I've always wondered why cells have only one nucleus, as having multiple would seemingly prevent mutation. Are there examples of organisms with multiple nucleuses? If not, is there a reason?
Are there examples of cells with more than one nucleus?
Yes, they are called Multinucleate cells. There are two types of multinucleated cells
I highly recommend having a look at this answer for the definitions.
Examples of Syncytia include
- Skeletal muscle fibers (thanks @kmm)
Examples of Coenocytes include
- Codium (Thanks @GerardoFurtado; see picture below)
- Blastoderms early in the development of a fruit fly
Are there examples of organisms with multiple nucleuses?
Side note: The plural of nucleus is nuclei
In many fungi, during sexual reproduction, a fusion of cytoplasm happen early in the mycelium but a fusion of the nucleus happens only very late (just before sporulation). This is a type of Coenocytic mycelium. In these species, non-negligible fractions of their cells are multinucleated.
There are endosymbiotic and endoparasitic eukaryotes in other eukaryotes that would result in a cell containing several nuclei but that would not count I would guess as the nuclei belong to different species.
Picture of Codium. The entire algea is a single multinucleated cell.
There is a branch of life called the Diplomonads, most of which have two nuclei. They are single cell organisms and an early offshoot of the eukaryotic linage. A good example is Giardia lamblia. https://en.wikipedia.org/wiki/Diplomonad Giardia lamblia
According to this article The hairy beast with seven fuzzy sexes
Tetrahymena thermophila has two: a large macronucleus and a small micronucleus. The macronucleus controls the everyday functions of the cell, while the micronucleus deals with its complicated sex life. In fact this is true for all ciliates.