Genetic and demographic signatures accompanying the evolution of the selfing syndrome in Daphne kiusiana, an evergreen shrub.

BACKGROUND AND AIMS: The evolution of mating systems from outcrossing to self-fertilization is a common transition in flowering plants. This shift is often associated with "selfing syndrome", which is characterized by less visible flowers with functional changes to control outcrossing. In most cases, the evolutionary history and demographic dynamics underlying the evolution of the selfing syndrome remain poorly understood.

METHODS: Here, we characterize differences in the demographic genetic consequences and associated floral-specific traits between two distinct geographical groups of wild shrub Daphne kiusiana, endemic to East Asia; plants in the eastern region (southeastern Korea and Kyushu, Japan) exhibit smaller, fewer flowers compared to those of plants in the western region (southwestern Korea). Genetic analyses were conducted using nuclear microsatellites and chloroplast DNA (Multiplexed Phylogenetic Marker sequencing: MPM-seq) datasets.

KEY RESULTS: A high selfing rate with significantly exceeded homozygosity characterized the eastern lineage, associated with lower levels of visibility and herkogamy in the floral traits. The two lineages harbored independent phylogeographic histories. In contrast to the western lineage, the eastern lineage showed a gradual reduction in the effective population size with no signs of a severe bottleneck despite its extreme range contraction during the last glacial period.

CONCLUSIONS: Our results suggest that the selfing-associated morphological changes in D. kiusiana are of relatively old origin (at least 100 Kya) and were driven by directional selection for efficient self-pollination. We provide evidence that the evolution of selfing syndrome in D. kiusiana is not strongly associated with a severe population bottleneck.