MadSci Network: Microbiology
Query:

Re: How does the paramecium reproduce?

Date: Sat Feb 7 19:06:46 1998
Posted By: Dean Jacobson, Faculty Biology, Whitworth College
Area of science: Microbiology
ID: 886728619.Mi
Message:

Kimmie,
	Great question!
	Paramecium is a ciliate, a group of single-celled animal-like creatures 
(protists) that have a rather unusual way of reproducing. Most protists 
(and cells in animals and plants) have a single nucleus which, once all the 
chromosomes have been duplicated, splits in half, along with the rest of 
the cell.  Ciliates, however, are different: they have at least two nuclei:  
a large macronucleus and a tiny micronucleus.  So, both nuclei must divide.  
But Paramecium is such a complicated cell that it can't just split in half 
like an amoeba; it has a mouth and an anus too, and its cilia are arranged 
in a complex, intricate pattern (which can be very nicely seen with a 
microscope if you mix some paramecium cells with india ink, carefully 
spread it on a slide in a thin layer, and let it air dry.)  Before a 
Paramecium reproduces, it must first grow a second mouth, below the 
original one!!  Then the cell divides in half, with one Paramecium above 
the other, like a short totem pole.

Paramecium also reproduce sexually, and this is really weird.  Two 
compatable cells join "belly to belly" and part of their cytoplasm fuses 
(kind of like a siamese twin).  For sex, they do not need their big 
macronuclues: it is slowly destroyed.  Their micronuclues goes through a 
long process where its chromosomes are reduced in number by half (a process 
called meiosis) and then the post-meiosis nucleus replicates, so each of 
the mating cells has an extra nucleus.  Then, each cell trades the extra 
nucleus with each other! 
 
Finally, they two cells separate; both now have a pair of "his and hers" 
nuclei, which fuse into one, mixing the chromosomes together.  Then the one 
nucleus divides, and the extra one grows into the macronucleus.

Hard to understand, but weird and wonderful!  It's actually not hard to see 
wild  Paramecium and other ciliates (from a pond sample to which rice or 
barley has been added) mating; look for two similar cells stuck together 
side by side.  

By the way, most protists simply fuse when they join in sexual union (like 
a sperm and an egg) but ciliates have such a complicated external stucture 
that they had to develope this odd way of mating without totally fusing to 
become one cell.  Pretty clever.

	Cheers,
Dean Jacobson, protist watcher



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