SYSTEMATIC BIOLOGY

THE EUKARYOMONADAE
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PHYLUM CENTROHELOMONADA

INTRODUCTION TO THE CENTROHELOMONADA

Centrohelomonada (sen-tro-he-lo-mo-NA-da) is made of three Greek roots that mean central (kentrokos -κεντρικός); sun (helios -ήλιος); and unit (monada -μονάδα).  The reference is to a cell with spike-like rays emanating from it (formerly one of the unicells called heliozoans, sun animacules).

The centrohelid heliozoans resemble other heliozoan taxa by having a round cell body with radiating axopods, and stiffened cytoplasmic extensions supported by an internal microtubular framework.   The centrohelids, however, have a central microtubule organizing center (MTOC) that may or may not have an axoplast, but is always associated with the nucleus.

A. Raphidiophrys showing the microtubule organizing center and axopods.
Image taken from:
 A: http://www.bch.umontreal.ca/protists/raphp/appearance.html

SYNOPTIC DESCRIPTION OF THE CENTROHELOMONADA

The following description comes from Patterson (1999) and Febvre-Chevalier (1990).

I. SYNONYMS: heliozoans, sun animals, sarcodina, actinopods.

II. NUMBER: >85 species?

III. PHYLUM CHARACTERISTICS:

A. Structure and Physiology

Cell Form: Unicellular.

Flagella: Naked flagella in the dimorphids.

Basal Bodies: Orthogonal or perpendicular when present.

Cell Covering: Naked with radiating axopods supported by microtubules in a double polygonal spiral and attached to or surrounded by the nucleus.

Chloroplasts: Not present.

Food Reserves: Not reported.

Mitochondria: Flattened cristae.

Golgi: Present.

Nucleus: Uninucleate to multinucleate.  Endosome?

Centrioles: Not reported.

Inclusions and Ejectile Organelles:  Food vacuoles.

B. Mitosis, Meiosis and Life History

Mitosis: Poorly known.  Closed?

Meiosis: Not reported.

Sexual Reproduction and Life History:  Not reported.

C. Ecology: Freshwater and marine; free-living, filter feeders.

SYSTEMATICS OF THE CENTROHELOMONADA

For some time they seemed to have found a home in the "sarcodina" because they produced axopods (e.g. Kudo 1966; and Grell 1973).  That also seemed to ally them with numerous other heliozoan-like organisms (e.g. Radiolaria, Actinophryids, Desmothoraceans, etc.) and led to the union of all axopod-bearing taxa into the phylum Actinopoda (Febvre-Chevalier 1990; and Margulis and Schwartz 1998).  Smith and Patterson (1986) reviewed ultrastructural and life history details and suggested that the groups of heliozoans were only superficially alike and in need of systematic revision.  Patterson (1999) separated the heliozoans into several sisterless groups, some of which were later incorporated into the Cercozoae, and the Heterokontae (=Stramenopiles).  Cavalier-Smith and Chao (2003) concluded that the centrohelids (sensu strictu) evolved from biflagellated forms, and the centroplast is just a modified centrosome.  They further claimed that the centrohelids formed an independent lineage.  Though other analyses confirmed that conclusion, they did show an association with the Cryptomonads (Nilolaev et al. 2004), and with the haptomonads (Cavalier-Smith 2003).  Thus, with some reservations, I have placed them in the same kingdom with the cryptomonads and haptomonads.  

Febvre-Chevalier (1990) unites the centrohelids (sensu strictu) with the dimorphids on the basis that both groups have microtubule organizing centers (MTOC) in the center of the cell, sometimes surrounded by the nucleus.  Although Patterson (1999) considers the two groups to be separate, I have retained them as a provisional group until further molecular/ultrastructural work can be done on them.

HIERARCHICAL CLASSIFICATION OF THE CENTROHELOMONADA

This system is a modification of Febvre-Chevalier (1990) and Patterson (1999) as informed by Nikolaev et al. (2004).

CLASS CENTROHELOMONADEA

Not flagellated.

ORDER AXOPLASTIALES.

Microtubule Organizing Center is centrally-located and the axoplast is spherical.  

Gymnosphaera, Actincoryne

ORDER CENTROPLASTIALES

Microtubule Organizing Center has a centroplast.  The central basic microtubule pattern has one distorted hexagon.  Some with silicaceous scales and/or spicules.

Acanthocystis, Raphidiophrys, Heterophrys, Cienkowskaya.

CLASS DIMORPHOMONADEA

Flagellated.  Microtubule Organizing Center is bulky and 3-parted.

ORDER ENDONUCLEOPLASTIALES

Microtubule Organizing Center axoplast is surrounded by the nucleus.  Two orthogonal flagella.

Dimorpha.

ORDER EXONUCLEOAXOPLASTIALES

Microtubule Organizing Center is a lens-shaped axoplast. outside of the nucleus.  Irregular pattern of microtubules.  Cells with 4 flagella.

Tetradimorpha.

This page is maintained by Jack R. Holt. Last revised: 03/17/2008.