Current Research

Functional and Ecological Genomics

Whole genome transcriptional studies have become a major tool in ecological genomics. In laboratory and wild populations of Drosophila mojavensis that exhibit low levels of sexual isolation and are characterized by use of different host cacti, life histories, and epicuticular hydrocarbon profiles, we have used both microarrays and transcriptome sequencing in collaboration with Allen Gibbs at UNLV, Mike Ritchie at the University of St. Andrews, and Shripad Tuljapurkar’s group at Stanford.

See Bill’s lightning talk at Evo-Demo 2013.

Transcriptional variation caused by host plants, temperature variation, desiccation regimes, mating status, and across different life stages and ages have revealed enormous differences across the genome and ubiquitous GxE interactions. Population and host plant-caused variation in the transcriptome should provide key genetic information on the evolution of ecological divergence and reproductive isolation. We have evaluated genomic responses to heat and desiccation stress as a model for understanding the consequences of global warming in this well studied desert species. Analysis of transcriptional variation over the entire life cycle has elucidated changing patterns of gene expression with stage and age.

Recently, we have discovered the causes of adult transcriptome variation due to egg to adult development time differences. Hundreds of genes are differentially expressed in aged adults that may explain why sexual behavior and premating isolation are genetically correlated with development time.

Working together with Axel Wiberg at the University of St. Andrews, we have developed new approaches to mapping groups of differentially expressed genes in D. mojavensis. See Bill’s talk at Evolution 2016 in Austin, Texas.

Ecological Divergence and Speciation

Genetic and ecological determination of sexual isolation among geographically isolated populations of Drosophila mojavensis has been analyzed in QTL studies of male courtship songs, epicuticular hydrocarbon variation, mating success, and egg to adult development time. Rearing substrate differences had significant effects on QTL detection for all traits and caused numerous GxE interactions. A single QTL was detected that influenced mating success: this QTL is 12.3 Kb from desat-2 that is involved in cuticular hydrocarbon production. We are optimistic that these and other candidate gene regions can be further exploited in future association studies.

We found strong effects of exposure to females on adult male hydrocarbon variation, and significant Cactus x Exposure (ExE) interactions for a large number of QTL. Four QTL influenced both development time and epicuticular hydrocarbon differences associated with courtship success. Since regional shifts in life histories have evolved since D. mojavensis invaded mainland Mexico from Baja California by shifting host plants, covariation of these QTL suggests there is a genetic correlation between development time and cuticular hydrocarbon-based mate preferences.

Spatial Variation and Identification of Novel Cuticular Hydrocarbons and Triacylglycerides

Collaboration with Joanne Yew at the University of Hawaii (previously at Temasek Life Sciences Laboratory-University of Singapore) and colleagues has yielded quantification of cuticular hydrocarbons and triacylglycerides in D. mojavensis, D. arizonae, and other repleta group species using direct ultraviolet laser desorption/ionization orthogonal time-of-flight mass spectrometry (UV-LDI-o-TOF MS) and other techniques. Joanne and students have identified many uncharacterized cuticular hydrocarbons, particularly in the anogenital and ventral abdomen regions of males, as well as the transfer of male cuticular hydrocarbons and novel triacylglycerides to females after mating. Characterization of these cuticular hydrocarbons and triacylglycerides is ongoing in order to understand which genes are responsible for their production, and their roles in male-female interactions before and after courtship.

Evolution of Reproductive Isolating Traits

We assessed the degree of phylogenetic conservatism in cuticular hydrocarbon profiles and courtship songs in the monophyletic Drosophila buzzatii species cluster from South America. Mapping these as continuous multivariate characters onto a phylogeny of this group revealed strong phylogenetic signals for these traits. Sexual isolation between species in courtship trials was strong among all populations and species tested suggesting that all seven currently recognized species are reproductively isolated in nature despite one report of introgressive hybridization. Additional species are likely to exist given the significant within species patterns of hydrocarbon differentiation.

Genomic Divergence Between and Within Species

Together with Konrad Lohse (Univ. Edinburgh), Mike Ritchie, and invaluable input from Alfredo Ruiz, the genomes of D. arizonae, D. mojavensis, and D. navojoa have been sequenced and compared for divergence in collinear and inverted chromosomal regions. Due to the number of fixed and polymorphic gene arrangements in these species, it is possible to assess the role of inversions during species divergence. Konrad has revealed that despite the absence of recent hybridization between D. arizonae and D. mojavensis, post-divergence gene flow occurred up until ca 250 kya, about the same time mainland and Baja California populations of D. mojavensis became isolated. Assessment of population-level genomic divergence among populations of D. mojavensis is underway.