The name "jakobid" is applied informally to a group of small, bacterivorous, heterotrophic flagellates found in freshwater and marine habitats. The flagellates are not uncommon, but are easily overlooked because of their small size and lack of color. The group contains (O'Kelly 1993) four genera (Histiona, 2-3 species; Jakoba, one species; Malawimonas, one species; Reclinomonas, one species). Additional species have been found and are being investigated. Jakoba and "jakobid" commemorate scientist Jakoba Ruinen, who first described (Ruinen 1938) the marine protozoon that now bears her name.
Besides small cell size, the jakobid flagellates are alike by having two flagella inserted near the apical end of the cell. The more posteriorly directed of these flagella possesses a vane and lies in a conspicuous ventral groove. The cell's shape is supported by a complex cytoskeleton. The architecture of this cytoskeleton, both at interphase and during cell division, is essentially the same for all jakobids, and differs from that in most other protists (O'Kelly 1993).
Jakobid protists have the most eubacterial-like mitochondrial genomes of any eukaryote known to date (Lang et al. 1997, Palmer 1997, Gray et al. 1998, 1999). They therefore may be among the most ancient of living eukaryotes. Moreover, their morphology, ultrastructure, and cell division patterns are most similar to those of certain groups of protozoa that do not have mitochondria or easily-visible endomembranes (endoplasmic reticulum, Golgi bodies). The parasitic protozoa in the genus Giardia belong to one of these groups, and have been assumed to be an appropriate model system for ancient eukaryotes. This assumption has been reexamined; cells like Giardia may have lost mitochondria and endomembranes, and undergone other modifications, from a jakobid-like ancestor.
Investigations are currently underway on the newly-discovered jakobid species, as well as two groups of protists that appear to be closely related to both the jakobids and Giardia: the retortamonads and Trimastix.
References:
O'Kelly CJ. 1993. The jakobid flagellates: structural features of Jakoba, Reclinomonas and Histiona and implications for the early diversification of eukaryotes. Journal of Eukaryotic Microbiology 40: 627-636. Abstract (local)
Gray MW, Lang BF, Cedergren R, Golding GB, Lemieux C, Sankoff D, Turmel M, Brossard N, Delage E, Littlejohn TG, Plante I, Rioux P, Saint-Louis D, Zhu Y, Burger G. 1998. Genome structure and gene content in protist mitochondrial DNAs. Nucleic Acids Research 26: 865-878. Abstract via NCBI PubMed
Gray MW, Burger G, Lang BF. 1999. Mitochondrial evolution. Science.
Lang BF, Burger G, O'Kelly CJ, Cedergren RJ, Golding B, Lemieux C, Sankoff S, Turmel M, Gray MW. 1997. An ancestral mitochondrial DNA resembling a eubacterial genome in miniature. Nature 387 (6632): 493-497. Abstract via NCBI PubMed
Palmer JD. 1997. The mitochondrion that time forgot. Nature 387 (6632): 454-455.
Ruinen J. 1938. Notizen über Salzflagellaten. II. Über die Verbreitung der Salzflagellaten. Archiv für Protistenkunde 90: 210-258.
|
Picture Gallery |