DESCRIPTION OF THE PHYLUM METAMONADA (GRASSE’ 1952)

EUKARYA> EXCAVATA> EUEXCAVATA> 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.

INTRODUCTION TO THE METAMONADA 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 (text with tooltip) An anteriorly-directed flagellum extends in the direction of the motion of the motile cell. The interpretation is that the flagellum functions by pulling the cell. and the fourth directed posteriorly ( recurrent (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 metamonads are made of 2 major groups. The trimastigids, represented by Trimastix (Figure 1), have only one set of flagella while the oxymonads have multiple sets of flagella and most have an anterior proboscis (Figure 2). Both groups have a longitudinal internal cable made of the microtubular flagellar roots (text with tooltip) Flagellar roots are microtubular structures that arise from the basal bodies and elaborate to the inside of the cell. They may connect to the nucleus (as part of a karyomastigont) or course their way into the cell such that they form other structures like axostyles. woven into an elongate axostyle (text with tooltip) An axostyle is rod-like microtubular structure that occurs in certain Metamonadotista and Parabasalotista. The axostyle runs through the axis of the cell from the anterior (usually anchored around the nucleus) to the posterior end, often forming an extended point at the posterior end. . We 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 work summarized by Baldauf (2003a, 2008) 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 1 and 2) 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 (2003a and 2003b) and Simpson (2003). Figure 3 shows the metamonads as part of a clade of symbiotic unicells. The basal position is tentative and our interpretation of Lara et al. (2006), Kolisko et al. (2008), and Malik et al. (2011).

FIGURE 1. Drawing of Trimastix. Image from http://microscope.mbl.edu/	FIGURE 2. Drawing of Oxymonas. Image from http://microscope.mbl.edu/

FIGURE 3. A cladogram showing the relationships between the phyla of the Euexcavatae (taxa in bold). The metamonads (in the shaded box) are part of the symbiotic clade. The relationships given here have been inferred from Lara et al. (2006), Kolisko et al. (2008), and Malik et al. (2011).

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By Jack R. Holt. Last revised: 02/17/2014