Genomics of speciation: dissecting mechanisms of reproductive barriers in fungi

Soppriket er en av de mest artsrike organismegruppene, men det vites lite om hvordan sopparter oppstår. For å forstå hvordan sopp bestemmer hvem de skal krysse seg med, hvordan arter består og nye arter oppstår har vi analysert reproduktive barrierer i soppslekten fiolkjuke (Trichaptum, Hymenochaetales, Basidiomycota). Fiolkjuke er en utbredt soppslekt som bryter ned døde bartrær i temperert og boreal sone. Vi har samlet fiolkjuker fra hele den nordlige hemisfære, laget kulturer og krysset de på lab. Videre har vi har så isolert DNA og sekvensert noen komplette genomer, og resekvensert over 400 individer. Med disse dataene har vi vist at: 1) fiolkjuke og tannet fiolkjuke ikke hybridiserer i dag (selv om de av og til kan være vanskelig å skille morfologisk), men at de har en komplisert historie med genflyt. 2) Kjønnsgener kontrollerer hvem som kan parre seg med hverandre. Vi har vist at stor diversitet opprettholdes med mekanismen balanserende seleksjon. 3) Vi har funnet flere genetiske grupper i dette datasetet som bare delvis passer med reproduktive barrierer. Dette indikerer at det ikke bare er slektskap som bestemmer reproduktive barrierer, men at barrierer kan oppstå hurtig når det er lite gunstig å krysse seg. Vi jobber nå videre med de komplette genomene for å forstå hvordan genomisk reorganisering, transposoner og kopitallsvariasjon som kan ha påvirket hvordan reproduktive barrierer har oppstått raskt. 4) I Europa har vi ikke observert reproduktive barrierer, men det er tydelig genetisk struktur som passer med ulike istidsrefugier. I Alpene har disse grupperingene møttes og hybridisert. Vi har også observert to ulike populasjoner i Norge, noe som kan indikere to ulike invandringsveier inn til Norge etter istidene. I løpet av prosjektet har vi utviklet metoder for å gjennomføre full livssyklus på lab. Det gjør det mulig å undersøke videre når i livssyklusen disse barrierene oppstår og hvordan den unike livssyklusen endrer potensialet for å skape diversitet i denne organismegruppen.

The project has been successful in that both the PI and the hired post doc on the project have got permanent scientific positions during the project period. Getting this funding, and the publications from the project supported these hirings and ensured a continued carriers for these researchers. A PhD student that has worked on this project, partially funded by the University of Oslo, will also soon deliver his thesis. Further, two master students have finished their thesis connected to the project, where both have continued as PhD students. Thus, the project has provided staff that will continue to use its knowledge on evolutionary genomics for the future. The project has provided and will continue to provide information about how species evolve, and lay the ground for more research in this field. Fungi are vital players in the carbon cycle by recycling plant carbon through decomposition, supporting plant growth through mycorrhiza and as key contributors to build up of soil organic carbon. Fungi are one of the most diverse eukaryotic groups on earth, still we have limited knowledge about the underlying speciation processes leading to the immense diversity, and to what degree fungal species are able to adapt to the changing climate. We now know a bit more about how species barriers form, but continued research is important to increasing the understanding of these processes.

What we know about speciation mechanisms, the processes that have generated the diversity of species on earth, are largely derived from studies of animals and plants. However, eukaryote organisms have a diversity of life histories and life cycles, and may speciate by a diversity of mechanisms. Basidiomycetes, the second largest fungal phylum, have a unique life cycle with a dominant dikaryotic phase where each cell possesses two haploid nuclei obtained from each parent mycelia or spore. To obtain knowledge on how species emerge in this phylum, we will analyse the evolution of pre- and postzygotic reproductive barriers in the genus Trichaptum (Polyporales, Basidiomycota) and determine where the break-down of the life cycle occurs. In the species T. abietinum two inter-sterile populations are reported in sympatry in North America, both being partially fertile to a European population. We hypothesize that reinforcement has been involved in creating the reproductive barriers in North America. The European population possesses two divergent but admixed genetic types and we hypothesize that this is due to a historic reverse speciation event where two divergent lineages have fused. In addition, hybrids are found between T. abietinum and its sister species T. fuscoviolaceum, but the level of introgression is unknown. We will produce high quality genome assemblies of the different populations of T. abietinum and its sister species T. fuscoviolaceum, using state of the art sequencing technology, to investigate genomic barriers to reproduction, e.g. genomic rearrangements, transposable elements, and gene loss and gain. We will also sequence populations of each group to investigate the divergence rate and time, selection pressure, and signals of demographic events. Lastly, we will investigate at what step in the life cycle reproductive barriers evolves, and how the unique dikaryotic stage alters the evolution of reproductive barriers compared to other organisms.