Evolution of Dimorphic Sex Chromosomes

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The proposed research will experimentally test predictions from recent theory, concerning the mechanisms that lead to the evolution of dimorphic sex chromosomes. Most animal species, and a smaller percentage of plant species, have a special pair of chromosomes, the X and Y sex chromosomes, that determine gender. The typical pattern is for males to carry two types of sex chromosomes, and X and a Y, and for females to carry two of the same type (X) of sex chromosome. Several lines of evidence now indicate that sex chromosomes are derived from a pair of chromosomes which formerly were not associated with the determination of gender. This transition first involves the evolution of a major sex determining gene, followed by the evolution of a sex-determining chromosomal segment, and finally in the evolution of sex-determining chromosomes. During the evolution of sex chromosomes, one chromosome, the X, evolves enhanced biochemical activity (dosage compensation) while the partner chromosome, the Y, is degraded and loses virtually all genetic activity. The sex chromosomes also lose their ability to exchange genetic material during this transition. An understanding of the genetic mechanisms responsible for the formation of sex chromosomes is the focus of the proposed research. The investigator will use a Drosophila melanogaster (common fruit fly) model system to experimentally create a new pair of sex- determining genes and chromosomes. The investigator will then use this model system to explore the mechanisms that lead to the breakdown in genetic exchange between the sex chromosomes, and to the mechanisms that lead to the deterioration (accumulation of mutations) of the genetic activity of the Y sex chromosome. An understanding of the evolution of dimorphic sex chromosomes has many practical applications. First, the presence of sex chromosomes causes males (more generally the heterogametic sex) to be far more susceptible to hereditary disease. Dimorphic sex chromosomes are the physical basis for sex-linked inheritance and cause males to inherit many hereditary diseases thousands of times more frequently than females. The evolution of dimorphic sex chromosomes also results in the phenomena of dosage compensation (a modified level of gene expression for the hereditary material located on the sex chromosomes). Biochemical irregularities with dosage compensation are responsible for some forms of hereditary disease. The proposed experiments will also monitor the accumulation of deleterious mutations on both the sex chromosomes and on ordinary chromosomes. An understanding of this accumulation process is important in understanding the risk to human populations of increasing levels of exposure to radiation (e.g. that produced from nuclear waste and the thinning of the ozone layer of the atmosphere). Recent applied theoretical work by the investigator also suggests that data from the experiments will be useful in determining the genetic risk (due to inbreeding depression and natural levels of background radiation) to wildlife populations that are maintained to small effective population size, such as occurs in parks and game refuges.