Myzozoa

Dinomonas vorax, copyright Won Je Lee.

Belongs within: Alveolata.
Contains: Perkinsidae, Dinoflagellata, Sporozoa.

Another non-missing not-quite-link
Published 21 February 2008
Chromera velia, from Nature News and Views.

After years of ignorance, we are slowly piecing together an understanding of the inter-relationships between the various groups of eukaryotes. In the process of doing so, researchers have confirmed some traditionally recognised groups, dismantled others, and recognised new groupings that were previously unsuspected. One well-supported group of protists that has emerged into the light is the Alveolata. Alveolates combine three superficially dissimilar groups—the ciliates, dinoflagellates and sporozoans—united by the possession of alveoli, flattened membrane-bound vesicles directly under the cell surface, supported by microtubules. Within the alveolates, it is also generally agreed that the dinoflagellates and sporozoans are more closely related to each other than to the ciliates.

The sporozoans are parasitic forms that include a number of significant pathogens such as Plasmodium, the cause of malaria, and Cryptosporidium. One of the interesting, relatively recent discoveries about this group of organisms was that some of them possess a remnant, colourless plastid (referred to as the apicoplast), which together with the presence of chloroplasts in the related dinoflagellates suggested a photosynthetic ancestor. Understanding the origin of apicoplasts has become particularly significant as it has been touted as a potential target in developing treatments for sporozoan infections that target the parasite without damaging the host.

Unfortunately, a direct connection between the apicoplast and the dinoflagellate chloroplast has remained largely theoretical. Not all sporozoans possess apicoplasts—only a particular clade (including coccidians and Plasmodium) does so, while other sporozoans such as gregarines and Cryptosporidium show no sign of them. Chloroplasts or chloroplast remnants are also absent in an assortment of alveolate flagellate taxa (such as Colpodella) that are regarded as falling within the dinoflagellate-sporozoan clade. Direct comparison of apicoplasts and dinoflagellate chloroplasts is pretty much impossible—apicoplasts have (unsurprisingly) lost all photosynthetic genes, while dinoflagellate chloroplasts have developed severe wierdnesses of their own where they have pretty much lost all genes except the photosynthetic ones*. As a result, researchers have been unable to entirely rule out the possible that sporozoans gained their plastids independently from dinoflagellates. This is where today’s announced discovery comes in.

*And severely altered what little they have left. In fact, dinoflagellate genomes as a whole are wierd beyond all belief—they’re the only eukaryotes to have lost histones, for instance. I have no idea why they’re so strange.

Chromera velia is a small photosynthetic eukaryote described by Moore et al. (2008). It is generally immotile, though an internal(!) cilium is present at one end of the cell, and motile stages were seen briefly in old cultures. Reproduction was mainly by binary division—frustratingly, Moore et al. add “not restricted to binary division”, but completely fail to explain what this means and how sexual reproduction occurred (if it occurred). In fact, the paper as a whole is frustratingly uninformative about the ultrastructure of Chromera*, being mostly dedicated to the molecular phylogeny of the new taxon relative to other alveolates.

*I mean, seriously, how can you refer to an internal cilium and not go further?

The molecular phylogeny quite strongly supports Chromera as more closely related to sporozoans than dinoflagellates, though less closely related to sporozoans than are colpodellids. The discovery of this photosynthetic member of the sporozoan line adds additional support to the idea that sporozoans are ancestrally photosynthetic.

However, this does not automatically mean that dinoflagellate and sporozoan plastids share a single origin, despite the authors’ conclusions. Analysis of plastid genes gave conflicting results—psbA supported a dinoflagellate-Chromera grouping, but SSU rDNA did not. Alveolates have also been suggested to be closely related to the chromists, another group of mostly photosynthetic eukaryotes (including brown and golden algae and diatoms), in a larger grouping called ‘chromalveolates’. The existence of the chromalveolate clade was first suggested by their mutual possession of chlorophyll c, a form of chlorophyll not found in any other organisms*. However, Chromera lacks chlorophyll c, and possesses chlorophyll a only, like the red algae from which chromalveolate plastids are derived.

*For those unfamiliar with the various chlorophylls, chlorophyll a is the ancestral form found in pretty much all chlorophyll-containing organisms. Chlorophyll b is found in green algae, land plants and organisms with green alga-derived plasmids, as well as a few Cyanobacteria. Chlorophyll c, as I’ve said, is found in chromists and dinoflagellates.

The authors of Chromera assume that this indicates a loss of chlorophyll c in the ancestor of Chromera, but I would say that this is too strong a conclusion. The possibility that the sporozoan + Chromera ancestor gained its chloroplast independently from the dinoflagellate ancestor remains alive and well, and, as always, we need to look further into this question.

Systematics of Myzozoa
<==Myzozoa [Myzomonadea, Perkinsemorpha, Perkinsozoa, Spiromonadaceae, Spiromonadida, Spiromonadidae, Spiromonadidomorphina]
    |--DinozoaC-S18
    |    |--PerkinsidaeC-SC04
    |    `--DinoflagellataC-SC04
    `--ApicomplexaC-S18
         |--SporozoaSM03
         `--Colpodellida [Apicomonada, Apicomonadea]AB19
              |  i. s.: Alphamonas Aléxéieff 1918C-SC04 [AlphamonaceaeAB19, Alphamonadidae]
              |           `--*A. edax (Klebs 1892)C-SC04 (see below for synonymy)
              |--Vitrella [Vitrellaceae, Vitrellida, Vitrelloidia]C-S18
              |    `--V. brassicaformisC-S18
              `--Myzomonadia [Chromovoridia]C-S18
                   |--Chromera Moore, Obornik et al. 2008MO08 [ChromeridaC-S18]
                   |    `--*C. velia Moore, Obornik et al. 2008MO08
                   |--Algovora Cavalier-Smith & Chao 2004C-SC04 [AlgovoridaC-S18, Algovoridae]
                   |    |--*A. pugnax Cavalier-Smith & Chao 2004C-SC04
                   |    `--A. turpis (Simpson & Patterson 1996) [=Colpodella turpis]C-SC04
                   |--VoracidaC-S18
                   |    |--Dinomonas Saville Kent 1880–1881 [Dinomonadidae]C-S18
                   |    |    `--*D. vorax Saville Kent 1880-1881C-S18 [=Colpodella voraxC-S18; incl. Alphamonas coprocolaC-SC04]
                   |    `--Microvorax Cavalier-Smith 2018 [Microvoracidae]C-S18
                   |         |--*M. angusta Cavalier-Smith 2018C-S18
                   |         |--M. gonderi (Foissner & Foissner 1984)C-S18 [=Spiromonas gonderiC-S18, Colpodella gonderiC-SC04]
                   |         `--M. tetrahymenae (Cavalier-Smith in Cavalier-Smith & Chao 2004) [=Colpodella tetrahymenae]C-S18
                   `--ColpodellaceaeAB19
                        |--Colpodella Cienkowski 1865FT93 (see below for synonymy)
                        |    |--*C. pugnax Kent 1880C-S18, C-SC04
                        |    |--C. angusta (Dujardin 1841) (see below for synonymy)C-S18
                        |    `--C. pseudoedaxC-S18
                        `--VoromonadidaC-S18
                             |--Voromonas Cavalier-Smith & Chao 2004AB19, C-SC04 [Voromonadidae]
                             |    `--*V. pontica (Mylnikov 2000) [=Colpodella pontica]C-SC04
                             `--Chilovora Cavalier-Smith & Chao 2004C-SC04 [Chilovorida, ChilovoridaeC-S18]
                                  `--*C. perforans (Hollande 1938) (see below for synonymy)C-SC04

*Alphamonas edax (Klebs 1892)C-SC04 [=Bodo edaxC-SC04, Colpodella edaxC-S18, Spiromonas edaxK92; incl. Nephromonas hyalina Droop 1953C-SC04]

*Chilovora perforans (Hollande 1938) [=Bodo perforans, Colpodella perforans, Spiromonas perforans]C-SC04

Colpodella Cienkowski 1865FT93 [incl. Dingensia Patterson & Zölfell 1991C-S18, Spiromonas Perty 1852FT93; Paraconoidia]

Colpodella angusta (Dujardin 1841) [=Heteromita angusta, *Dingensia angusta, Spiromonas angustata]C-S18

*Type species of generic name indicated

References

[AB19] Adl, S. M., D. Bass, C. E. Lane, J. Lukeš, C. L. Schoch, A. Smirnov, S. Agatha, C. Berney, M. W. Brown, F. Burki, P. Cárdenas, I. Čepička, L. Chistyakova, J. del Campo, M. Dunthorn, B. Edvardsen, Y. Eglit, L. Guillou, V. Hampl, A. A. Heiss, M. Hoppenrath, T. Y. James, A. Karnkowska, S. Karpov, E. Kim, M. Kolisko, A. Kudryavtsev, D. J. G. Lahr, E. Lara, L. Le Gall, D. H. Lynn, D. G. Mann, R. Massana, E. A. D. Mitchell, C. Morrow, J. S. Park, J. W. Pawlowski, M. J. Powell, D. J. Richter, S. Rueckert, L. Shadwick, S. Shimano, F. W. Spiegel, G. Torruella, N. Youssef, V. Zlatogursky & Q. Zhang. 2019. Revisions to the classification, nomenclature, and diversity of eukaryotes. Journal of Eukaryotic Microbiology 66: 4–119.

[C-S18] Cavalier-Smith, T. 2018. Kingdom Chromista and its eight phyla: a new synthesis emphasising periplastid protein targeting, cytoskeletal and periplastid evolution, and ancient divergences. Protoplasma 255: 297–357.

[C-SC04] Cavalier-Smith, T., & E. E. Chao. 2004. Protalveolate phylogeny and systematics and the origins of Sporozoa and dinoflagellates (phylum Myzozoa nom. nov.) European Journal of Protistology 40: 185–212.

[FT93] Fensome, R. A., F. J. R. Taylor, G. Norris, W. A. S. Sarjeant, D. I. Wharton & G. L. Williams. 1993. A classification of living and fossil dinoflagellates. Micropaleontology Special Publication 7: i–viii, 1–351.

[K92] Krylov, M. V. 1992. The origin of heteroxeny in Sporozoa. Parazitologiya 26 (5): 361–368.

[MO08] Moore, R. B., M. Oborník, J. Janouškovec, T. Chrudimský, M. Vancová, D. H. Green, S. W. Wright, N. W. Davies, C. J. S. Bolch, K. Heimann, J. Šlapeta, O. Hoegh-Guldberg, J. M. Logsdon, Jr. & D. A. Carter. 2008. A photosynthetic alveolate closely related to apicomplexan parasites. Nature 451: 959–963.

[SM03] Saldarriaga, J. F., M. L. McEwan, N. M. Fast, F. J. R. Taylor & P. J. Keeling. 2003. Multiple protein phylogenies show that Oxyrrhis marina and Perkinsus marinus are early branches of the dinoflagellate lineage. International Journal of Systematic and Evolutionary Microbiology 53: 355–365.

Leave a comment

Your email address will not be published. Required fields are marked *