Collaborative research, SG, RUI: The importance of the X-chromosomes in speciation: are genes that do not introgress concentrated on the X chromosome?

Williams College, Williamstown MA

Investigators

Abstract

To understand how new species evolve we need to understand the genes involved in the process and where these genes are located in the genome. Speciation results from genes that cause reproductive incompatibilities between groups of individuals, and earlier studies have suggested these genes are located on the X chromosome. This project will investigate the role of the X chromosome in the evolution of two closely related species of field crickets. These two cricket species are still capable of producing some hybrid offspring and exchanging genetic information. Populations of these species co-occur in the Northeastern U.S., where they form an extensive natural hybrid zone, in which genes have been exchanged for thousands of generations. The researchers will investigate rates of genetic mixing between the species, and determine where it occurs across the genome. This research will address the hypothesis that the X chromosome contains the genes that lead to reproductive isolation, by testing whether the X chromosome shows a lower rate of genetic exchange between species. Specific genes located within regions of low genetic exchange will also be identified to further our understanding of what is required for new species to evolve. This is a collaborative research project that will be mainly conducted at a liberal arts, undergraduate college and will provide substantial opportunity for training and mentoring of undergraduate students. Outreach includes science education workshops and activities, and will target underrepresented, middle and high school students in STEM fields. The proposed research will test both the importance of the X chromosome in speciation and whether observed islands of differentiation in the field crickets, Gryllus firmus and G. pennsylvanicus, reflect differential introgression and/or recent selection. The project has three main objectives. First, the X chromosome will be mapped using hybrid backcrosses and within species crosses; patterns of differentiation for these X-linked chromosome markers will be assessed. Second, gene sequences will be obtained from genome regions that show varying rates of gene exchange and differentiation between species. Genome regions with low rates of gene flow and high differentiation might harbor genes responsible for reproductive isolation and species identities. Absolute and relative measures of genetic differentiation and estimates of selection will be calculated for high differentiation regions and for control regions with low differentiation. These results will be used to test if the genome is composed of a patchwork of high and low differentiation regions. The third objective is based on the observation that there is a one-way incompatibility between the two cricket species, but that this 'rule' is broken in some population crosses. If regions of high differentiation are responsible for reproductive isolation, then there should be consistent genomic differences between pairs that produce offspring and those that do not. Results will provide important insights into the genomic architecture of speciation, the role of the X chromosome in reproductive isolation and divergent adaptation, and will contribute to ongoing debates about genomic islands of differentiation.