Rodent's bizarre traits deepen mystery of genetics, evolution

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"Nobody has posters of voles on their wall," said J. Andrew DeWoody, associate professor of genetics in the Department of Forestry and Natural Resources, whose study appears this month in the journal Genetica. "But when it comes down to it, voles deserve more attention."

Small rodents often confused for mice, except with shorter tails and beady eyes, voles live throughout the Northern Hemisphere and are often considered agricultural pests because they eat vegetation. Nevertheless, voles are an "evolutionary enigma" with many bizarre traits, DeWoody said. Understanding the basis for these traits could lead to better understanding of the same phenomena in human genetics and genetic disorders, and could have implications for gene therapy, he said.

The study focuses on 60 species within the vole genus Microtus, which has evolved in the last 500,000 to 2 million years. This means voles are evolving 60-100 times faster than the average vertebrate in terms of creating different species. Within the genus (the level of taxonomic classification above species), the number of chromosomes in voles ranges from 17-64. DeWoody said that this is an unusual finding, since species within a single genus often have the same chromosome number.

Among the vole's other bizarre genetic traits:

- In one species, the X chromosome, one of the two sex-determining chromosomes (the other being the Y), contains about 20 percent of the entire genome. Sex chromosomes normally contain much less genetic information.

- In another species, females possess large portions of the Y (male) chromosome.

- In yet another species, males and females have different chromosome numbers, which is uncommon in animals.

A final "counterintuitive oddity" is that despite genetic variation, all voles look alike, said DeWoody's former graduate student and study co-author Deb Triant.

"All voles look very similar, and many species are completely indistinguishable," DeWoody said.

In one particular instance, DeWoody was unable to differentiate between two species even after close examination and analysis of their cranial structure; only genetic tests could reveal the difference.

Nevertheless, voles are perfectly adept at recognizing those of their own species.

"I have seen absolutely no evidence of mating between different species," Triant said. "We don't know how they do this, but scent and behavior probably play a role."

DeWoody said, "The vole is a great a model system that could be used to study lots of natural phenomena that could impact humans."

His research focuses on the mitochondrial genome, the set of DNA within the cellular compartment responsible for generating energy (the mitochondria). Some of Triant's additional work explores the unique ability of vole's mitochondrial DNA to insert itself within DNA in the cell nucleus. The nuclear genome, as it is known, contains the vast majority of a cell's DNA and is responsible for controlling cellular function and development.

"Deb's work in this area could potentially have some basic science impact on gene delivery mechanisms, such as those used in gene therapy," DeWoody said.

In this relatively new therapy, treatment involves the insertion of a gene into human patients' cells in order to counter some illness or disease like hemophilia. However, it is often difficult to insert the desired gene in the "correct" location, or a location where it does what it is supposed to do. A better understanding of the unusual prevalence of this activity in voles, and the manner in which it happens, could have important human implications.

DeWoody's research was funded by the National Science Foundation and the U.S. Department of Agriculture. DeWoody hopes to continue his work on vole genetics at some point in the future.

Purdue University