Inter- and intra-island divergence in Odorrana ishikawae (Anura, Ranidae) of the Ryukyu Archipelago of Japan, with description of a new species

Abstract

The endangered frog, Odorrana ishikawae (Anura, Ranidae), is a species endemic to the Amami and Okinawa Islands of the Ryukyu Archipelago, Japan. Segmentation of these islands has been considered to occur middle or upper Pleistocene. Our morphometric analyses revealed obvious differences between the Amami and Okinawa populations. Two distinct morphotypes were also recognized from the Amami Island (Amami common and Amami large types). Furthermore, the Amami and Okinawa populations could be distinguished clearly by coloration and dorsal tuberculation. Based on 16S rRNA gene data, the Okinawa and Amami populations were phylogenetically separated but the genetic divergence (1.44– 2.16%) was lower than the value suggested as species threshold in anurans (> 3% in 16S). Individuals of the Amami common and large types were nested within a single clade. Artificial hybridization experiments revealed normal hybrid viability between the two Amami types, with one exception. By contrast, between Okinawa females and two Amami type males, complete hybrid inviability was observed at early embryonic stages in the hybrids contrary to expectations from their low divergence in 16S. The reciprocal hybrids between two Amami type females and Okinawa males were viable, but spermatogenesis in the hybrid males showed some degree of abnormality. These results strongly indicate specific separation of the Amami population from the Okinawa population of O. ishikawae. Thus, we describe the Amami population as a new species, which is readily distinguishable from O. ishikawae by smaller ruggedly edged dorsal spots and an immaculate ventral surface.