Speciation genetics

Our main research effort is focused on understanding the molecular and evolutionary genetics of the origin of new species. Speciation is caused by the evolution of reproductive isolating barriers that prevent genetic exchange between populations. We focus on the evolution of hybrid incompatibilities— deleterious epistatic interactions— that cause postzygotic reproductive isolation, causing hybrid inviability or hybrid sterility. In particular, we would like to identify the genes involved in hybrid sterility and inviability and to understand the evolutionary basis of the strong rules that characterize the evolution of postzygotic isolation.

Finding the genes

With few notable exceptions, little is known about the genes involved in hybrid incompatibilities (Orr and Presgraves 2000; Orr, Masly, and Presgraves 2004). Our first goal is to address this problem by identifying many such “speciation genes” and to answer several longstanding questions in evolutionary biology: What are the normal cellular or developmental functions of speciation genes within species? Are certain classes of genes more likely to cause hybrid fitness problems than others? What evolutionary forces cause the functional divergence of these genes? Are the relevant substitutions concentrated in coding or in regulatory regions? We are currently mapping and characterizing genes that cause inviability or sterility in hybrids between the fruitfly species, Drosophila melanogaster and D. simulans (Presgraves 2003; Presgraves et al. 2003; Presgraves and Stephan 2007; Tang and Presgraves, in prep.) and between D. mauritiana and its sibling species, D. sechellia and D. simulans (Masly and Presgraves 2007; Cattani and Presgraves, in prep.).

Rules for speciation

Our second goal is to understand the evolutionary and genetic causes of the strong patterns that characterize speciation, including its Haldane’s rule and the large X-effect. To study these patterns we use comparative (Presgraves and Orr 1998; Presgraves 2002) and genetic approaches (Masly and Presgraves 2007). Currently, we are testing hypotheses that might explain why the X chromosome is a hotspot for hybrid male sterility factors (Presgraves 2008).

Recombination and natural selection

We use molecular population genetics to ask how natural selection might shape features of the genome and, conversely, how genomic context might influence the efficacy of natural selection. We have found for instance, that the power of positive and negative selection is compromised in genomic regions with low recombination rates (Betancourt and Presgraves 2002; Presgraves 2005).

We have also found that, contrary to the faster-X theory, genes on the X chromosome do not show accelerated protein evolution, suggesting that new beneficial amino acid mutations are not, on average, recessive (Betancourt, Presgraves, and Swanson 2002). However, the X and autosomes differ when it comes to the lengths of non-coding DNA: the non-neutral evolution of small insertions and deletions has led to longer introns on the X chromosome in D. melanogaster (Presgraves 2006).

Selfish genes

We are studying the evolutionary history and genetics of Segregation Distorter (SD), an autosomal meiotic drive gene complex in D. melanogaster. Fifty years of continuous intensive work, culminating in recent breakthroughs on the molecular genetic and cell biological basis of distortion, make SD the best characterized meiotic drive element known. However, important evolutionary questions concerning its origin and population genetic history remain (e.g., Presgraves 2007, 2008; Presgraves et al., in prep).