Format: ORAL

Sebastian Mortimer and Aaron Liston

Oregon State University, Oregon, USA

Allopolyploidy is widespread among plant lineages and is an important driver of speciation. Understanding how allopolyploid species have formed through reticulate evolution is a challenging task. How can we infer the ancestry of extant allopolyploids when many ancestral diploids and lower-level polyploid ancestors are extinct? Moreover, genomic signal is often eroded in the allopolyploid genome through homeologous exchange. Using a suite of genomic tools, we developed a pipeline for identifying the progenitor lineages of high-level allopolyploids (6x and above). We test our pipeline on Fragaria cascadensis, a recently described allo-decaploid which is narrowly endemic to a single mountain range in western North America. The species has acquired five subgenomes through sequential interspecific hybridization events likely taking place in Beringia in the last 1 million years. Using whole genome alignments between extant diploid and polyploid strawberry species, a phylogenetic hypothesis testing framework, and k-mer based inferences of transposon activity, we reconstruct the allopolyploid ancestors and identify putative diploid progenitors of the decaploid strawberry F. cascadensis. Our new genomic evidence suggests a revision of the current ancestry hypothesis. We characterize the extent of asymmetric subgenome evolution and compare the decaploid strawberry to closely related octoploid species. Finally, we consider the implications of a new allo-decaploid hypothesis on the biogeography, systematics, and evolution of mating systems in polyploid Fragaria. We anticipate that our pipeline for reconstructing the origin of polyploids will be informative in other lineages with complex allopolyploids.