Background: ‘Coevolution’ was coined by Ehrlich and Raven in 19641 to describe the reciprocal adaptations of plants and herbivorous insects. Despite five decades of work on coevolution, its genomic mechanisms remain difficult to disentangle. Two main competing models exist. The Arms race model posits that (counter)adaptations evolve under directional selection leading to selective sweeps and fixation, while the Red Queen dynamics advances that (counter)adaptations evolve under balancing selection, thus not reaching fixation. Moreover, the links between microevolutionary coevolutionary processes and its macroevolutionary consequences remain elusive2.
Objectives and research questions: This project will test these two mechanisms in the iconic passionflower (Passiflora)/Heliconius butterfly system, which coevolved defence (cyanogenic glycosides [CNglc]) and counter-defence (P450 detoxifying enzymes) for ~30 million years. Specifically, the project will focus on the subgenus Astrophea (~60 species) and the sara-sapho complex and relatives in Heliconius (~15 species). This group is ideal given its workable size and its evolutionary replication with many host-plant shifts and highly-specialized interactions.
Obj1: Is host-plant shift associated with gene copy number expansion, contraction or shifts in selection regimes and do these coincide with diversification rate shifts in the plant or/and butterfly side? This question will be addressed by sequencing all Astrophea and Heliconius species interacting with Astrophea plus their appropriate outgroups using Illumina to (i) provide a solid phylogenomic framework, (ii) perform diversification analyses and (iii) perform comparative genomics of CNglc biosynthetic pathway genes (Astrophea) and detoxifying enzymes (P450) in Heliconius.
Obj2: What coevolutionary mechanisms operate in the Passion flowers – Heliconius butterfly system? Microevolutionary Illumina datasets for multiple individuals representing four pairs of species for both the Passiflora and butterfly each representing independent host-plant shifts will be generated to perform tests for selective sweeps and directional vs. balancing selection to test the arms race vs. Red Queen dynamics hypotheses.
Novelty and timeliness: The proposed PhD project will provide one of the first links between coevolution models at microevolutionary scale, genomic adaptations, and macroevolution. The availability of reference genomes and understanding of the genetic basis of CNglc biosynthesis and P450 detoxifying enzymes provide a unique opportunity to investigate the genomics of plant-herbivore coevolution in a macroevolutionary framework. The expertise of supervisory team, together with the secured access to samples guarantee the success of this project.