Person:

Roda, Federico

Hybridization has played an important role in the evolution of many lineages. With the growing availability of genomic tools and advancements in genomic analyses, it is becoming increasingly clear that gene flow between divergent taxa can generate new phenotypic diversity, allow for adaptation to novel environments, and contribute to speciation. Hybridization can have immediate phenotypic consequences through the expression of hybrid vigor. On longer evolutionary time scales, hybridization can lead to local adaption through the introgression of novel alleles and transgressive segregation and, in some cases, result in the formation of new hybrid species. Studying both the abundance and the evolutionary consequences of hybridization has deep historical roots in plant biology. Many of the hypotheses concerning how and why hybridization contributes to biological diversity currently being investigated were first proposed tens and even hundreds of years ago. In this Update, we discuss how new advancements in genomic and genetic tools are revolutionizing our ability to document the occurrence of and investigate the outcomes of hybridization in plants.

Gene flow can impede the evolution of reproductive isolating barriers between species. Reinforcement is the process by which prezygotic reproductive isolation evolves in sympatry due to selection to decrease costly hybridization. It is known that reinforcement can be prevented by too much gene flow, but we still do not know how often have prezygotic barriers evolved in the presence of gene flow or how much gene flow can occur during reinforcement. Flower colour divergence in the native Texas wildflower, Phlox drummondii, is one of the best-studied cases of reinforcement. Here we use genomic analyses to infer gene flow between P. drummondii and a closely related sympatric species, Phlox cuspidata. We de novo assemble transcriptomes of four Phlox species to determine the phylogenetic relationships between these species and find extensive discordance among gene tree topologies across genes. We find evidence of introgression between sympatric P. drummondii and P. cuspidata using the D-statistic, and use phylogenetic analyses to infer the predominant direction of introgression. We investigate geographic variation in gene flow by comparing the relative divergence of genes displaying discordant gene trees between an allopatric and sympatric sample. These analyses support the hypothesis that sympatric P. drummondii has experienced gene flow with P. cuspidata. We find that gene flow between these species is asymmetrical, which could explain why reinforcement caused divergence in only one of the sympatric species. Given the previous research in this system, we suggest strong selection can explain how reinforcement successfully evolved in this system despite gene flow in sympatry.