THE EUEXCAVATAE
| SYSTEMATIC BIOLOGY |
THE EUEXCAVATAE |
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| PHYLUM METAMONADA | |||||
INTRODUCTION TO THE METAMONADA
Metamonada (me-ta-mo-NA-da) is made from two Latin roots that mean change (meta); and unit (monus). The reference is to the variation in form that some cells can exhibit.
The metamonads occupy a large group of multiflagellated cells which are parasites or commensals of the guts of insects. Their flagella occur in groups of four, usually with 3 directed anteriorly and the fourth directed posteriorly (recurrent). The metamonads are made of 2 major groups. The trimastigids, represented by Trimastix (Figure A), have only one set of flagella while the oxymonads have multiple sets of flagella and most have an anterior proboscis (Figure B). Both groups have a longitudinal cable made of the microtubular flagellar roots woven into an elongate axostyle.
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A. Drawing of Trimastix. |
B. Drawing of Oxymonas. |
| Images taken from: A&B: http://microscope.mbl.edu/scripts/ |
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SYNOPTIC DESCRIPTION OF THE METAMONADA
| The following descriptions come from Vickerman (1990), Brugerolle and Mignot (1990), Dyer (1990d), Lee et al. (1985), Grell (1976), Kudo (1966), Margulis and Schwartz (1998), Patterson (1999), Taylor (1999), Cavalier-Smith (2003), and Simpson (2003). |
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I. SYNONYMS: pyrsonymphids, some polymastigotes, axostylids. II. NUMBER: >300 species known. III. PHYLUM CHARACTERISTICS: A. Structure and Physiology Cell Form: Unicellular. Flagella: Occur in groups of four; usually 3 anteriorly and the 4th posteriorly-directed or recurrent. The posterior flagellum has two vanes, longitudinal flanges. Basal Bodies: Basal bodies orthogonal; microtubular rootlets from the basal bodies form bundles passing into the cytoplasm, and microtubular and/or striated roots connect the basal bodies to the nucleus thus forming a karyomastigont unit. Cell Covering: Naked but pellicular microtubules may be present. Chloroplasts: Not present. Food Reserves: Glycogen. Mitochondria: Usually not present. When present, mitochondria do not have cristae. May possess hydrogenosomes, likely mitochondria that are modified for anaerobic conditions. Golgi: Present but not attached to a parabasal band (as in parabasalids). Nucleus: Connected to basal bodies via basal body roots to form a karyomastigont unit; Four flagellar basal bodies associated with a nucleus. Centrioles: Not reported. Inclusions and Ejectile Organelles:Food vacuoles. B. Mitosis, Meiosis and Life History MITOSIS: Intranuclear spindle. MEIOSIS: Presumed. SEXUAL REPRODUCTION AND LIFE HISTORY: Reported in some. C. Ecology: Mostly endobiotic and phagotrophic. |
I defined the metamonads as those organisms having one to many karyomastigonts without parabasal bodies as did Margulis and Schwartz (1988) and Sleigh et al. (1984) who considered the organisms to be a coherent group. The taxonomy of this group has been in flux for many years. Lee et al. (1985) lumped the metamonads together with the parabasalids (and other flagellated unicells) as did Margulis and Schwartz (1988) and Margulis et al. (1990) who regarded the metamonads as part of a large, heterogeneous collection of flagellated organisms called the "Zoomastigina". Later, Margulis and Schwartz (1998) grouped these with other amitochondriates in a large, diverse phylum called Archaeprotista. Earlier, Sleigh et al. (1984) raised the classes of the "Zoomastigina" to the phylum level in recognition of their disparity in structure. Taylor (1976) showed that the metamonads were allied with the parabasalids all of which he supposed had a loose association with the chrysophytes. Tudge (2000) considered this phylum to arise from the very base of the Eukaryote Domain. This view was supported by the Roger (1999) modification of the Archezoa Hypothesis.
However recent work summarized by Baldauf (2003) presents a consensus view based on molecular and ultrastructural characters in which this group is no longer considered primitive. In this system, the metamonads (trimastigids and pyrsonymphids; Figures A&B) are considered part of a natural group called the excavates which includes the parabasalids. The phylum as it is defined here is a modification of Cavalier-Smith (2003) and Simpson (2003).
HIERARCHICAL CLASSIFICATION OF THE METAMONADA
| The general organization of the following taxonomy was influenced by Cavalier-Smith (2003), Simpson (2003), and Taylor (1999). |
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CLASS OXYMONADEA (AXOSTYLIDA OR PYRSONYMPHIDS)
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This page is maintained by Jack R. Holt. Last revised 02/14/2008.
