Recent advances including the use of DNA sequencing in systematics have made Darwin's vision of a Tree of Life, showing evolutionary relationships among all organisms, a feasible goal. However, there are still challenges that must be addressed before this vision can become reality. One of the main challenges is that virtually all current molecular phylogenies (evolutionary trees) for closely related animals are based on maternally inherited mitochondrial DNA sequences. Mitochondrial DNA works well for many reasons, but independent nuclear sequences are needed to test these mitochondrial trees where they are controversial. The published mitochondrial tree for orioles is one of the most controversial in all of bird systematics, so it is an ideal phylogeny to test using nuclear sequences. Five different nuclear intron regions will be sequenced to evaluate the oriole evolutionary tree. Nuclear introns are free to mutate more rapidly than protein coding gene regions, so they are good candidate regions for rapidly evolving sequences in the nuclear genome. The resulting evolutionary tree will be used to infer the history of plumage color differences (dimorphism between the sexes) in orioles. In some groups of animals, changes in female color may be the main cause of gains and losses of dimorphism. In orioles and many other groups of birds, females in temperate migratory species are generally dull colored, whereas females in tropical non-migratory species are generally elaborately colored. These orioles therefore present an excellent opportunity to reconstruct changes in plumage color dimorphism, and investigate the possible causes of changes in female plumage brightness. In addition, this project will test and demonstrate the value of combining data from introns, mitochondrial DNA and morphology. Combining multiple nuclear intron sequences may provide a basis for constructing whole genome trees for all closely related animal species. This research program by Prof. Kevin Omland at University of Maryland Baltimore County provides many opportunities for education and outreach activities. The project includes an international student exchange program between the University of Maryland Baltimore County (UMBC) and the National Autonomous University of Mexico (UNAM). Over the last six years, Prof. Omland has been developing a research and undergraduate exchange program with researchers at UNAM. This exchange model will be expanded to include UMBC students, especially from UMBC's well-known minority science programs, who will assist with field and museum research in Mexico. Undergraduate students from Maryland and Mexico will have the opportunity to travel internationally and participate in a wide range of systematics research. The research team will produce a bilingual website on orioles to help disseminate findings to both English and Spanish speakers, and to promote understanding of evolution, systematics and biodiversity. In the classroom at UMBC the oriole research program will be used as an extended case study in active learning exercises to help illuminate the process of science. The Baltimore Oriole is the state bird and the mascot of the local baseball team, which helps make this research accessible to students and the public. This international research and exchange program will foster US-Mexico collaboration, and promote systematics research to a diverse audience.
The research funded by this NSF CAREER grant addressed two questions: 1) How can DNA sequence data be used to determine evolutionary relationships among very closely related species? (molecular phylogeny) 2) How can evolutionary relationships be used to infer whether differences in characteristics between closely related species result from gains or losses of traits? (character evolution) We addressed these questions using a well-known songbird group, the New World orioles (genus Icterus). The research addressed specific basic questions applied to coloration in these orioles, but the approaches and results are applicable to a wide range of basic and applied questions across broad fields of biological and medical research. I. Evolutionary Trees for Closely Related Species: Beginning with Darwin, biologists have sought to understand the evolutionary relationships among all species on Earth, a "Tree of Life". However, determining evolutionary trees for closely related species remains a major obstacle. For example, phylogenies of closely related animal species have generally relied on a single gene region - mitochondrial DNA. But mitochondrial DNA has severe limitations, especially because it is just one linked segment of DNA. Our project helped demonstrate the utility of combining DNA sequences from multiple independent "introns" (segments of DNA within gene regions that do not code for proteins). These introns freqently do have much variation compared to mitochondrial DNA, but in spite of their low sequence variability and frequent shared variation between species, we found strong support for the evolutionary trees that we obtained. Thus, simply combining data from multiple introns from throughout the genome provided lots of information about the phylogeny for closely related animal species that have only diverged from each other for a few million years. II. Evolution of Sexual Dimorphism in Orioles: Understanding the causes of elaborate bird coloration has been a focus of ornithology since Darwin. Most of this research has asked why elaborate male color has been favored. Researchers assumed that species with colorful male plumage have evolved from species in which both sexes were drab. However, a modern evolutionary perspective indicates the differences between males and females in species like the Baltimore Oriole could actually result from changes in FEMALES. Using the evolutionary tree that we developed, we inferred that the ancestral oriole was likely a tropical species that had elaborate coloration in both sexes. Male only elaborate color has repeatedly evolved in orioles due to losses in females in temperate species. Modern evolutionary approaches based on molecular phylogenies led us to consider both temperate and tropical species help avoid biases and preconceptions determined by which bird species we are most familar with (e.g., which species breed in the US). Such comparative phylogenetic approaches can be used to address a wide range of questions, from our basic research on animal coloration, to applied questions such as whether a disease that affects humans and monkeys crossed into humans from monkeys or vice versa.
