My research program has two broad areas of inquiry. The first is chromosome evolution, specifically sex chromosome and chromosome number evolution. The second is the development of methods and databases that accelerate the analysis of data within a quantitative genetic or phylogenetic framework. To address these topics, I use a broad range of approaches including theoretical population genetics, bioinformatics, and molecular cytogenetics.
Fragile Y Hypothesis
Chromosomal sex determination is phylogenetically widespread, having arisen independently in many lineages. Decades of theoretical work provide predictions about sex chromosome differentiation that are well supported by observations in both XY and ZW systems. However, the phylogenetic scope of previous work gives us a limited understanding of the pace of sex chromosome gain and loss and why Y or W chromosomes are more often lost in some lineages than others, creating XO or ZO systems. Contrary to our initial expectations, we find that highly degenerated Y chromosomes of many members of the Coleoptera suborder Polyphaga are rarely lost, and that cases of Y chromosome loss are strongly associated with chiasmatic segregation during male meiosis. We propose the "Fragile Y Hypothesis", that recurrent selection to reduce recombination between the X and Y chromosome leads to the evolution of a small pseudoautosomal region, which, in taxa that require XY chiasmata for proper segregation during meiosis, increases the probability of aneuploid gamete production, with Y chromosome loss. This hypothesis predicts that taxa that evolve achiasmatic segregation during male meiosis will rarely lose the Y chromosome.
The pace and direction of evolution are governed by the genetic architecture of trait variation. Evolutionary biologists have disagreed about whether genes can be considered to act in isolation, or in the context of their genetic background (Fisher Wright debate). Line cross analysis (LCA) estimates genetic architecture parameters conditional on the best model chosen from a vast model space using relatively few line means and ignores uncertainty in model choice. To address these issues we introduce an information theoretic approach to LCA, which comprehensively assesses the potential model space, quantifies model selection uncertainty, and uses model weighted averaging to accurately estimate composite genetic effects. Using simulated data and previously published LCA studies we have shown the utility of our approach to define the components of complex genetic architectures. Our analysis of 20+ previously published datasets also shows that traditional approaches have underestimated the importance of epistasis.
Coleoptera exhibit variation in both chromosome number and sex chromosome systems. However, the last compilation of Coleoptera karyotype information was completed in 1978. Since that time the number of species that have been studied has doubled. Unfortunately this data is often scattered among journals and dissertations that are not accessible without subscriptions. We created the Coleoptera Karyotype database to make this data open and available to all researchers. Our database currently has over 4,861 records and has proven to be a valuable resource to explore and test ideas about the evolution of high-level genome evolution.
Chromosome Number and Eusociality
It has been suggested that in Eusocial insect selection to increase genetic diversity within a colony may indirectly select for increases in the number of chromosomes. To test this long-standing hypothesis we investigated the relationship between eusociality and chromosome number across Hymenoptera. We found that solitary and social Hymenoptera do not have significantly different numbers of chromosomes. However, we did find that chromosome number evolves more quickly in social than solitary Hymenoptera. It remains unclear whether variable selection pressure or drift are responsible for this difference.