plants”. An example is the extraordinary variation in the type of flagellar insertion (apical or lateral), some from an anterior pit (crypt?). The arrangements of the flagellar roots are equally varied. Some possess a multilayered structure (text with tooltip) A multilayered structure (MLS) is a microtubular array that is at the base of the basal bodies in the motile cells of certain green algae and embryophytes. The MLS seems to be part of the microtubular organizing center for the cytoskeleton of the cell. (MLS) at the base of the flagella. This is characteristic of most members of the Streptobionta and all of the embryophytes that produce flagellated cells. Characters such as these confirm that the prasinophytes are artificial and paraphyletic, as remnant groups of early radiation events tend to be (Proschold and Leliaert 2007, and Turmel et al. 2009). Indeed, the Prasinophytes could be broken into as many as four to six to nine clades (Marin and Melkonian 1999; Turmel et al. 2009 and Proschold and Leliaert 2007, respectively). Indeed, the analysis of Proschold and Leliaert (2007) suggests that the Prasinophyta could be separated into as many 8 monophyletic groups (Figure 5). |
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| FIGURE 1. A DIC micrograph of Nephroselmis showing its unequal flagella. Image from http://www.uio.no/miljoforum/m_alge/art/nephroselmis.htm | FIGURE 2. An SEM micrograph of Pyramimonas. Note the cell and flagella are covered in scales. Image from http://www.aad.gov.au/asset/MME/images/P_motile.jpg | FIGURE 3. TEM micrograph of Micromonas which has a single flagellum that lacks scales. Image from http://www.biol.tsukuba.ac.jp/~inouye/ino/g/pr/pr_pic.html | FIGURE 4. Prasinocladus, which in the motile stage is known as Tetraselmis, lives as an attached form that produces branched gelatinous filaments. Image from http://www.bio.mtu.edu/the_wall/phycodisc/CHLOROPHYTA/ |
 | FIGURE 5. This is an adaptation of Proschold and Leliaert (2007) in which the Prasinophyceae forms 9 different clades, many of them nested. |
| LITERATURE CITED Christiansen, T. 1962. Botanik. Vol 2. Systematic Botanik. No. 2. Alger. Munksgaard. Copenhagen. 178pp. Marin, B. and M. Melkonian. 1999. Mesostigmatophyceae, a new class of streptophyte green algae revealed by SSU rRNA sequence comparisons. Protist. 150: 399-417. Melkonian, M. 1989. D. Taxonomy I. Systematics and Evolution of the Algae. Progress in Botany. 50: 214-245. Melkonian, M. 1990a. Chlorophyta, Introduction. 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. 597-599. Melkonian, M. 1990b. Chlorophyceae. 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. 608-616. Melkonian, M. and B. Surek. 1995. Phylogeny of the Chlorophyta: congruence between ultrastructural and molecular evidence. Bull. Soc. Zool. Fr. 120: 191-208. Not, F., M. Latasa, D. Marie, T. Cariou, D. Vaulot, and N. Simon. 2004. A single species, Micromonas pusilla (Prasinophyceae), dominates the eukaryotic picplankton in the Western English Channel. Applied Environmental Microbiology. 70(7): 4064-4072. Pröschold, T. and F. Leliaert. 2007. Systematics of the green algae: conflict of classic and modern approaches. In: Brodie, J. and J. Lewis, eds. Unraveling the Algae: The Past, Present, and Future of Algal Systematics. The Systematics Association Special Volume Series. 75: 123-153. Suda, S., M. M. Watanabe, and I. Inouye. 1989. Evidence for sexual reproduction in the primitive green alga Nephroselmis olivacea (Praesinophyceae). Journal of Phycology. 25: 596-600. Sym, S. D. and R. N. Pienaar. 1993. The Class Praesinophyceae. In: F. E. Round and D. J. Chapman, eds. Progress in Phycological Research. 9: 281-376. Turmel, M., M.-C. Gagnon, C. J. O’Kelley, C. Otis, and C. Lemieux. 2009. The chloroplast genomes of the green algae Pyramimonas, Monomastix, and Pycnococcus shed new light on the evolutionary history of prasinophytes and the origin of the secondary chloroplasts of euglenids. Molecular Biology Evolution. 26(3): 631-648. Van den Hoek, C., D. G. Mann, and H. M. Jahns. 1995. Algae, An Introduction to Phycology. Cambridge University Press. Cambridge. |
| By Jack R. Holt. Last revised: 03/22/2014 |
