How fast does the speciation clock tick in selfing versus outcrossing lineages?

Species are the most fundamental unit in nature, yet we know surprisingly little about how long it takes for new species to arise and what factors influence the rate of speciation (the ticking of the speciation clock). Recently, we found that new species may develop at astonishing speed in Arctic self-pollinating plants. This is important because many wild and cultivated plants are more or less self-fertilizing. In this project, we develop and test theoretical models of how self- and cross-fertilization can affect genetic architecture and rate of speciation. We work with a large set of different species. For each species, we cross different populations and examine the resulting hybrids for fertility. If the hybrids are fully or partly sterile, the parental populations are on their way to evolve into new species. We use genomic analyses to estimate parental divergence time and rate of self-fertilization. We also raise second-generation hybrids to enable more detailed genomic analyses. We use the African sky archipelago as the main study system, because the populations on these isolated high mountains represent a wide range of divergence times. We arranged extensive field expeditions to six of the highest African mountains, and the collected plants were cultivated at UiO. All crossing experiments and fertility analyses are finalized, and the genomic data are collected and partly analyzed (except for the second-generation hybrids, currently under sequencing). Delays caused by the corona epidemic made it necessary to merge the current datasets with some unpublished datasets from our previous RCN project. Preliminary analyses of this merged dataset suggest that self-fertilization speeds up the speciation process, in agreement with our predictions developed in the theoretical part of the project. The project has already resulted in a number of scientific and popular contributions, including several spin-off scientific papers on sky island evolution, speciation, and development of genomic resources (e.g. in PNAS, Molecular Ecology). One of the core theoretical papers is accepted in PLOS Genetics. We are confident that the project makes a significant contribution to our understanding of the speciation process, and it may also help to improve crop breeding. Many crops are self-pollinating and may quickly develop crossing barriers towards their wild relatives, from which we need to collect new genes to improve resistance against diseases and environmental change.

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Species are the most fundamental unit in nature, yet we know surprisingly little about how long it takes for new, reproductively isolated species to arise and what factors influence the rate of speciation. Recently, we found that postzygotic reproductive isolation (RI) may develop at astonishing speed in lineages that are self-fertilizing, a common mating system both in wild and cultivated plants and also found in animals. We hypothesize that although antagonistic effects can be expected, selfing may overall accelerate postzygotic RI accumulation rates. In this project, we will develop and empirically test theoretical models of the impact of mating systems on the genetic architecture and rate of speciation (i.e. the ticking of the 'speciation clock'). We will 1) establish a theoretical framework to understand and predict the effects of mating system, 2) measure the rate of intraspecific postzygotic RI accumulation (i.e. incipient speciation) in a large set of species representing the selfing-outcrossing spectrum and divergence times spanning the last ~1 million years, 3) test if the rate at which RI loci accumulate is higher in selfers, and 4) quantify the role that selection has played on RI loci using population genomic analyses in one selfing and one outcrossing species. We have selected the African 'sky archipelago' as a study system because the populations in these isolated high mountains represent a wide range of divergence times and levels of intermountain gene flow. We use this optimal system to study the rate and mode of plant speciation with a unique combination of novel genomic technologies, niche modeling through time, and classical field and crossing experiments. This project will make a significant contribution to our understanding of the process and speed of biodiversity generation, which has been a core issue in biology since Darwin and is especially relevant today, when we see accelerating extinction of species due to global change.