Dwarfism of Ficus microcarpa in the Ryukyu islands, Okinawa, Japan

Abstract

Ficus microcarpa, belonging to Moraceae, is an evergreen tree that can grow on tropical or subtropical rocky coasts. Recently, dwarf F. microcarpa individuals have been found on Nakanokamishima Island, Okinawa, Japan, but it remains unclear whether this dwarf trait is based on environmental plasticity or represents an intermediate stage of ecological speciation. To clarify the morphological and genetic traits of dwarfism and consider the process of ecological divergence, we conducted a common garden experiment and performed population genetic and structure analyses using 11 microsatellite markers. Moreover, we conducted a coalescent analysis to estimate the demographic parameters of two genetic clusters: dwarf and tree groups. Common garden experiments clearly classified the samples into two groups: dwarf and tree. In the STRUCTURE analysis, the highest ΔK value was obtained when K = 2, suggesting the existence of two genetic clusters: A and B. All samples collected on Nakanokamishima Island were classified into cluster B. Most samples from the other islands were classified into cluster A. Additionally, it was found that dwarf and tree lineages had diverged from an ancestral population hundreds or thousands of years ago. These results indicate that the dwarfism of F. microcarpa can be considered an ecotype defined as the intermediate stage of ecological speciation, and that dwarf individuals diversified very recently from an ancestral population with the existence of gene flow.