James Forney, a professor of biochemistry at Purdue University, says "Perhaps the most unusual characteristic of paramecia is their nuclei. "Paramecium along with the other ciliates have this rather unique feature, they have two types of nuclei, which differ in their shape, their content and function."
The two types of nuclei are the micronucleus and macronucleus. The micronucleus is diploid; that is, it contains two copies of each paramecium chromosome. Forney notes that the micronucleus contains all of the DNA that is present in the organism. "It's the DNA that is passed from one generation to the another during sexual reproduction," he said. On the other hand, the macronucleus contains a subset of DNA from the micronucleus, according to Forney. "It is the transcriptionally active nucleus," he added. "So it's the nucleus that is transcribed to make mRNAs and proteins from those mRNAs." The macronucleus is polyploid, or contains multiple copies of each chromosome, sometimes up to 800 copies.
All Paramecium species have one macronucleus, according to Forney. However the number of micronuclei can vary by species. He gives the example of the Paramecium aurelia species complex, which have two micronuclei and Paramecium multimicronucleatum, which have several.
Why the presence of two distinct nuclei? One evolutionary reason is that it is a mechanism by which paramecia and other ciliates can stave off genetic intruders: pieces of DNA that embed themselves into the genome. "In the case of ciliates, there's a mechanism in which, if a piece of DNA is in the micronucleus but it's not in the macronucleus, it will be removed from the next macronucleus that is made," Forney explained. "In other words, if something foreign got into the micronuclear genome, then when the next macronucleus is made, it would removed and not included in the expressed version [transcribed] of the genome." Forney notes that this has been described by some as a primitive DNA immune system; that is, surveying the genome and trying to keep out invading elements.
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