Didinium

NCBI: Taxonomy Genome

Didinium nasutum. The Ciliates Lab Home Page by Shivar Nanthakumar et. al.

Classification

Higher order taxa:

Eukaryota; Alveolata; Ciliophora; Litostomatea; Haptoria; Haptorida; Didiniidae

Species:

Didinium nasutum, D. gargantua

Description and Significance

Didinium are members of the phylum Ciliophora. It shares many of the common characterstics of other ciliates, such as having two nuclei and being a model organism. However, this is a unique organism as well. Didinium are most famous for their predator-prey relationship with Paramecium, described below.

Genome Structure

Didinium is one of the Ciliophora with a unique genetic structure. In these organisms, the traditional stop codons UGA and UAG code for other chemicals. UAA is used as a stop codon, but there is no in-frame UGA or UAG.

Cell Structure and Metabolism

Didinium cysts. "Didinium: A Wolf Comes Calling" by Bruce J. Russell.

Didinium have about 3,000 cilia. These cilia are arranged in bands around the body. One is located around the middle, and the other at the front. The front ends in a pointed snout (cytosome). Didinium are unicellular and have an oval shape.

Didinium are heterotrophic organisms. They only have one type of prey; the much larger cilate Paramecium. When a Didinium finds a Paramecium, it ejects poison darts (trichocysts) and attachment lines. The Didinium then proceeds to engulf its prey. Although Paramecium are large, Didinium are voracious eaters and will be ready to hunt for another meal after only a few hours. If Paramecium become depleted, the Didinium encyst themselves until its food source becomes replenished.

Didinium, like all members of Ciliophora, can reproduce sexually or asexually. Nucleoli degranulate before binary fission; the bands are dispersed throughout the macronucleus. These bands are segregated during division, but then re-form granular parts. The nucleoli are separated into granular and fibrillar parts during conjugation. The fibrillar parts are usually eliminated.

Ecology

Didinium engulfing Paramecium. From Nanthakumar et. al.

Didinium engulfing Paramecium. From Mike Dingley.

Didinium engulfing Paramecium. From Bruce J. Russell.

Didinium are commonly found in freshwater, particularly still water. However, the species Didinium gargantua exists primarily in marine environments; it was first recorded in the Arctic Sea in 1910.

Like other members of Ciliophora, Didinium are model organisms. The relationship between Didinium and Paramecium is regularly used as a model for research in predator-prey relationships as well as biodiversity studies.

References

Dingley, Mike. "Didinium the master feeder." Microscopy UK. Accessed 20 June 2005.

Karajan, Bella P., Vladimir I. Popenko, and Olga G. Leonova. "Fine structure of nucleoli in the ciliate Didinium nasutum." Protistology 3 (2), 99-106 (2003).

Kim OT, Yura K, Go N, Harumoto T. "Highly divergent actins from karyorelictean, heterotrich, and litostome ciliates." The Journal of eukaryotic microbiology. 2004 Mar-Apr;51(2):227-33.

Kim OT, Yura K, Go N, Harumoto T. "Newly sequenced eRF1s from ciliates: the diversity of stop codon usage and the molecular surfaces that are important for stop codon interactions." Gene. 2005 Feb 14;346:277-86. Epub 2005 Jan 4.'

Nanthakumar, Shivar, et. al. The Ciliates Lab Home Page. Accessed 20 June 2005.

Pernberg J, Machemer H. Voltage-dependence of ciliary activity in the ciliate Didinium nasutum. The Journal of experimental biology. 1995;198(Pt 12):2537-45.

Russell, Bruce J. "Didinium: A Wolf Comes Calling." The Lives of Micro-Organisms: A MicroNaturalist's Notebook. October 2003. Accessed 17 June 2005.

Strüder-Kypke, MC, ER Kypke, S Agatha, J Warwick, and DJS Montagnes. The Planktonic Ciliate Project on the Internet. Accessed 20 June 2005.