Sex Chromosome Macroevolution

Sex chromosomes are thought to arise from ancestral autosomes, usually by selection acting on sexually antagonistic mutations that initiate divergence between homologous chromosomes. The origins of sex chromosomes are not well understood experimentally, or theoretically, and are a fundamental focus in modern biology. We are using broad comparative data to figure out how sex chromosomes evolve.

Sex chromosome evolution is a research subject of great current interest, but how sex chromosomes acquire their distinctive structure and function is not well understood. This project has three objectives. In the first objective, we will identify the decay rate of heterogametic sex chromosomes (Y or Z chromosomes) over macroevolutionary scales using karyotype data from over 500 vertebrate species. This analysis will be the first large-scale macroevolutionary analysis testing the hypothesis that heterogametic sex chromosomes readily decay over evolutionary timescales. sexchrommacroObjective two is determine which physiological model of sex determination is most consistent with macroevolutionary models. That is, does the physiological process of sex determination influence macroevolutionary patterns?  Objective three is to predict what type of sex chromosomes dinosaurs had using a new variable rate model of evolution.

To the right is a phylogeny of 220 amniotes with branch lengths in units of time. Colors correspond to the evolutionary rate of sex determination (temperature dependent, XX/XY, or ZZ/ZW). The varying rates in this model are consistent with a physiological temperature-dosage model (Quinn et al. Biology Letters, 2011). A log Bayes factor of 74 represents substantial phylogenetic support for the temperature-dosage model over a model where the rates of evolution are uniform across the tree.