AbstractA transition to selfing can be beneficial when mating partners are scarce, for example, due to ploidy changes or at species range edges. Here, we explain how self-compatibility evolved in diploid SiberianArabidopsis lyrata, and how it contributed to the establishment of allotetraploidArabidopsis kamchatica. First, we provide chromosome-level genome assemblies for two self-fertilizing diploidA. lyrataaccessions, one from North America and one from Siberia, including a fully assembledS-locus for the latter. We then propose a sequence of events leading to the loss of self-incompatibility in SiberianA. lyrata, date this independent transition to ∼90 Kya, and infer evolutionary relationships between Siberian and North AmericanA. lyrata, showing an independent transition to selfing in Siberia. Finally, we provide evidence that this selfing SiberianA. lyratalineage contributed to the formation of the allotetraploidA. kamchaticaand propose that the selfing of the latter is mediated by the loss-of-function mutation in a dominantS-allele inherited fromA. lyrata.