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article imageThe first speciation gene in mammals has just been discovered

By Bart B. Van Bockstaele     Dec 12, 2008 in Science
The search for genes that drive speciation is a difficult one. No more than a handful of these have been found. Now, the first of these genes in mammals has been found. It is a very important step.
There are several ways to define species, but the most accepted definition would be that a species is a group of organisms that can interbreed to produce fertile offspring.
The word "fertile" is very important here, because interbreeding of different species is often quite possible. Two examples that are very well known are mules, the offspring of female horses and male donkeys, and hinnies, the offspring of male horses and female donkeys. Both are, however, not fertile. This means that mules cannot have offspring, and the same is true for hinnies. Even this is not entirely true however. Cases of fertile offspring do occur. Nature does not conform to human categorizations and there are very few absolutes.
That said, interspecies breeding is problematic, and because of this, it is a powerful force in furthering speciation. There are several causes that make interspecies breeding difficult, but one of them is that there are genes that actually block reproduction in hybrids.
Now, Nature reports that Jiri Forejt, a geneticist at the Academy of Sciences of the Prague-based Czech Republic, and his colleagues have found the very first of these genes in a mammal, namely in a mouse. Forejt says that it was terrible work. He has spent the last 30 years trying to find a speciation gene in two mouse subspecies.
They crossed the animals, they changed their genome, and in combination with data of an entire genome that has recently become available, Forejt was finally able to determine that the male hybrids of these two subspecies were infertile because of the Prdm9 gene.
They found that the gene has the code for a protein that silences genes. This confirms also the hypothesis that certain traits that can be inherited without changing the underlying DNA sequence, called epigenetic changes, are important in speciation.
The genetics of speciation are extremely difficult to figure out. The reason for that is simple enough. Since speciation means that interbreeding is not possible, it becomes very hard to interbreed the different species an use the findings to determine the speciation genes. That is the opinion of Nitin Phadnis, a geneticist at the Fred Hutchinson Cancer Research Center in Seattle.
Together with his colleague H. Allen at Rochester University in New York had been looking for speciation genes that caused peculiar hybrids when two related subspecies of fruitflies (Drosophila) were crossed. The male hybrids turn out to be infertile for most of their lives, but when they get older, they get some fertility back, but they can only produce female offspring at that time.
Because of these hybrids, Phadnis and Orr thought that a segregation distorter could be linked to speciation. Segregation distorters are genes that lead the chromosomes carrying them to be passed more often to the offspring. In this specific example, they do this by determining the sex of the offspring.
Phadnis also says that segregation distorters can be involved in an arms race with genes that try to prevent this from happening, and that because of this, segregation distorters may evolve quite rapidly. It had already been suggested that this rapid evolution of cheating genes and the genes repressing them could lead to large functional differences between populations and that this could result in speciation, but there was not much evidence for this hypothesis.
However, by using techniques similar to those Forejt used, Phadnis was able to show that a single gene caused sterility in hybrids and segregation distortion.
"What this work shows you is that speciation can happen not only because of adaptation to the external environment, but also because of adaptation to the internal genomic environment," Phadnis says.
"Since there are so few speciation genes identified to the sequence level, adding one more to the list is exciting," says Roger Butlin, a geneticist at Sheffield University, UK. "I think there will be a lot more interest in segregation distortion and its relationship to speciation."
"There is no question that in this era of whole-genome sequences and genomic data it is much easier to identify speciation genes than it used to be" says Michael Nachman. He studies mouse genetics at the University of Arizona in Tucson.
It is now hoped that these discoveries, and the discovery of more of these genes would provide more insight into speciation. For example, it is possible that only particular types of genes play an important role in speciation, such as the epigenetic genes and segregation distorters that are described in this article. It is, of course, also possible that there are many types of genes that can (help) cause speciation.
"What is surprising about the speciation genes that have been identified [so far] is that there is a whole hodgepodge of different kinds of genes with different functions," says Nachman. "I don't think we're going to see [trends] until dozens of genes are identified, and there's just a handful now."
More about Genes, Speciation, Mammals mice fruitflies
 
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