Abstracts (first author)
Genome-wide patterns of admixture between species during an adaptive radiation
Most speciation events probably occur gradually, without complete and immediate reproductive isolation, but the full extent of gene flow between diverging species has rarely been characterized on a genome-wide scale. Documenting the extent and timing of admixture between diverging species can clarify the role of geographic isolation in speciation. Here we use new methodology to quantify admixture at different stages of divergence in Heliconius butterflies, based on whole genome sequences of 31 individuals. Comparisons between sympatric and allopatric populations of H. melpomene, H. cydno and H. timareta revealed a genome-wide trend of increased shared variation in sympatry, indicative of pervasive interspecific gene flow. Up to 40% of 100 kb genomic windows clustered by geography rather than by species, demonstrating that a very substantial fraction of the genome has been shared between sympatric species. Analyses of genetic variation shared over different time intervals suggested that admixture between these species has continued since early in speciation. Alleles shared between species during recent time intervals displayed higher levels of linkage disequilibrium than those shared over longer time intervals, suggesting that this admixture took place at multiple points during divergence and is probably ongoing. The signal of admixture was significantly reduced around loci controlling divergent wing patterns, as well as throughout the Z chromosome, consistent with strong selection for Müllerian mimicry and with known Z-linked hybrid incompatibility. Overall these results show that species divergence can occur in the face of persistent and genome-wide admixture over long periods of time.
Defining the regulatory regions that control Heliconius butterfly colour pattern mimicry
Genomic studies of natural populations are offering novel insights into adaptation and diversification. In particular, recent studies of parallel evolution of similar phenotypes in divergent lineages have commonly shown the utilization of shared genetic variation. Heliconius butterflies represent a recently documented example of shared allelic variation across species boundaries.
Heliconius display bright wing patterns that warn predators of distastefulness and also act as mating cues. The diversity of patterns displayed within and between the hundreds of forms is remarkable, as is the convergence between species onto near-perfect mimetic patterns. Recent field studies have identified a group of populations along the eastern slopes of the Andes that are allied to H. timareta and share wing phenotypes with sympatric H. melpomene. Genomic studies have shown that the populations with similar phenotypes also share allelic variation at wing patterning loci, with adaptive introgression across the species boundary providing the most likely explanation for this pattern.
We sequenced a 600 Kb genomic region that regulates diverse red wing pattern phenotypes, using 80 Heliconius samples. Genomic intervals associated with at least three independent red colour pattern phenotypes were resolved using sequence comparisons that grouped similar wing phenotypes, irrespective of species. By comparing the level of nucleotide variation within each colour pattern interval, we estimate the time in generations when introgression events occurred between H. melpomene and H. timareta. Gene exchange after speciation has resulted in the adaptive spread of colour pattern alleles. Here we have identified narrow genomic regions that must act through cis-regulatory control of the transcription factor optix, in order to control complex phenotypes.