People

Research Overview

We study the evolution & consequences of selfish genetic elements. Recurrent invasion & innovation by selfish genetic elements and suppression & counter-innovation by hosts are major, if still unappreciated, drivers of genome evolution. Our lab combines classical, molecular, and evolutionary genetics & genomics to study how selfish genetic elements— such as transposons and meiotic drivers— affect speciation, sex chromosome evolution, and meiotic recombination.

Hybrid incompatibility.  Historically, one of the major gaps in speciation genetics research was the limited number of hybrid incompatibility genes identified at the molecular level. This situation began to change ~15 years, with a surprising discovery: hybrid incompatibility genes evolve via recurrent positive selection as they mediate molecular arms races between hosts and their selfish genetic elements. Two hybrid incompatibility genes that we identified between Drosophila melanogaster and its sibling species interact with the host transposon surveillance machinery.  In new work involving the much younger species of the D. simulans clade species— D. mauritiana, D. simulans, and D. sechellia— we have identified a direct role for transposons in hybrid incompatibility. 

Sex chromosomes and speciation in flies.  Sex chromosomes play a special role in speciation, as evidenced by three major “rules” of speciation: Haldane’s rule, the large X-effect, and the tendency for X and Y (Z and W) chromosomes to show reduced interspecific gene flow. Our genome-wide introgression experiments and population genomics analyses in the D. simulans clade species show that the rapid accumulation of hybrid male sterility factors on the X can explain Haldane’s rule, the large X-effect, and its reduced propensity for natural introgression. This work has revealed a surprising interplay between gene flow, hybrid male sterility, and meiotic drive.   

Neo-sex chromosomes and speciation in birds. In collaboration with Al Uy’s lab, we are studying the evolutionary genomics of neo-sex chromosomes and speciation in oceanic island birds.  About a century ago, two species of Myzomela honeyeaters established secondary contact and began to hybridize regularly. We are now investigating how gene flow, natural selection, and behavior have shaped the amount, direction, and distribution of genomic admixture. 

Meiotic drive.  We study the genetics, molecular biology, evolution, population dynamics, and genomic consequences of meiotic drive elements— selfish genes that enhance their own transmission by distorting fair Mendelian inheritance— in two systems.  The autosomal Segregation Distorter (SD) of Drosophila melanogaster is a classic selfish supergene that comprises multiple, linked, epistatically interacting loci and suppressors of recombination.  And the recently evolved, cryptic sex-ratio drive systems of D. simulans clade species each comprises multi-copy sets if X-linked drivers and specialized endogenous small RNA-encoding autosomal suppressors.

The evolution of meiotic recombination rate & patterning.  Crossing over between homologous chromosomes during meiosis repairs DNA double-strand breaks, ensures proper segregation, and shapes the genomic distribution of genetic variability.  We have also shown that recombination enhances the efficacy of natural selection in Drosophila genomes.  But why species with identical karyotypes evolve big differences in rates of crossing over is unclear.  We found that a single meiosis gene with a history of positive selection mediates most of the species difference in crossing over between D. melanogaster and D. mauritiana.  Optimal crossover rates may evolve continuously to balance the benefits of recombination against the ever-shifting risk of ectopic exchange posed by transposons.

Research Interests

• Evolutionary genetics
• Speciation genetics
• Molecular population genetics
• Selfish gene complexes