Why Are Mules Sterile?

Why mules and other interspecies hybrids have trouble with fertility.
By Jennifer Nemec
January/February 2012
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Meiosis, four reproductive cells are created.
Nate Skow
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According to a friend of mine, mules are pretty much the best thing since sliced bread. He says they are smarter, more patient, and easier to work with than their horse mother and donkey father. After spending time around his beauties, I tend to agree. Unfortunately, I can’t just wait for his molly to foal – because she most likely never will. All male mules (johns) and most female mules (mollies) can’t reproduce. But why are mules sterile?

The key is in the chromosomes. Return with me now to junior high biology. Remember DNA, mitosis and meiosis? No? I didn’t either, so here’s a refresher: DNA (deoxyribonucleic acid – don’t worry, there won’t be a test) contains the genetic instructions for all living things. DNA is organized into chromosomes, the number of which varies from species to species (humans have 46 chromosomes; an earthworm has 36; a goldfish, 100 to 104). You might notice all these numbers are even; this is because chromosomes are usually organized in sets of homologous pairs (this will be important later). They are “homologous” because they contain the same genes in the same order. For example, humans have 23 pairs, and the gene for brown eyes is located on both chromosomes of pair 15.

For organisms with a cell nucleus, mitosis is the process of cell division needed to create new cells. In mitosis, the chromosomes are duplicated, the nuclear membrane dissolves, and then the cell splits in two, with each new cell getting half of the duplicate chromosomes. This is how organisms form and grow. Starting from a fertilized egg, cell division occurs, then happens again and again, until you have the trillions of cells that make up a dog, a goldfish or a human.

But it’s what happens before that first division that makes mollies babyless. For sexual reproduction, you need a cell from each parent that contains half of the necessary chromosomes (one from each homologous pair). These cells are created through a process of cell division called meiosis (see illustration in Image Gallery).

First, all the chromosomes are duplicated, then these duplicate duos align with their original homologous partner and its duplicate. While they are aligned, some crossover between the pairs occurs (you can have both your maternal grandfather’s eyes and your maternal grandmother’s smile). At this point the homologous chromosomes are separated, and the cell divides, giving the intermediate cells two versions of a chromosome from each homologous pair. Then these cells divide, separating the duplicates, and creating four cells with one chromosome from each original homologous pair.

But, what happens if the pairs aren’t so homologous? This is where the process breaks down in interspecies hybrids like the mule. The chances that two different species can form working homologous pairs are slim. Males of interspecies hybrids are usually sterile (their sperm doesn’t develop completely), and fertile females appear rarely.

Our molly mule has an extra strike against her because horses have 64 chromosomes (32 pairs), and donkeys have 62 chromosomes (31 pairs). When they mate, a mule receives 32 chromosomes from her mother and 31 from her father. This leaves a chromosome without a partner, and adds one more possibility for error. (Once in a great while, all the possible errors are avoided, and a molly foals.) Though even hybrids from species with the same number of chromosomes, like tigers and lions (whose offspring are called ligers or tigons), have trouble with fertility.

All of this adds up to mean I’ll likely have to find a willing horse and donkey if I want a mule of my own.

Novice rider and Web Editor Jenn Nemec mostly admires horseflesh from the ground. 


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