Pollination and reproduction of Psychotria homalosperma,an endangered distylous tree endemic to the oceanic Bonin (Ogasawara) Islands,Japan

Distyly is usually rare or not observed in species thriving on oceanic islands. The rarity of this breeding system is probably because of the difficulty of colonization for distylous plants and the paucity of pollinators on oceanic islands. However, the endemic and endangered tree Psychotria homalosperma has maintained its distylous nature in the oceanic Bonin Islands, Japan. To understand how the distylous breeding system of P. homalosperma has been maintained on these islands and to characterize the reproduction systems, we studied the pollination and reproductive biology of this species. Specifically, we observed current flower visitors and estimated their effects on plant reproduction. We also examined the floral traits and floral volatiles of P. homalosperma to infer its original pollinators, because plant–pollinator relationships in the Bonin Islands have recently been disrupted by anthropological activities. Finally, we examined the fruit set and pollen tube growth in the stigmas under hand and open pollination. Although original pollinators were presumed to be moths with long proboscises, the introduced honeybee, Apis mellifera, was the most common flower visitor. The honeybee carried pollen grains only unidirectionally, from the short‐ to long‐styled morphs, because it could not reach the hidden stigmas of the short‐styled flowers, and long‐styled flowers set fruits 1.7–38 times more than short‐styled ones. This case study indicates that the instability of pollinator fauna can cause distylous species to be rare on oceanic islands.     

Origin and evolution of endemic plants of the Bonin (Ogasawara) Islands

The Bonin Islands are typical oceanic islands, located at the western part of the North Pacific Ocean and approximately 1,000 km south of mainland Japan. This archipelago consists of about 20 small islands. Although floristic diversity is low due to the small area and limited environmental diversity, the Bonin Islands harbor unique endemic flora as in other well-known oceanic islands. This paper presents a brief summary of the results obtained from recent studies on the endemic flora of the Bonin Islands. The results are reviewed in relation to the four stages of the evolution of endemic flora in the oceanic islands; migration, establishment, enlargement and diversification. The ancestors of the flora originated mostly from tropical and subtropical Southeast Asia or mainland Japan by rare events of long distance dispersal. The proportion of bird-dispersed species is relatively high as for other oceanic islands. Genetic data sets obtained from allozyme variation in some endemic species suggest that migration occurred several million years ago and genetic diversity is correlated with current population size. At the time of establishment, self-compatible plants are expected to have an advantage. However, the percentage of dioecious plants is relatively high. This is partly due to evolutionary changes from hermaphroditic ancestors to dioecy which occurred in two genera in the Bonin Islands. In addition, there are some examples of evolutionary changes from herbaceous ancestors to woody endemics. Adaptive radiation is found in some genera, although the number of congeneric endemic species is less than five. Studies of allozyme variation inPittosporum, Symplocos andCrepidiastrum showed that genetic identity is generally very high between congeneric species in spite of their distinct morphologies. This result suggests that divergence of these species occurred rather recently and distinct morphological differences are based on a limited number of genetic changes.     

Distyly and incompatibility in Psychotria homalosperma (Rubiaceae), an endemic plant of the oceanic Bonin (Ogasawara) Islands

Distyly rarely occurs on oceanic islands. Indeed, this form of polymorphism is absent on the Hawaiian Islands and in New Zealand. However, Psychotria homalosperma (Rubiaceae), which is endemic to the oceanic Bonin Islands of Japan, was presumed to maintain a distylous breeding system and floral morphology. We investigated the floral and reproductive characteristics of this species and examined incompatibility in self- and intramorph pollination. Psychotria homalosperma is morphologically and functionally distylous, based on findings with the examined populations, but its stigma and anther heights are not exactly reciprocal between the two morphs. This is the second example of distyly among the flora from the Bonin Islands, the other being the endemic P. boninensis. Both of these distylous species contrast with the Hawaiian Psychotria species, which have already become dioecious. The flowers of the long (L)-styled (pin) morph of the Bonin Islands plants are completely self- and intramorph incompatible, while the flowers of the short (S)-styled (thrum) morphs are partially intramorph compatible. The S-styled morph was more abundant in the examined populations from three islands of the archipelago, probably due to the existence of partial S–S intramorph compatibility.     

Allozyme diversity and the evolution ofSymplocos (Symplocaceae) on the Bonin (Ogasawara) Islands

To study the origin and speciation of plants in oceanic islands, electrophoretic analyses have been done on three endemic species ofSymplocos in the Bonin Islands as well as on three other species;S. kuroki, S. nakaharae andS. tanakae which are considered to be closely related to the Bonin endemics. There occur three species:S. kawakamii, S. pergracilis andS. boninensis in Bonin. The genusSymplocos is one which is considered to be diversified in the Bonin Islands. Seven enzyme systems presumed to be encoded by 18 loci were examined. The genetic diversity was low in the island species, as reported in some oceanic island plants of Hawaii and the Bonin Islands. The three endemics share high genetic identities and they clustered together in the tree drawn by the UPGMA method, suggesting that they are a monophyletic group, that is, they result from a single introduction.     

Dioecy in the endemic genusDendrocacalia (Compositae) on the Bonin (Ogasawara) Islands

Dendrocacalia crepidifolia Nakai (Compositae, Senecioneae), the only species of this arboreal genus endemic to Haha Island in the Bonin Islands, was found to be dioecious. Male flowers differ from female ones in having a stunted style (style in female exserted from corolla and deeply bifurcating) and anthers filled with fertile pollen (anthers in the female lacking pollen). The size of the corolla and number of florets per head were similar between male and female flowers. The crown area of this arboreal species was also similar in male and female plants. The sex ratio was 0.55 male, not significantly different from 0.5. Both sexes produced nectar of similar sugar concentration (ca. 50%). The flowers are pollinated by feral honeybees (Apis mellifera), but they are thought to have been pollinated by small, lesshairy, endemic solitary bees before honeybees were introduced and subsequently became the dominant bee species on the island. The evolution of dioecy ofDendrocacalia on the island is thought to stem from the deleterious effects of inbreeding that are inherent in plants with geitonogamy. The increased geitonogamy on the island has resulted from increased woodiness (i.e., increased number of flowers per plant) and the original dependence on endemic bee pollinators, which are now endangered.     

Origin and diversification of Hibiscus glaber, species endemic to the oceanic Bonin Islands, revealed by chloroplast DNA polymorphism

Abstract Two woody Hibiscus species co-occur in the Bonin Islands of the northwestern Pacific Ocean: Hibiscus glaber Matsum. is endemic to the islands, and its putative ancestral species, Hibiscus tiliaceus L., is widely distributed in coastal areas of the tropics and subtropics. To infer isolating mechanisms that led to speciation of H. glaber and the processes that resulted in co-occurrence of the two closely related species on the Bonin Islands, we conducted molecular phylogenetic analyses on chloroplast DNA (cpDNA) sequences. Materials collected from a wide area of the Pacific and Indian Oceans were used, and two closely related species, Hibiscus hamabo Siebold Zucc. and Hibiscus macrophyllus Roxb., were also included in the analyses. The constructed tree suggested that H. glaber has been derived from H. tiliaceus, and that most of the modern Bonin populations of H. tiliaceus did not share most recent ancestry with H. glaber. Geographic isolation appears to be the most important mechanism in the speciation of H. glaber. The co-occurrence of the two species can be attributed to multiple migrations of different lineages into the islands. While a wide and overlapping geographical distribution of haplotypes was found in H. tiliaceus, localized geographical distribution of haplotypes was detected in H. glaber. It is hypothesized that a shift to inland habitats may have affected the mode of seed dispersal from ocean currents to gravity and hence resulted in geographical structuring of H. glaber haplotypes.     

Allozyme diversity ofPittosporum (Pittosporaceae) on the Bonin (Ogasawara) Islands

The genusPittosporum includes about 160 species. Four species ofPittosporum occur in the Bonin Islands, and all of these are endemic to the islands. Electrophoretic studies of the four endemic species,P. tobira, from the Japanese mainland, andP. lutchuense var.denudatum from the Ryukyu Islands, were used to determine the origin and speciation pattern of the endemic species. 259 individuals were sampled from ten populations. Twenty loci in nine enzyme systems were resolved and used to calculate the gene frequencies for each population. A low genetic diversity was observed in three of the Bonin Island species, as is reported for other oceanic island plants. The exception,P. boninense, has the largest population size and widest distribution. A dendrogram generated by the UPGMA method shows two clusters. One consists of only the Bonin endemics, suggesting a monophyletic origin for these species.     

Isolation and characterization of 10 microsatellite loci in Callicarpa subpubescens (Verbenaceae), an endemic species of the Bonin Islands

Ten microsatellite loci were isolated and characterized for Callicarpa subpubescens (Verbenaceae), an endemic tree species of the Bonin Islands. The observed number of alleles at each locus ranged from two to eight with an average of 4.9, and the expected heterozygosity ranged from 0.238 to 0.690 with an average of 0.483. All 10 loci were screened in cross-amplification tests for two other endemic Callicarpa species that also inhabit the Bonin Islands. All loci were successfully amplified in these species.     

Development of EST-SSR markers for Elaeocarpus photiniifolia (Elaeocarpaceae), an endemic taxon of the Bonin Islands

? Premise of the study: Expressed sequence tag (EST)-derived microsatellite markers were developed for Elaeocarpus photiniifolia, an endemic taxon of the Bonin Islands. ? Methods and Results: Initially, a complementary DNA (cDNA) library was constructed by de novo pyrosequencing of total RNA extracted from a seedling. A total of 267 primer pairs were designed from the library. Of the 48 tested loci, 25 loci were polymorphic among 41 individuals representing the entire geographical range of the species, with the number of alleles per locus and expected heterozygosity ranging from two to 14 and 0.09 to 0.86, respectively. Most loci were transferable to a related species, E. sylvestris. ? Conclusions: The developed markers will be useful for evaluating the genetic structure of E. photiniifolia.     

Morphological Variation of Three Endemic Species of Callicarpa (Verbenaceae) in the Bonin (Ogasawara) Islands

Abstract A total of 22 population samples of three endemic species of Callicarpa in the Bonin Islands (the Chichijima Islands and the Hahajima Islands) were examined for nine leaf characters to clarify the present status of morphological variations. Results showed that three species are distinetly recognizable and have their own patterns of variation. Callicarpa glabra and C. nishimurae were relatively less variable and adapted to limited habitats only in the Chichijima Islands. Thus these species were regarded as specialized species. C. subpubescens was more variable than the above two species and distributed widely in both of the two island groups. This species was morphologically differentiated between the two island groups. In the Hahajima Islands where two other species were not distributed, several plants of C. subpubsecens were similar to C. glabra. The populations of C. subpubescens in the Hahajima Islands were inter-populationally more variable than those in the Chichijima Islands. C. subpubescens occupied various habitats within the Bonin Islands and was regarded as a diversified species.     

Dioecism of the genusCallicarpa (Verbenaceae) in the Bonin (Ogasawara) Islands

The morphology and function of the flowers of three species of the genusCallicarpa, C. glabra, C. nisnimurae andC. subpubescens, endemic to the Bonin Islands, revealed that all three species were dioecious. Male plants had short-styled flowers with sterile ovaries, while female plants had long-styled flowers with non-germinating inaperturate pollen grains. Such dioecism has never been reported from the family Verbenaceae. This curious sexual system with non-germinating pollen grains may have been maintained as a reward to insect pollinators.     

Development of SCAR markers distinguishing pure seedlings of the endangered species Morus boninensis from M. boninensis × M. acidosa hybrids for conservation in Bonin (Ogasawara) Islands

To support the recovery of Morusboninensis Koidz. (Moraceae), a criticallyendangered species distributed in the BoninIslands of Japan, we need to distinguishbetween pure M. boninensis and hybrid(M. boninensis × M. acidosa Griff.)seedlings within populations derived from M. boninensis mother trees. Therefore, wedeveloped three co-dominant SCAR (SequenceCharacterized Amplified Region) markers todistinguish between M. boninensis, M. acidosa and their hybrids. All threemarkers could clearly identify the species andthe hybrids, although we observed one case of aputative null allele or a backcross at onelocus (Mos0251-1). Hence, we suggest that pureand hybrid seedlings could be unambiguouslyidentified by the simultaneous use of the threemarkers. When we investigated a seedlingpopulation produced by a single mother treegrowing in the Iguma-bay population ofHaha-jima Island, the proportion of embryosformed by pollination from M. acidosapollen was about 13%. All hybrid seedlingsidentified according to the molecular markersshowed incisions in their leaves, amorphological character that has never beenobserved in pure M. boninensisindividuals. Our results suggest that M.boninensis can potentially be multiply byexcluding hybrids from the seedling pools,using incisions on the leaves as morphologicalmarkers to identify them.     

Phylogenetic relationships ofCrepidiastrum (Asteraceae) of the Bonin (Ogasawara) Islands based on chloroplast DNA restriction site variation

RFLPs of cpDNA were investigated for seven species ofCrepidiastrum, of which three are endemic to the Bonin Islands. As an outgroup for rooting the tree,Paraixeris denticulata was also examined. Approximately 350 restriction sites were surveyed using 16 restriction enzymes. A total of 26 restriction site mutations were detected, and seven of them were phylogenetically informative. Wagner parsimony analysis resulted in four most parsimonious trees. In the tree obtained, the Bonin endemics are monophyletic with four mutations and a bootstrap value of 0.98 for the branch. This result agrees with that obtained from a previous electrophoretic analysis (Ito and Ono 1990), and supports the hypothesis that the three Bonin endemics have been evolved from a common ancestor. The Bonin endemics cluster together withC. lanceolatum, suggesting that the endemics andC. lanceolatum share a common ancestor, although the bootstrap probability is not very high (0.78) and thus other possibilities cannot be rejected.     

关于樟科润楠属和鳄梨属的分类界线问题

讨论了润楠属Machilus Nees与鳄梨属Persea Mill.的分类界线, 认为两属的主要区别如下: 在润楠属中, 花被裂片相等或近相等, 极少外轮明显短于内轮, 在果期为纸质, 极少为薄革质, 伸长, 强度反曲, 极少张开与不反曲, 几乎宿存, 极少脱落, 如果脱落时则内、外轮从基部完全脱落; 花柱早落; 分布亚洲热带及亚热带地区。而在鳄梨属中, 花被裂片极不相等, 外轮明显短于内轮, 很少近相等, 在果期为革质到木质, 很少为薄革质, 张开或直立, 几乎不反曲, 绝大部分宿存或内轮在距基部的1/3到1/2处脱落; 花柱有时宿存; 分布美洲。两属的界线清楚, 不宜合并。把产于越南北部的Persea balansae Airy Shaw和产于苏门答腊的P. sumatrana Kosterm.转移到Machilus中, 从而提出Machilus balansae (Airy Shaw) F. N. Wei &; S. C. Tang和M. sumatrana (Kosterm.) F. N. Wei &; S. C. Tang两个新组合。     

Impact of introduced honeybees, Apis mellifera, upon native bee communities in the Bonin (Ogasawara) Islands

The Bonin (Ogasawara) Islands are oceanic islands located in the northwest Pacific, and have ten native (nine endemic) bee species, all of which are nonsocial. The European honeybee (Apis mellifera), which was introduced to the islands for apiculture in the 1880s, became naturalized in a few islands shortly after introduction. To detect the impact of the honeybees upon native bee diversity, we analyzed pollen harvest by honeybees and surveyed the relative abundance of honeybees and native bees on flowers on several islands. Both hived and feral honeybee colonies were active throughout the year, harvesting pollen of both native and alien flowers and from both entomophilous and anemophilous flowers. Honeybees strongly depended on the alien plants, especially during winter to spring when native melittophilous flowers were rare. From June to November, honeybees exhaustively utilized native flowers, which had originally been utilized and pollinated by native bees. On Chichi and Haha Islands, where human disturbance of forests has been severe, both native and alien flowers were dominated by honeybees, and native bees were rare or extinct even in well-conserved forests. In contrast, on Ani Island and Haha's satellite islands where primary forests were well conserved and honeybees were still uncommon or absent, native bees remained dominant. These results suggest that competition for nectar and pollen of the native flowers between honeybees and native bees favors honeybees on the disturbed islands, which are thoroughly invaded by alien nectariferous, sometimes aggressive, weedy plants. Received: May 8, 1998 / Accepted: May 6, 1999     

Allozyme diversity and the evolution ofCrepidiastrum (Compositae) on the Bonin (Ogasawara) Islands

The genusCrepidiastrum is distributed in East Asia and includes 7 species. In the Bonin Islands, three species ofCrepidiastrum occur, and all of them are endemic to the islands. For detecting the origin and speciation of these endemic species, electrophoretic studies have been done in three endemic species of the Bonin Islands as well as in the remaining four species ofCrepidiastrum, andYoungia denticulata which is considered to be closely related toCrepidiastrum. A total of 386 individuals were sampled from 14 populations. As a result, 17 loci of 10 enzyme systems were resolved and gene frequencies for each population were calculated. The genetic variability was low in island species, as reported in some oceanic island plants. Four groups were recognized in the dendrogram generated by the UPGMA method. The Bonin endemics were clustered together, suggesting a monophyletic origin.C. ameristophyllum andC. linguaefolium were found to be genetically very similar, and this may suggest recent and rapid speciation within the islands.     

Isolation and characterization of microsatellite loci in <Emphasis Type="Italic">Melicope quadrilocularis</Emphasis> (Rutaceae), an endemic plant species of the Bonin Islands,Japan, and cross-species amplification in closely related taxa

Twelve microsatellite loci were isolated and characterized for Melicope quadrilocularis, an insular endemic tree species of the Bonin Islands. The observed number of alleles at each locus ranged from 1 to 18. The range of expected heterozygosity was 0.0000–0.9445. The inter-specific applicability of these loci was evaluated by analyzing two other endemic species and one endemic variety of Melicope that are also distributed on the Bonin Islands. All primer pairs for the 12 loci tested successfully amplified the loci in all taxa, except for primers for four loci in M. nishimurae.     

Threatened pollination systems in native flora of the Ogasawara (Bonin) Islands

• Background and Aims Various alien species have been introducedto the Ogasawara Islands (Japan). A survey was made investigatingwhether the native pollination systems fit an ‘islandsyndrome’ (biasing the flora to dioecy, with subdued,inconspicuous flowers) and whether alien species have disruptedthe native pollination network. • Methods Flower visitors and floral traits were determinedin the field (12 islands) and from the literature. Associationsamong floral traits such as sexual expression, flower colourand flower shape were tested. • Key Results Among the 269 native flowering plants, 74·7% are hermaphroditic, 13·0 % are dioecious and 7·1% are monoecious. Classification by flower colour revealed that36·0 % were white, 21·6 % green and 13·8% yellow. Woody species (trees and shrubs) comprised 36·5% of the flora and were associated with dioecy and white flowers.Solitary, endemic small bees were the dominant flower visitorsand visited 66·7 % of the observed species on satelliteislands where the native pollination networks are preserved.In contrast to the situation on the satellite islands, introducedhoneybees were the most dominant pollinator (visiting 60·1% of observed species) on the two main islands, Chichi-jimaand Haha-jima, and had spread to satellite islands near Chichi-jimaIsland. • Conclusions The island syndrome for pollination systemsin the Ogasawara Islands was evident in a high percentage ofdioecious species, the subdued colour of the native flora andsolitary flower visitors on satellite islands. The shape andcolour adaptations of several flowers suggested native pollinationniches for long-proboscis moths and carpenter bees. However,the domination and expansion of introduced honeybees have thepotential for disruption of the native pollination network inthe two main, and several satellite, islands of the OgasawaraIslands.     

Re-established mutualism in a seed-dispersal system consisting of native and introduced birds and plants on the Bonin Islands,Japan

The disruption of plant–animal interactions such as seed dispersal is one of the most critical effects of biological invasions. To understand the role of introduced species in current seed-dispersal systems, we conducted fecal analyses of the most common resident land birds on the Bonin Islands, Japan, and estimated their relative importance as seed-dispersal agents. Two native birds, the brown-eared bulbul and the Bonin Islands white-eye, and the introduced Japanese white-eye were the primary seed dispersers in secondary forest sites. Because the seed species composition in the feces of native and introduced white-eyes was similar, the latter may be replacing the former as a seed-dispersal agent. Introduced plants did not decrease the number of seed-dispersal opportunities for native species through competition for seed dispersers. Because some bird species have already become extinct on the Bonin Islands, their ecological functions may also have been permanently lost; however, the introduced white-eye may be compensating for this loss of function. In addition, new mutualistic relationships involving native and introduced birds and plants have already been established. In order to control introduced species while having the least impact on the native biota, interspecific interactions must be thoroughly understood before initiating control efforts.