Abstract
Most ‘Sohayaki elements’ have been considered to be relict species that originated from ancestors in eastern to southwestern China during the Tertiary Period. Impatiens hypophylla Makino (Balsaminaceae), an endemic Japanese species, is considered to be a representative Sohayaki element. To assess the evolutionary history of the Sohayaki elements, we determined the phylogenetic position of I. hypophylla and estimated its divergence time as well as its ancestral distribution. Three species of Impatiens (I. hypophylla, I. textorii Miq., and I. ohwadae M. Watan. et Seriz.) in Japan form a clade (hereafter called ‘Japanese Impatiens’), which is sister to I. tienmushanica Y. L. Chen, a species endemic to eastern China. Japanese Impatiens and eastern Chinese species likely diverged approximately 5.68 million years ago (mya) (atpB–rbcL) and 7.37 mya (ITS), followed by further divergence of Japanese Impatiens (I. hypophylla, I. textorii, and I. ohwadae) at approximately 3.96 mya (atpB–rbcL) and 5.27 mya (ITS). An estimation of ancestral distribution using S-DIVA revealed that the common ancestor of Japanese Impatiens and the eastern Chinese species occurred in eastern China and the Japanese islands, with the most recent common ancestor of Japanese Impatiens being in the Japanese islands. As a consequence, the common ancestor of Japanese Impatiens and the eastern Chinese species would have widely expanded their distribution in eastern China and the Japanese islands through the East China Sea region in the Tertiary Period. The ancestral range was probably divided into Chinese and Japanese populations due to a vicariance event between mainland China and the Japanese islands. Thereafter, I. hypophylla and other Japanese Impatiens differentiated in the Japanese islands.
