Department of Biology

Widespread variation exists between species in the size, shape, and number of chromosomes, and these differences can have profound influences on genetic variation and reproductive compatibility. Ryan Bracewell, an assistant professor in the College of Arts and Sciences’ Department of Biology at Indiana University Bloomington, and members of his research lab focus on sex chromosomes in their quest to gain a better understanding of the genomic outcomes of transitions in chromosome number and structure.

One of the most pronounced genomic changes that can occur is when a chromosome—the fundamental structure necessary for the transmission of genetic information—becomes involved in sex determination. Over time, sex chromosomes typically diverge quite noticeably in gene content, gene expression, transposable element content, and levels of genetic diversity. These types of chromosomal changes can be the root of a surprising amount of variation.

National Institutes of Health has awarded nearly $2 million to Bracewell through a Maximizing Investigators' Research Award for Early Stage Investigators (R35) to investigate chromosome evolution and consequences of rapid Y chromosome degeneration. The MIRA, as the award is known, is presented by the National Institute of General Medical Sciences, a section of the NIH. It provides support for an investigator's research that falls within the mission of NIGMS. The MIRA provides researchers with greater stability and flexibility which in turn enhances scientific productivity and the chances for important breakthroughs. The award is for five years.

The Bracewell lab will use genomics and bioinformatics-based approaches to examine chromosome evolution and changes in genome structure in fruit flies (Drosophila), one of the most powerful and heavily studied systems in genetics. The unique features of Drosophila—numerous species, small genomes, few chromosomes, ease of karyotyping—make a large-scale comparative analysis tracking the fates of chromosomes and their associated genes possible. The analyses will for the first time systematically identify genus-wide chromosome evolution and constraints on gene order and organization.

Bracewell will also be studying bark beetles as a model system in the project.

“We're initially looking at the mountain pine beetle and some of its close relatives within the genus since several have weird sex chromosomes,” Bracewell explained. “The bark beetle system is one-of-a-kind and gives us the unique opportunity to peek into a narrow window of time when multiple neo-Y chromosomes are present and degenerating.”

Another aspect of Bracewell’s research will be to investigate multiple Y chromosome types that are likely responsible for the evolution of reproductive incompatibilities between populations of beetle. The research will use several cutting-edge genomics techniques to help characterize these recently formed Y chromosomes and will then be coupled with population genomics to help characterize genetic exchange (or lack thereof) in natural populations.

Together, these projects will take advantage of the unique attributes of two insect systems to understand the processes that lead to major changes in karyotype as well as variation in degeneration and gene regulation of young sex chromosomes. More broadly, Bracewell's investigation will provide a deeper understanding of the maintenance of, and variation in, chromosome structure and function.