High speciation rates in Arctic plants: genomic mechanisms and relevance to the latitudinal diversity gradient

Decreasing species diversity from the Tropics to the Arctic is a major pattern in ecology, but typically based on species we easily can recognize by their external characteristics (morphology). However, in previous work we found that the supposedly species-poor Arctic flora contains numerous ?cryptic? species as defined by reproductive isolation (i.e. using the biological species concept). Thus, within what we have recognized as single, well-defined arctic plant species, we find that every local population as a rule represent a distinct, reproductively isolated species, although it is not possible to distinguish based on morphology. In this project, we aimed to identify the genomic mechanisms responsible for rapid speciation in arctic plants. We have obtained high-quality whole-genome sequences of two arctic species (as traditionally defined by morphology). These are the first arctic plant species to be fully genome-sequenced, and we identified a large number of genes that can be associated with adaptation to the harsh Arctic environment. We have used these genomes to construct detailed ?genetic maps? based on more or less sterile hybrids between populations. We are now finishing work to identify genes and mechanisms causing sterility. We also test whether similar ?cryptic? species occur further to the south, but have been overlooked. Our data from the Arctic suggest that this kind of speciation may be associated with inbreeding via self-pollination, which is common in the insect-poor Arctic environment, but also characteristic of many southern species. To test this we collected populations of both self-pollinating and cross-pollinating species from Greece and Spain, planted them in a phytotron, and crossed the Greek and Spanish populations using hand-pollination. The resulting seeds were sown and the adult hybrids tested for degree of fertility. We found reduced fertility in six of the totally 15 species for which the crosses were successful. We currently use population genomic analyses of the parental populations to assess to what degree reduced fertility is associated with self/cross-pollination and with how long time the populations have been separated. The project provides new insights into the dynamics of the latitudinal diversity gradient and the fundamental question of how species arise. The project is also highly relevant for cultivated crops: many of these are self-pollinating, and it is important to know how and how fast they develop crossing barriers towards their wild relatives. To cross crop plants with their wild relatives is a well-known method to improve their resistance against diseases and environmental change.

Dette prosjektet har resultert i svært gode og spennende resultater som er godt underveis mot høynivå-publisering. Det har medført et stort løft for vårt forskningsmiljø i Oslo, især innen genom-analyser, bioinformatikk og artsdannelsesgenetikk. Flere yngre medarbeidere har utviklet videre kvalifisering for akademiske stillinger. Prosjektet har medført betydelig input fra utenlandske toppmiljøer til vårt miljø. Prosjektet bringer ny innsikt i gradienten i artsrikdom fra sør til nord, og bidrar til å besvare et av de mest grunnleggende spørsmål i biologien: hvordan oppstår arter? I tillegg har prosjektet klar relevans for våre dyrkete matplanter: mange av disse er selv-pollinerende, og det er viktig å vite hvor fort og hvordan disse utvikler krysningsbarrierer mot sine ville slektninger. Å krysse matplanter tilbake til sine ville slektninger er en velkjent metode for å forbedre motstandsevnen mot plantesykdommer og miljøendringer.

Decreasing species diversity from low to high latitudes is a major pattern in ecology, but the extent to which this pattern depends on morphologically rather than biologically defined species is unclear. We have recently shown that the supposedly species-poor Arctic flora displays exceptionally high rates of formation of cryptic biological (i.e., reproductively isolated) species, possibly associated with a selfing mating system and based on multiple genetic mechanisms. In this project, we will identify the genomic mechanisms of postzygotic reproductive isolation in two Arctic systems, and test whether more southern floras contain similarly high, but hitherto undetected, cryptic biological species diversity. In particular, based on crossing experiments and high-throughput sequencing we will address whether selfers contain more cryptic species than outcrossers when controlled for lineage age. The project will provide new and potentially ground-breaking insights into both the dynamics of the latitudinal diversity gradient and the fundamental process of species divergence. Furthermore, the results will facilitate a re-evaluation of how conservation assessments and evolutionary studies are impacted by the widespread use of non-biological species concepts. The international project team will consist of one postdoc and one technician funded by RCN and one postdoc funded by NHM Oslo, in addition to leading experts on speciation genetics, mating system evolution, polyploid speciation, genome sequencing, bioinformatics, flow cytometry, experimental plant culturing and crossing, morphometrics and plant systematics.