Map showing the location of the Ryukyu Archipelago and Taiwan 

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... Island of the southern Ryukyus and northward, while I. repens occurs on all islands except for Iriomotejima and Yonagunijima Islands. I. nakazonei , comprises six polyploid cytotypes, 3 x , 4 x , 5 x , 6 x , 7 x and 8 x , based on x = 8. Four cytotypes from 3 x to 6 x occur in the central Ryukyus, while four cytotypes from 5 x to 8 x occur in the southern Ryukyus. The higher polyploids of I. nakazonei tend to be distributed in the more southerly area. Tetraploids of I. nakazonei always co-occur with I. debilis and I. repens , supporting the hybrid origin of this cytotype. Considering the chromosome number, octoploids, which predominate in the southern Ryukyus and Taiwan, may have derived directly from hybridization between I. debilis and I. repens . Odd- numbered polyploids of I. nakazonei , 3 x , 5 x and 7 x , are relatively rare. Their chromosome numbers indicate that triploids and heptaploids are hybrids between the tetraploid of I. nakazonei and I. repens , and between the octoploid of I. nakazonei and I. debilis , respectively. Pentaploids of I. nakazonei in the central and southern Ryukyus are, respectively, hybrids between the tetraploid of I. nakazonei and I. debilis and between the octoploid of I. nakazonei and I. repens , indicating that pentaploids of I. nakazonei have at least two independent origins. Key words Cytogeography · Hybridization · Ixeris nakazonei · Polyploid complex · Reticulate evolution · Ryukyu Archipelago The Ryukyu Archipelago comprises approximately 200 subtropical islands spanning 1300 km between Taiwan and Kyushu Island of Japan. More than 1600 vascular plants, including a number of endemics or taxa of phytogeographical interest, are distributed in this area (Hatusima 1975; Hatusima and Amano 1994). This marvelous diversity of plants in the current Ryukyu Archipelago must have been established due to the influence of the paleogeographical dynamics of this area. In turn, these dynamics very probably enhanced colonization and migration through land bridge connections and vicariance through insular isolations in many plant taxa. In addition, such speciation pro- cesses as interspecific hybridization, introgression, and polyploidization appear to play an important role in creat- ing and maintaining plant diversity in the Ryukyus. An example of interspecific hybridization has been reported from Ixeris debilis A. Gray and I. repens (L.) A. Gray in the Ryukyus (Denda and Yokota 1999, 2003). I. debilis is a stoloniferous perennial herb that is widely distributed from Japan and Korea, through Taiwan, to southern China (Kitamura 1956; Pak and Kawano 1992). Although it usually prefers to grow in inland habitats, I. debilis is a common weed on the coastal dunes in the southern portion of its distribution range, including the Ryukyu Archipelago and Taiwan (Hatusima 1975; Peng et al. 1998). I. debilis in this habitat of the Ryukyu Archipelago often co-occurs and crosses with I. repens , a typical maritime plant that is widely distributed from Kamchatka, through Japan and Taiwan, into China (Kitamura 1956; Pak and Kawano 1992). Because of such morphological char- acteristics as palmate radical leaves and large imbricate outer involucral bracts, I. repens is clearly distinguished from I. debilis , which has spathulate radical leaves and small calyculate outer involucral bracts. I. repens is also distinguished from I. debilis based on the chromosome number; that is, the former is diploid with 2 n = 16 and the latter is hexaploid with 2 n = 48 (e.g., Pak and Kawano 1990). One putative hybrid between I. debilis and I. repens has been described as I. nakazonei (Kitam.) Kitam. (Kitamura 1942). I. nakazonei is an endemic to the Ryukyu Archipelago (Hatusima 1975), but is locally found in the Atsumi Peninsula of central Japan (Denda and Yokota 2000). This species is characterized by irregularly pinnatifid leaves. However, various degrees of morphological intermediacy are found between I. nakazonei and I. debilis . Denda and Yokota (1999) reported the chromosome numbers 2 n = 4 x = 32, 5 x = 40 and 6 x = 48 for I. nakazonei . Based on the molecular data and chromosome number intermediacy (Denda and Yokota 1999, 2003), the tetraploid of I. nakazonei is a hybrid between I. debilis and I. repens . Molecular data further suggest that I. debilis and I. repens are also involved in the origin of the hexaploid of I. nakazonei , although the establishment process for this cytotype is not clear (Denda and Yokota 2003). Because the hexaploid of I. nakazonei is cytologically similar to I. debilis (Denda and Yokota 1999), they can easily cross with each other. This may make the circumscription of species between I. nakazonei and I. debilis rather obscure at the hexaploid level. This study aims to elucidate the cytogeographical structure of I. nakazonei in the Ryukyu Archipelago and Taiwan. The origins of various polyploid cytotypes are also discussed based on the detailed chromosome counts. As shown in Fig. 1, the Ryukyu Archipelago is often subdi- vided into five areas from both the paleogeographical and phytogeographical points of view (Hatusima 1975). These are: the northern, central, and southern Ryukyus, the Daito Islands and the Senkaku Islands. In the present study, plant materials were collected mainly from three of these five areas (the northern, central and southern Ryukyus), and partly from Taiwan. Because of many intermediate forms, we could not dis- tinguish between Ixeris nakazonei and I. debilis at the time of sampling in the fields. Thus, these two species were treated together until their chromosome numbers were investigated. In total, 856 plants of I. nakazonei and/or I. debilis were collected from five localities on two islands of the northern Ryukyus (Yakushima and Tanegashima Islands), from 39 localities on eight islands of the central Ryukyus (Amamioshima, Tokunoshima, Okinoerabujima, Okinawajima, Iejima, Ikeijima, Hamahigajima and Kumejima Islands), from 83 localities on eight islands of the southern Ryukyus (Ikemajima, Miyakojima, Kurimajima, Irabujima, Taramajima, Ishigakijima, Iriomotejima and Yonagunijima Islands), and from two localities in Taiwan (Appendix). Hereafter, Okinawajima Island and its sur- rounding islets (Iejima, Ikeijima and Hamahigajima Islands) are referred to as the Okinawa Islands. Likewise, Miyakojima Island and its surroundings (Ikemajima, Kurimajima and Irabujima Islands) are referred to as the Miyako Islands. The 127 collection sites in the Ryukyu Archipelago are coastal dunes, except for the following ten localities: one (TAN-3) on Tanegashima Island, four (MIY-11, -15, -17 and -24) on the Miyako Islands, two (ISH-13 and -14) on Ishigakijima Island, one (IRI-3) on Iriomotejima Island, and two (YON-2 and -6) on Yonagunijima Island. Six of these ten collection localities, TAN-3, MIY-11, MIY-17, ISH-13, IRI-3 and YON-2, were beside sugar cane or paddy fields, and MIY-15 and YON-7 were on exposed grassland on top of limestone sea cliffs. Two other localities, MIY-24 and ISH-14, were under roadside trees near the coast. The distribution of I. repens was also surveyed at 114 of 117 coastal dune localities in the Ryukyu Archipelago. The voucher specimens were deposited in the herbarium of the Faculty of Science, University of the Ryukyus ( RYU ). The collected plants were transplanted to a greenhouse at the University of the Ryukyus for cytological investiga- tion. Root tips were obtained from cultivated plants, and mitotic chromosomes were observed using the aceto-orcein squash method. The gross morphology of leaves represen- tative of major polyploid cytotypes collected from Okinawajima, Miyakojima and Ishigakijima Islands was also investigated. Six different chromosome numbers, 2 n = 24, 32, 40, 48, 56 and 64, were found in 856 plants collected from the Ryukyu Archipelago and Taiwan (Fig. 2 and Appendix). They are 3 x , 4 x , 5 x , 6 x , 7 x and 8 x based on the basic chromosome number of x = 8, respectively. Even-numbered polyploids, 4 x , 6 x and 8 x , all showed conspicuous variation in leaf morphology. These variations included spathulate to oblong leaves with entirely or sparsely dentate margins, pinnatisect to irregularly pinnatifid leaves, and tripartite to trilobed leaves (Fig. 3). Similar variations were observed in odd- numbered polyploids, 3 x , 5 x and 7 x (data not shown). Because of these variations, it was very difficult to distin- guish some hexaploids of Ixeris nakazonei from I. debilis (6 x ) based on leaf morphology. Thus, for convenience, we will hereafter treat hexaploids with entire to pinnatisect leaves as I. debilis and treat those with irregularly pinnatifid to tripartite and trilobed leaves as I. nakazonei . Other polyploids, 3 x , 4 x , 5 x , 7 x and 8 x , will be treated as I. nakazonei irrespectively of their leaf morphology or distribution. The frequencies of the polyploid cytotypes of Ixeris nakazonei and I. debilis at each locality in the northern and central Ryukyus are shown in Fig. 4. Previous chromosome counts on the Okinawa Islands (Denda and Yokota 1999) are pooled and displayed in the same figure. Only I. debilis was found with or without I. repens from five collection localities in the northern Ryukyus. In the central Ryukyus, intra-populational variations in ploidy level of I. nakazonei were observed in 14 of the 39 collection localities: (a) four cytotypes, 3 x –6 x , occurred at two collection localities on Amamioshima Island, (b) three cytotypes, 4 x –6 x , occurred at four collection localities on Okinoerabujima Island and the Okinawa Islands, and (c) tetraploids and hexaploids occurred at eight collection localities on Amamioshima Island, Tokunoshima Island, Okinoerabujima Island, the Okinawa Islands and Kumejima Island. I. debilis and I. repens always co-occurred with I. nakazonei in all of these 14 collection localities. Finally, I. debilis solely occurred or co-occurred with hexaploids of I. nakazonei in ...