DESCRIPTION OF THE PHYLUM DIPLONEMEA (CAVALIER-SMITH 1993)

EUKARYA> EXCAVATA> DISCICRISTATAE> DIPLONEMEA

Diplonemida (di-plo-NE-ma-da) is made of two Greek roots meaning two -diplo (διπλό) and thread -nema (νήμα). The reference is to the two short flagella in the genus, Diplonema, from which the phylum gets its name.

INTRODUCTION TO THE DIPLONEMEA This “phylum” includes only two genera described so far (Diplonema and Rhynchopus). The organisms are unicellular, dorsoventrally flattened, and somewhat metabolic. Most species are free-living, but one has been implicated as a parasite of marine diatoms and other taxa. In general, they seem to be phagotrophic flagellates that are opportunistic. For such low diversity, their distribution is quite interesting. They have been found associated with plankton, sediment, and plants in both freshwater and marine environments. In many ways, members of this group resemble a bodonid (see Trypanosomatids) without a kinetoplast (text with tooltip) A kinetoplast is a modified mitochondrion that is associated with the basal body (kinetosome) of kinetoplastids. Their mitochondrion had a very large amount of DNA (called k-DNA) and stains almost as a second nucleus in the cell. . When feeding, the cells of Rhynchopus (Figure 1) creep over a substrate, which could be another living thing. In that feeding or trophic stage, the flagella are very short and do not emerge from the gullet (text with tooltip) A flagellar pocket is an invagination of the cell within which the flagellar insertions occur. Extensions of the flagellar pocket form the undulating membrane in trypanosomatids. This is sometimes called the reservoir, the crypt, the cytostome (inappropriately), or gullet. . However, the cell can begin to swim during a dispersive stage in which there is an obvious anteriorly-directed flagellum (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 a ventral posteriorly-directed or recurrent flagellum (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. . Diplonema does not seem to produce a swimming swarmer stage. Some species have flagella that are short and move very slowly, much like walking legs pulling the cell over the substrate. Others have no flagella that emerge from the gullet and seem to undergo a writhing metabolic movement. Initially, Rhynchopus and Diplonema were considered euglenoids (discussed in Roy et al. 2007), but they were unusual in that they had no pellicular strips or paraflagellar rod. In recognition of the difference, Cavalier-Smith (1993), placed them as a separate class within the Euglenozoa together with the Euglenida and the Trypanosomatida. Patterson (1999) also includes the diplonemids within the Euglenozoa. Adl et al. (2005) show them in the clade of the Euglenozoa, but at a taxonomic level equal to Euglenida and Kinetoplastea, the same relative position that we have placed them. Some molecular systematic treatments place the diplonemids as sisters to the trypanosomatids (e.g. Lara et al. 2006, Simpson et al. 2002, Simpson et al. 2004, Simpson and Roger 2004a, and Marande et al. 2005). However, Busse and Preisfeld (2002) show the diplonemids as sisters to the euglenid + trypanosomatid clade. Moreira et al. (2004), Von der Heyden et al (2004), and Marin et al. 2003) determined that the euglenid + diplonemid clade was a sister group to the trypanosomatids (see Figure 2). Either of the latter views seems more likely to us because paraflagellar rods occur in both the euglenoids and the trypanosomatids, but they are absent in the diplonemids. Clearly, the diplonemids are separate from all other major taxa within the Euglenozoa clade. Thus, we have placed them at the phylum-level. That is despite the suggestion by Vickerman (2000c) that two genera deserve no more than a familial designation.

FIGURE 1. A light micrograph of Rhynchopus. The cell on the right is in the creeping trophic stage. The cell on the left is in the swarmer stage. Image from Roy et al. (2007), Figure 9.

FIGURE 2. Cladogram of relationships between phyla of the Discicristatae (taxa in bold). Diplonemea is the sister to Euglenida according to Von der Heyden et al. (2004) and Marin et al. (2003).

LITERATURE CITED Adl, S. M., A. G. B. Simpson, M. A. Farmer, R. A. Andersen, O. R. Anderson, J. R. Barta, S. S. Bowser, G. Brugerolle, R. A. Fensome, S. Fredericq, T. Y. James, S. Karpov, P. Kugrens, J. Krug, C. E. Lane, L. A. Lewis, J. Lodge, D. H. Lynn, D. G. Mann, R. M. McCourt, L. Mendoza, O. Moestrup, S. E. Mozley-Standridge, T. A. Nerad, C. A. Shearer, A. V. Smirnov, F. W. Spiegel, and M. F. J. R. Taylor. 2005. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology. 52(5):399-451. Busse, I. and A. Preisfeld. 2002. Phylogenetic position of Rhynchopus sp. and Diplonema ambulator as indicated by analyses of euglenozoan small subunit ribosomal DNA. Gene. 284: 83-91. Cavalier-Smith, T. 1993. Kingdom protozoa and its 18 Phyla. Microbiological Reviews. 57: 953-994. Cavalier-Smith, T. 2003a. Protist phylogeny and the high-level classification of Protozoa. European Journal of Protistology. 39:338-348. Lara, E., A. Chatzinotas, and A. G. B. Simpson. 2006. Andalucia (n. gen.) – the deepest branch within Jakobids (Jakobida; Excavata), based on morphological and molecular study of a new flagellate from soil. Journal of Eukaryotic Microbiology. 53(2): 112-120. Marande, W., J. Lukes, and G. Burger. 2005. Unique mitochondrian genome structure in diplonemids, the sister group of the kinetoplastids. Eukaryotic Cell. 4(6): 1137-1146. Marin, B., A. Palm, M. Klingberg, and M. Melkonian. 2003. Phylogeny and taxonomic revision of plastid-containing euglenophytes based on SSU rDNA sequence comparisons and synapomorphic signatures in the SSU rRNA secondary structure. Protist. 154: 99-145. Moreira, D., P. Lópex-García, and K. Vickerman. 2004. An updated view of kinetoplastid phylogeny using environmental sequences and a closer outgroup: proposal for a new classification of the class Kinetoplastea. International Journal of Systematic and Evolutionary Microbiology. 54: 1861-1875. Patterson, D. J. 1999. The diversity of eukaryotes. American Naturalist. 154 (Suppl.): S96–S124. Roy, J., D. Faktorova, O. Benada, J. Lukes, and G. Burger. 2007. Description of Rhynchopus euleeides n.sp. (Diplonemea), a free-living marine euglenozoan. Journal of Eukaryotic Microbiology. 54(2): 137-145. Simpson, A. G. B., J. Lukes, and A. J. Roger. 2002. The evolutionary history of kinetoplastids and their kinetoplasts. Molecular Biology and Evolution. 19(12): 2071-2083. Simpson, A. G. B., E. E. Gill, H. A. Callahan, R. W. Litaker, and A. J. Roger. 2004. Early evolution within kinetoplastids (Euglenozoa) and the late emergence of the trypanosomatids. Protist. 155:407-422. Simpson, A. G. B., and A. J. Roger. 2004a. Protein phylogenies robustly resolve the deep-level relationships within Euglenozoa. Molecular Phylogenetics and Evolution. 30:201-212. Vickerman, K. 2000c. Diplonemids. In: Lee, J. J., G. F. Leedale, P. Bradbury. An Illustrated Guide to the Protozoa. 2nd edition. Society of Protozoologists. Lawrence, KS. pp. 1157-1159.

By Jack R. Holt. Last revised: 02/17/2015