Relicts of the Mid-Miocene Climatic Optimum may contribute to the floristic diversity of Japan: a case study of Pinus mikii (Pinaceae) and its extant relatives.

The epidermis of Pinus mikii leaves was studied. Pinus mikii is a fossil species from the lower Miocene to lower Pleistocene of Japan. In P. mikii, the stomata are closely set and guard cells have polar extensions of cuticle on their inner cell walls. These features suggest that P. mikii is closely related to P. luchuensis, an extant species endemic to the Ryukyu Islands of Japan. Pinus mikii also shares some epidermal characters with P. thunbergii, which is semiendemic to Japan. It is possible that P. mikii is a common ancestor of both of these extant species. The distribution of P. mikii expanded during the Mid-Miocene Climatic Optimum (MMCO), but its distribution shifted southwards as global temperatures declined. Pinus luchuensis likely speciated from the retreating population, whereas P. thunbergii arose from a population that adapted to the cooler climate. This study provides a new perspective on the contribution of MMCO relicts to the floristic diversity of Japan.