Scientists Say ... Thorpe and Microscopic Image Coffee Mug

This mug features a quotation by British scientist William Homan Thorpe and a microscopic image of a fantastic creature (image courtesy of Gregory Antipa). Amazed by the complexity of a single cell? Didinium is a genus of unicellular ciliates with at least ten accepted species. All are free-living carnivores. A voracious predator, D. nasutum, employs specialized structures called toxicysts (some sources refer to these structures as trichites) to ensnare and paralyze the ciliary movement its prey, Paramecium. Once captured, the Paramecium, is engulfed whole by D. nasutum through its anterior cytostome. Didinium have two ciliary bands, called pectinellae, encircling the organism. This distinguishes them from the related genus Monodinium, which only have a single band except during cell division. The pectinellae are used to move through water by rotating the cell around its axis. Didinium are rounded, oval, or barrel-shaped and range in length from 40 to 200 µm. At the anterior end, a cone-shaped structure protrudes which encloses the cytostome or "mouth" opening, as found in other haptorian ciliates. The dimension of cone varies among the different species. The nucleus may also appear curved in an arched, U-shape. Didinium reproduce via binary fission asexually, or sexually via conjugation. In the absence of its Paramecium prey, D. nasutum will encyst, lying dormant within a protective coating. The cysts have been shown to remain viable for at least 10 years. When the cyst form of D. nasutum is exposed to its exclusive Paramecium food source it will excyst. At least one laboratory study showed that other conditions can trigger encystment in D. nasutum, such as the build-up of excessive waste products due to a high-density population. Phylogenetic analysis of the ciliates has presented a challenge to researchers due to the presence of both a "germ line" micronucleus containing the genes passed from generation to generation and a "somatic" macronucleus within each organism. Products of conjugation, micronuclear meiosis and fusion to form the zygote are followed by mitotic division and the formation of a macronucleus. Throughout this complex process the chromosomes undergo many processes that affect their composition, making phylogenetic analysis more difficult. In fact, at least two articles conclude that the Haptoria are not a monophyletic group.