Constraints on the evolution of chromosome dosage compensation: a test in butterflies and moths

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This research addresses the question of how genes evolve when individuals have different numbers of copies of each gene. In many species, the sex of the individual is determined by how many copies of the sex-determining chromosomes one has. In humans, females have two copies of the X chromosome and males have just one. Levels of gene expression usually reflect gene dosage: how many gene copies are present in an individual. Disrupting the balance of gene dosage between chromosomes typically has severe negative consequences. Understanding how and why mechanisms evolve to compensate for unbalanced gene dosage is an important goal both for medical and evolutionary research. This project focuses on moths and butterflies in which males have two copies of the sex-determining chromosome and females have just one. This project will systematically survey dosage compensation associated with sex-determining chromosomes in moths and butterflies. It will also test how factors like mutation rates and natural selection play a role in determining the evolution of balanced gene expression. The researchers will use the wide public appeal of moths and butterflies as a means to increase public engagement and knowledge about scientific research. Theory predicts that a mechanism should exist for compensating X-linked gene expression between the sexes. Observations from different species reveal that often such mechanisms have evolved, but sometimes they do not. The reasons for this difference are not understood. Preliminary results suggest moths and butterflies harbor intriguing variation in the magnitude of dosage compensation. The research will combine transcriptome and genome sequencing to characterize patterns of dosage compensation in 10 phylogenetically diverse moth or butterfly species. RNA-sequencing will provide genome-wide estimates of gene expression to test for differences between sexes in sex-linked gene expression. This broad survey of dosage compensation will resolve questions regarding the presence of sex-determining chromosome dosage compensation in this prominent female-heterogametic taxon. Comparative genomic analyses and resequencing of targeted species will be used to infer patterns of male mutation bias and the relative effective population sizes of sex-determining chromosomes and other chromosomes. These population genetic factors are expected to correlate with the presence of dosage compensation and will be compared across species to test for such a relationship.