Periodic Reporting for period 4 - BARRIERS (The evolution of barriers to gene exchange)
Reporting period: 2021-03-01 to 2022-02-28
The focus of this project is on the accumulation of barriers to gene exchange and the processes underlying increasing reproductive isolation between populations that have gone part of the way to being new species. I use the power of natural contact zones, where these populations hybridize, combined with novel manipulative experiments and modern genetic techniques, to separate the processes that underlie differentiation between populations and the barrier effects of differentiated traits. The model system is a common coastal snail, Littorina saxatilis. In many places around Europe, this snail forms distinct populations adapted to different parts of the shore environment, with areas of hybridization where environments meet. Our objective is to understand the traits that reduce interbreeding and lower fitness of hybrids, the genetic basis of the traits and the way they combine to limit gene exchange. We then model the underlying processes to predict the circumstances in which populations are most likely to evolve complete reproductive isolation and so become new species.
Our project has advanced understanding of the speciation process. We have shown that it is nearer to completion in Spanish populations than in Swedish populations, probably because the contact there is older and more extensive. A particular form of genetic variant, chromosomal inversions, have been shown to be critical to the progress of adaptation and speciation through their effects of traits that fit organisms to their environment and also impact on mating behaviour.
We have also studied mate choice, both directly by observing mating behaviour and indirectly through collecting offspring for genotyping. Combined with modelling results, this has provided novel insights into the way sexual selection can enhance barriers to gene flow, even when it operates in the same direction in both diverging populations. We have conducted a series of experiments aimed at understanding habitat choice. We have also analysed embryo abortion, a possible form of incompatibility that might also contribute to reducing gene exchange. Modelling of the system is continuing, now focusing on the role of chromosomal rearrangements.
These results have been published in 20 scientific articles to date and an animated video explaining our results to a wider audience is freely available to the public and for educational purposes (https://www.youtube.com/watch?v=XDAuQQzQuWg&t=1s).
Our work on assortative mating and sexual selection adds a new dimension to the role of sexual selection in speciation and introduced novel approaches to documenting mating patterns and assessing their impact on gene exchange.
We are also finding surprising evidence for very strong barriers to gene flow in the Spanish populations but need further work to understand this properly.
We have generated a new method for deriving shell growth parameters from two-dimensional images that will be useful to many other groups as well as giving us new insight into the basis of shape divergence in our system.
We have discovered a surprising association between local adaptation, sex-specific divergence and sex determination in Swedish snails. This promises to provide new insight into sexually-antagonistic selection during divergence and speciation.