The white-throated sparrow has generated a great deal of interest among behavioral biologists because it occurs in two ""morphs"", or types, that have different social behavior. Dozens of behavioral studies, conducted in both field and laboratory settings, have established that individuals with a white stripe on the crown tend to be more aggressive, whereas birds with a tan stripe exhibit more parental care. By conducting the studies in this project, the neurochemical reasons why white birds are more aggressive than tan birds will be revealed. Because the steroid hormone testosterone is higher in white than tan males, investigating the biological ways in which testosterone may alter aggression and thus cause behavioral differences between morphs is an essential feature of this project. The approach used by the PI is powerful because it will comprehensively examine both behavior and neurochemistry in the same individuals, thus enabling detailed study of individual variation. Because the biological basis of aggressive behavior is similar in most vertebrate animals, the results from these studies will facilitated a better understanding of the neurochemical underpinnings for aggression in tens of thousands of species, including humans. A graduate student will receive training in behavioral biology, neuroscience, and molecular biology, and will thus be well prepared for career that integrates all three fields. The PI will also mentor several undergraduate students, and will encourage them to present their findings at national meetings and in published journal articles.
Every now and then, nature provides a perfect experiment for us to discover and study. One example is the white-throated sparrow (Zonotrichia albicollis), a common North American songbird. In exactly half of the individuals in any population, one copy of chromosome 2 is rearranged. The birds with the rearrangement (ZAL2m), are different, both in plumage coloration and social behavior, from the ones with two copies of the standard chromosome (ZAL2). ZAL2m, or "white-striped" individuals are more aggressive than ZAL2, or "tan-striped" birds (Figure 1). This model thus allows a wonderful opportunity to study how the genes inside the rearrangement have been altered during evolution, and how those changes may contribute to plumage coloration and aggression. In this NSF-funded project, we have asked the following questions: 1. Do the morphs differ in their expression of neuropeptide receptors that control aggression? • To develop tools to answer this question, we sequenced four neuropeptide receptors, called vasotocin receptors, that are implicated in aggression in birds and other vertebrates. We then mapped the distributions of these receptors in the brain (see Figure 2). • We found that the expression of these receptors differs according to morph, but depends also on hormone levels. Morph differences in behavior are seen only during the breeding season, so reproductive hormone levels are very important. 2. Are morph differences in behavior caused by differences in gonadal hormones? • Because the white-striped males have higher levels of testosterone than the tan-striped males, we conducted a study to test whether testosterone causes differences in aggression. We found that even when we equalized testosterone experimentally, the white-striped males were still more aggressive. This shows that morph differences in behavior are not caused entirely by morph differences in hormone levels. • We suspect that the brains of the two morphs are differentially sensitive to the effects of gonadal steroids. We are now exploring the regulation of other receptors, such as serotonin receptors, in the two morphs. 3. Exactly where is the rearrangement and what genes have been affected by it? • We mapped the rearrangement (Figure 3) and found several neuroendocrine genes known to contribute to aggression. We are now quantifying the expression of those genes in both morphs to test for differences. • We found that the ZAL2m chromosome recombines very little with the ZAL2, which means that all of the genes inside the rearrangement are linked together and inherited as a unit. • We found that ZAL2m is differentiating from ZAL2, but it is not degenerating like the mammalian Y chromosome. • Two genes that are differentiating are bitter taste receptors, which means that taste perception could vary between the morphs. 4. Are there any birds with two copies of ZAL2m? The only way an individual could have two copies of ZAL2m is if its parents are both white-striped. But white-striped birds almost never mate with each other, a phenomenon that is still poorly understood. • In 2011, we collected a bird with two copies of ZAL2m (a "homozygote". She was the first homozygote out of more than 500 birds we have collected and genotyped over the years, and only the third in history to be reported. • We performed the first systematic characterization of the plumage and aggressive behavior of a ZAL2m homozygote. Her plumage was strikingly bright, resembling that of an adult male (Figure 4). Her behavior was extremely aggressive. She dominated every other bird in behavioral studies, showing that the level of aggression in this species depends on the number of copies of ZAL2m. • We sequenced the mRNA for every gene expressed in her brain. This information will help us understand which genes on the ZAL2m are important for aggression and other traits. • We prepared the skin as a museum specimen that is now part of the Smithsonian collection (USNM 627866). In addition to the above, the project had the following impacts: • Largely because of our work, there are more genomic resources available for the white-throated sparrow than any other North American songbird. • Two post-docs, 11 graduate students, 22 undergraduates, and one high school student were mentored in the lab. 75% were women or underrepresented minorities, and more than half were or will be co-authors on published work. • I received a Distinguished Mentor Award from the Howard Hughes Medical Institute. • Students in our lab have the opportunity to learn behavioral observation in both lab and field, molecular cloning, real-time qPCR, in situ hybridization, immunohistochemistry, microscopy, and fMRI. They thus acquire a tremendous set of tools with which to approach genetic-physiological-environmental influences on behavior. Learning a wide variety of biological concepts and techniques prepares students to excel in many different disciplines and provides them with options as they choose career paths.
