Recently, designer mutts like the Labradoodle—a cross between a Labarador retriever and a poodle—have become popular. A listener wanted to know if some kinds of dogs are just too different to make puppies.
Are there some dogs that don't mix? I'm Bob Hirshon and this is Science Update.
Perhaps hoping to cross-breed a Chihuahua and a pit bull, Mary Beckman of Idaho Falls, Idaho recently called the Why Is It line.
Species are partly defined by the ability to interbreed. With all the dog breeds in the world, are there any two dog breeds that could be considered different species?
We asked Gregory Acland, of Cornell University's Center for Canine Genetics and Reproduction. He says you're right to use the term "partly."
But that rule, even if it was true, isn't a rule that divides species anyway, because there are many species of birds for example, or fish, or lots of organisms, that if you put them in captivity and give them the opportunity to do, they will breed. So that you could cross wolves with dogs. You can certainly cross wolves with coyotes.
That said, he's found that there are certain combinations of dogs that don't cross-breed easily: for example, beagles and Irish setters.
These were dogs with family lines, where they routinely produce big litters, and yet when we tried to breed these fertile beagles to fertile setters, we got no pups at all, despite many attempts to do so, and then eventually, we were able to produce one litter with two pups in it.
He says subtle genetic incompatibilities between these breeds may get in the way.
If you've got a science question, then doggone it, call us at 1-800-WHY-ISIT. If we use it on the show, you'll get a free Science Update mug. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
We often think of species as reproductively isolated: in other words, you can't cross one species with another and get viable, fertile offspring. You certainly can't cross a cat with a chicken, for example—assuming you could figure out a way to mate them. And the poster child for cross-species reproduction, the mule, can't have offspring of its own.
But as Acland explains, that's not a hard and fast rule. In the wild, most species will not normally interbreed with any other species. But this isn't necessarily because they're genetically incompatible. Wolves and coyotes don't breed in the wild primarily because they antagonize and compete with each other. But throw a male wolf and a female coyote into a zoo cage for a few months, and eventually they just might mate. When they do, their offspring are not only viable but also fertile. Many species of birds or fish that simply don't cross paths in the wild can mate and produce offspring in captivity.
So the fact that most dog breeds can easily interbreed isn't reason enough to consider them all the same species, although that's a large part of it. It comes down to how you define "species." Many scientists use the rule of thumb that if two organisms can interbreed, they are of the same species. But that doesn't apply in every case, to say nothing of species like bacteria that don't "breed" at all, but reproduce asexually. The fact is that there are many definitions of "species" accepted and debated in the scientific community, and each has its exceptions.
But if even some separate species can cross-breed, what keeps some dog pairs, like beagles and Irish setters, from producing pups? Acland says no one knows for sure; they discovered these mismatches by accident while studying genetic diseases, not while formally studying dog breeding itself. But he says it may have something to do with a kind of DNA called "junk DNA."
Junk DNA is a kind of DNA that takes up space in the genome but doesn't serve an obvious function for the organism. The term "junk," however, is probably a misnomer that reflects our limited understanding of genetics. Strands of junk DNA appear in repetitive patterns on an organism's genome, and many of these patterns are unique to a particular strain or species.
In a species of fruit fly called Drosophila, there is a particular kind of junk DNA that tends to vary from strain to strain. Sometimes, when you try to breed a pair of fruit flies from two different strains, and these strains have mismatched patterns of this junk DNA, you get deformed offspring or no offspring at all. This is despite the fact that the flies belong to the same species.
It may be possible that junk DNA mismatches like this can cause infertility between certain pairs of dog breeds. Does that mean that these breeds are on their way to becoming separate species? Again, it depends how you define "species." And since the process of becoming a separate species takes much longer than a human lifetime, the answer won't be known anytime soon.
Now try and answer these questions:
- Are all dogs the same species? Why or why not?
- If you define a species by the ability to cross-breed and produce fertile offspring, what are some exceptions to this definition?
- How else might you define a "species?"
- What is junk DNA? How might it relate to dog breeding?
- Suppose human couples with unexplained fertility problems were found to be more likely to have mismatched junk DNA patterns. How would that affect the answer to this question?
Understanding Evolution teaches students what evolution is, how it works, how it factors into our lives, how research in evolutionary biology is performed, and how ideas in this area have changed over time.
The National Human Genome Research Institute offers this Genetics Primer for Owners to provide more information about dogs' genes and why it's so important to study them.
The DNA Learning Center, at Cold Spring Harbor Laboratory, offers a variety of resources on genetics for lay people of all ages.