The California mouse (Peromyscus californicus) is a species of rodent in the family Cricetidae found in northwestern Mexico and central to southern California. Most rodents are polygamous, but the California mouse pair bonds, making it a model organism for researchers studying the genetics and implications of partner fidelity. Credit: The University of California, Berkeley

In the foothills of the Santa Cruz Mountains two closely related species of mice share a habitat and a genetic lineage, but have very different social lives. The California mouse (Peromyscus californicus) is characterized by a lifetime of monogamy; the deer mouse (Peromyscus maniculatus) is sexually promiscuous.

Researchers at the University of California Berkeley recently showed how these differences in sexual behavior impact the bacteria hosted by each species as well as the diversity of the genes that control immunity. The results were published in the May 2012 edition of PLoS One.

Monogamy is a fairly rare trait in mammals, possessed by only five percent of species. Rarely do two related, but socially distinguishable, species live side-by-side. This makes these two species of mice interesting subjects for Matthew MacManes, a National Institutes of Health-sponsored post-doctoral fellow at UC Berkeley.

Through a series of analyses, MacManes and researchers from the Lacey Lab examined the differences between these two species on the microscopic and molecular levels. They discovered that the lifestyles of the two mice had a direct impact on the bacterial communities that reside within the female reproductive tract. Furthermore, these differences correlate with enhanced diversifying selection on genes related to immunity against bacterial diseases.

Bacteria live on every part of our bodies and have distinctive ecologies. The first step of MacManes project involved testing the bacterial communities that resided in the vaginas of both species of micethe most relevant area for a study about monogamous and promiscuous mating systems.

Next, MacManes performed a genetic analysis on the variety of DNA present, revealing hundreds of different types of bacteria present in each species. He found that the promiscuous deer mouse had twice the bacterial diversity as the monogamous California mouse. Since many bacteria cause sexually transmitted infections (like chlamydia or gonorrhea), he used the diversity of bacteria as a proxy for risk of disease. Results of the study were published in Naturwissenschaften in October 2011.

But this wasn't the end of the exploration.

"The obvious next question was, does the bacterial diversity in the promiscuous mice translate into something about the immune system, or how the immune system functions?" MacManes asked.

MacManes hypothesized that selective pressures caused by generation after generation of bacterial warfare had fortified the genomes of the promiscuous deer mouse against the array of bacteria it hosts.

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