Rapid evolution of reproductive isolation in newly allopatric invasive populations

Invasive populations are ideal model systems to understand the early stages of allopatric speciation, a keystone concept of evolutionary biology. Divergent selection to different environments can drive to reproductive isolation and, eventually, speciation. The annual, Centaurea solstitialis, (yellow starthistle) is an ideal system to study selection and speciation during a biological invasion. It is native from Eurasia, and invasive in Australia and the Americas. It developed different sets of local adaptations to different regions, including some degree of reproductive isolation. The fundamental research undertaken in this PhD project, will address the emergence and maintenance of reproductive isolation in yellow starthistle, and investigate how the balance of selective or neutral processes can lead to incipient speciation in a new allopatric range. This research will contribute substantially to our understanding of how biodiversity is generated and maintained , as well as establishing the likely impacts of invasive populations on biodiversity.

This project builds on an established research collaboration involving the first supervisor at Durham. Seed samples representing both the native range (Turkey and Spain) and the neo-allopatric range (Argentina, Chile, California, and Australia) were collected. Plants were grown under common garden conditions and intra- and inter-regional crosses were performed among plants from each region as well as to quantify reproductive isolation between regions. Genotyping by sequencing was performed to investigate population genetic structure. The next F1 generation of within and between region crossed individuals were then grown under common garden conditions to measure the trait inheritance and differentiation. More genotyping by sequencing data was collected and quantitative trait locus analysis to measure the genetic control of trait and reproductive isolation is underway. Further crosses were performed so that there are now a unique F2 generation resources of intra- and inter-regional crossed seed available for study that have taken years to develop. Studying the F2 generation will allow observation of more recombination and segregation between parental genomes originating in different regions leading to greater sensitivity and precision in determining the quantitative genetic architecture of reproductive isolation and invasive traits. With these unique resources, we will be able to study the worldwide geographic mosaic of local adaptation and reproductive isolation in this invasive species.

The genetic architecture of quantitative trait variation can give insights into its evolutionary origins; it will be critically important to assess the relative importance of chance genetic drift versus selection. For example, simple genetic control by few loci of major effect and acting in the same direction indicate strong recent selection while a shared genetic basis between traits indicate constraints on the independent evolution of those traits. The genetic basis of emerging RI might either be due to fixation by drift of genomic incompatibilities between regions or a pleiotropic by-product of selection in the form of associations with locally adapted traits. This aspect of the study will develop genetic markers associated with important invasive traits to allow monitoring of invasive threats for early and targeted management interventions as well as providing genetic tools for more detailed studies of the key genes involved.