If individuals of different species mate with each other, the hybridizing species can become more similar and eventually collapse into one species. However, chromosomal rearrangements can allow hybridizing species to persist because large blocks of the genome do not recombine. Previously funded work examined the hybridization process in two Drosophila species. Genes conferring differences between coexisting species were contained within chromosomal rearrangements while genes conferring differences between geographically separated species mapped throughout the genome, thus demonstrating the important of chromosomal rearrangements in prevening hybridization. The work in this project will fine-tune the earlier work by examining the fine details of the genetic architecture of these chromosomal rearrangements. Estimates will be made of whether and how much genetic material passes between these species at points in the genome varying in distance from chromosomal rearrangement breakpoints. This work will thus elucidate of the efficacy of chromosomal rearrangements in maintaining the distinct entities classified as species.
With the continued loss of species worldwide from human activities, this work will become increasingly important in identifying the processes that generate and maintain biodiversity on the planet. The work proposed will involve training of undergraduate and graduate students, specifically those from underrepresented groups, and will foster continued collaborations of an empirical lab with others focusing primarily on theory.
We are very grateful to NSF for this award, and we feel it has had an impact both on our scientific discipline and on science education more broadly. Within the discipline, this renewal award aimed to understand genetic elements that make new species likely to form. We specifically tested published descriptions suggesting chromosomal inversions (flips in DNA sequence that happen naturally) may allow species to maintain their identity even if they occasionally mate with each other. Looking at two fruit fly species, we found that very few genes move between the species when they hybridize either in or near (within 2.5 million bases) of the chromosomal inversions, suggesting these regions "define" the species at some level. We also examined exchange between chromosomes (crossing over) more broadly and identified a lot of variation within species and between pure-species and hybrids. These studies, and several subsidiary ones associated with either crossing over or molecular evolution, add a lot to our understanding of how newly evolved species can persist shortly after forming. We also received 2 "Research Experience for Teachers" supplements to our award. With these awards, we developed an activity called "Witnessing Evolution First-Hand: K-12 and College Laboratory Exercises in Genetics and Evolution Using Drosophila." Briefly, we had students start a population of white-eyed fruit flies, and added one "mutant" red-eyed male. Over a few generations, they saw that consistently more and more flies had red-eyes, suggesting the red-eye-gene-variant spread via natural selection. For the upper-level versions of this exercise, the students further isolated DNA and looked at the molecular evolutionary effects of the spread of this red-eye-gene-variant at genes near and far from it. The activity was conducted in middle schools, a high school, and a university, and different versions were developed as a result. It was presented at the North Carolina Science Teachers workshop as well as at a professional conference for Evolutionary biologists, and Carolina Biological Supply is now producing a kit to allow teachers around the world to be able to apply this exercise.
