Article

Vincetoxicum siamicum sp. nov. (Asclepiadeae, Asclepiadoideae, Apocynaceae), a rheophyte from Northeastern Thailand and preliminary observation on its in situ pollen germination

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Abstract

Vincetoxicum siamicum A.Kidyoo, newly described here, is a rheophytic herb growing in dense clumps near or in streams in northeastern Thailand. Here I provide a diagnosis and detailed description of this new species, as well as illustrations and photographs. Several characters of V. siamicum are compared with those of V. stauntonii, considered to be a close relative. These two species display consistent differences in leaf arrangement and corona structure. A comparison with other rheophytic species, V. glaucescens, V. hydrophilum, V. riparium, V. verticillatum, is also provided. They differ in several traits: shape, size and texture of leaves, hairiness of corolla lobes, branching pattern of inflorescences and elevations at which they occur. In addition, in situ pollen germination observed in the flowers of V. siamicum is reported.

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... Lately, based on morphological and chemical data, Liede (1996) recognized Vincetoxicum as a separate genus from Cynanchum and treated Tylophora Brown (1810:460) as the closest relative of Vincetoxicum. Vincetoxicum usually has colourless latex, fascicled roots, patent to horizontal corolla lobes, and a fleshy staminal corona, while Tylophora is characterized by twining or rarely erect habit, small flowers with rotate or subrotate and deeply 5-lobed corollas, a corona of five erect, turgid lobes, and small globose to subglobose pollinia in horizontal to erect position (Forster 1991, 1992, Kidyoo 2016. ...
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Vincetoxicum xinpingense, a new species from the Ailao Mountains in Yunnan Province, China, is described and illustrated. It is morphologically similar to V. sublanceolatum, Tylophora forrestii, and T. chingtungensis, but can be distinguished by several characters: shape and size of leaves, size of corolla lobes, length and shape of corona, attachment position of corona to the anthers, and orientation of pollinia. In addition, conservation status and other relevant notes are provided.
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An unknown Vincetoxicum species has recently been discovered in northeastern Thailand. It is a twiner that resembles in its morphology and ecology V. flexuosum var. flexuosum , a variable species widely occurring from tropical Asia to Australia. Comprehensive morphological examination showed that despite similarities in growth habit, leaf shape, branching of the inflorescence and flower colour, these two taxa exhibit substantial differences in both vegetative and reproductive traits that clearly distinguish one from the other, particularly shape of the flower bud and indumentum on the corolla lobes. In addition, we also evaluated phylogenetic relationships based on DNA sequence data for ITS, trn T-L, trn L, and trn L-F markers of this new Vincetoxicum sp. with congeners (including, inter alia, new sequences of V. flexuosum var. flexuosum and also those of V. flexuosum var. tenuis , the other variety occurring in Thailand). The analyses demonstrated that the new Vincetoxicum sp. is not closely related to the taxa recognized as V. flexuosum. The new species was instead retrieved as sister to a clade containing the African taxa, V. caffrum , V. lycioides and V. fleckii . Therefore, integrated analyses of morphology and molecular phylogeny revealed Vincetoxicum sp. to be a well-defined species clearly distinct from V. flexuosum , as well as from all other known congeners. The morphological similarity between the new Vincetoxicum sp. and V. flexuosum var. flexuosum likely resulted from convergence, leading to various taxonomic complications. We here describe it as a new species, V. sangyojarniae , sp. nov., and provide a detailed description, illustration, photographs, and comparison to the morphologically similar V. flexuosum var. flexuosum . A preliminary taxonomic reconsideration of the infraspecific taxa under V. flexuosum is also suggested.
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The phylogenetic structure of Asclepiadoideae (Apocynaceae) has been elucidated at the tribal and subtribal levels in the last two decades. However, to date, the systematic positions of seven Asian genera, Cosmostigma, Graphistemma, Holostemma, Pentasachme, Raphistemma, Seshagiria and Treutlera, have not been investigated. In this study, we examine the evolutionary relationships among these seven small enigmatic Asian genera and clarify their positions in Asclepiadoideae, using a combination of plastid sequences of rbcL, rps16, trnL and trnL- F regions. Cosmostigma and Treutlera are resolved as members of the non-Hoya clade of Marsdenieae with strong support (maximum parsimony bootstrap support value BSMP=96, maximum likelihood bootstrap support value BSML=98, Bayesian-inferred posterior probability PP=1.0). Pentasachme is resolved as sister of Stapeliinae to Ceropegieae with moderate support (BSMP=64, BSML=66, PP=0.94). Graphistemma, Holostemma, Raphistemma and Seshagiria are all nested in the Asclepiadeae-Cynanchinae clade (BSMP=97, BSML=100, PP=1.0). The study confirms the generally accepted tribal and subtribal structure of the subfamily. One exception is Eustegia minuta, which is placed here as sister to all Asclepiadeae (BSMP=58, BSML=76, PP=0.99) and not as sister to the Marsdenieae+Ceropegieae clade. The weak support and conflicting position indicate the need for a placement of Eustegia as an independent tribe. In Asclepiadeae, a sister group position of Cynanchinae to the Asclepiadinae+Tylophorinae clade is favoured (BSMP=84, BSML=88, PP=1.0), whereas Schizostephanus is retrieved as unresolved. Oxystelma appears as an early-branching member of Asclepiadinae with weak support (BSMP=52, BSML=74, PP=0.69). Calciphila and Solenostemma are also associated with Asclepiadinae with weak support (BSMP=37, BSML=45, PP=0.79), but all alternative positions are essentially without support. The position of Indian Asclepiadoideae in the family phylogeny is discussed.
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We investigated the effect of self-pollination on fruit- and seed-set in Asclepias exaltata, a self-incompatible milkweed. A total of 1,380 hand-pollinations were performed on 138 flowers in each of five treatments. In each treatment, two pollinia were placed in adjacent stigmatic chambers, which transmitted pollen tubes to the same ovary. Treatments 1 and 2 involved simultaneous placement of two self-pollinia (treatment 1) or two cross-pollinia (treatment 2) into adjacent stigmatic chambers. No fruits were produced after self-pollinations, but 23% of the cross-pollinated flowers matured fruits. The three remaining experimental pollination treatments were self-pollination preceding cross-pollination by 24 hr (treatment 3), simultaneous self- and cross-pollination (treatment 4), and cross-pollination preceding self-pollination by 24 hr (treatment 5). Compared with that in flowers receiving only cross-pollen, fruit-set in treatments 3-5 was reduced 81%, 49%, and 29%, respectively. Seed-set was also significantly reduced in flowers receiving self-pollen 24 hr in advance of the cross-pollen. Using genetic markers, we observed that only 0.6% of seeds resulted from self-pollination. Our data strongly suggest that self-pollination in milkweeds not only wastes pollen but also greatly reduces the number of ovules and ovaries that might otherwise mature fruits and seeds after cross-pollination.
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Asclepiadaceae are the dicot counterparts to the Orchidaceae, which also transmit their pollen grains in large groups within pollinia. Unlike many terrestrial, nectar-producing orchids, however, milkweeds are characterized by low fruit-set, typically averaging 1-5%. Transfer of hundreds of pollen grains as a unit makes it possible to quantify pollinator activity and male and female reproductive success more directly and more easily in milkweeds than in plants with loose pollen grains. It also leads to the production of fruits whose seeds all share a single father, thus simplifying paternity analysis. Recent anatomical work has demonstrated that three of the five stigmatic chambers of milkweed flowers transmit pollen tubes to one of the two separate ovaries, whereas the other two chambers transmit only to the second ovary. Milkweed flowers are long-lived and produce copious nectar, which flows from nectaries within the stigmatic chambers to fill the hoods, which serve as reservoirs. Nectar also serves as the germination fluid for pollen grains, but concentrations above 30% inhibit germination. Most milkweeds are genetically self-incompatible and express an unusual late-acting form of ovarian rejection. Some weedy milkweeds, however, are self-compatible, and levels of self-insertion of pollinia are apparently high in these, as well as in self-incompatible, species. Early attempts to explain the evolution of inflorescence size in milkweeds were hampered by failure to consider the genetic basis of the variation observed and by failure to determine the unit on which selection should act. Direct tests of the ''pollen donation'' hypothesis have cast doubt on the validity of the view that flower number and other floral traits evolved primarily to enhance male reproductive success. Milkweeds are pollinated by a diverse array of large bees, wasps, and butterflies, and these generalist pollinators effect extensive gene flow within and between populations, augmented by wind dispersal of comose seeds. Morphological and biochemical evidence support the view that limited, localized hybridization occurs between sympatric species of milkweeds.
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Scanning electron and fluorescence microscopy were used to clarify some aspects of the floral morphology ofApocynum cannabinum. Insects are required for pollination, since the floral morphology prevents autogamy and minimizes intrafloral self-pollination. Flowers hand-pollinated with self-pollen never set fruit, but 10.6% of cross-pollinations produced fruit. Self-pollen did germinate, however, and produced abundant tubes that grew through the pistil and entered the ovule micropyles. The proportion of ovules penetrated by self- and outcross-pollen tubes was not statistically significantly different. These results suggest thatA. cannabium possesses late-acting self-incompatibility, similar to that in the closely related Asclepiadaceae.
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Reproductive character and genetic diversity were investigated for an autogamous species of Tylophora matsumurae endemic to the Ryukyu Islands and its progenitor species T. tanakae. Approximately one-fourth of bagged flowers set fruits in T. matsumurae, although no fruits were obtained from bagged flowers in T. tanakae. In situ pollen tube germination was observed with high frequency in all five populations examined in T. matsumurae. Furthermore, in T. matsumurae, anther sacs were not dehisced even at anthesis. These results suggest the highly autogamous nature of T. matsumurae. No allozymic variation was detected in all seven populations examined in T. matsumurae. The phenogram constructed using the neighbor-joining method based on Nei's unbiased genetic distance indicated that T. matsumurae clustered with the Okinoerabu Island population of T. tanakae. The estimated outcrossing rate of four populations of T. tanakae varied from 0.18 to 0.59. It is probable that T. matsumurae had been derived from the predominantly self-pollinating population of T. tanakae, and rapidly enlarged its distribution area.
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The phylogeny of the genus Cynanchum s. str. is studied using cpDNA spacers and ITS. Morphological, anatomical and latex triterpenoid data are interpreted in light of the molecular results, and discrepancies are discussed. Vegetative characters are better indicators of relationship than floral characters, especially corona characters. The monophyly of all Malagasy species and, nested within the latter, of all stem-succulent taxa is ascertained and the genera Folotsia, Karimbolea, Platykeleba and Sarcostemma are subsumed under Cynanchum. One African species, C. galgalense, is excluded from Cynanchum.
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Comparisons of the causes and consequences of cross- and self-fertilization have dominated research on plant mating since Darwin's seminal work on plant reproduction. Here, I provide examples of these accomplishments, but also illustrate new approaches that emphasize the role of floral design and display in pollen dispersal and fitness gain through male function. Wide variation in outcrossing rate characterizes animal-pollinated plants. In species with large floral displays, part of the selfing component of mixed mating can arise from geitonogamy and be maladaptive because of strong inbreeding depression and pollen discounting. Floral strategies that separate the benefits of floral display from the mating costs associated with geitonogamy can resolve these conflicts by reducing lost mating opportunities through male function. The results from experiments with marker genes and floral manipulations provide evidence for the function of herkogamy and dichogamy in reducing self-pollination and promoting pollen dispersal. Evidence is also presented indicating that increased selfing resulting from changes to floral design, or geitonogamy in large clones, can act as a stimulus for the evolution of dioecy. The scope of future research on mating strategies needs to be broadened to include investigations of functional links among flowers, inflorescences and plant architecture within the framework of life-history evolution.
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Molecular phylogenetic analyses of Vincetoxicum and Tylophora (Apocynaceae-Asclepiadoideae) were conducted based on the nucleotide sequences of cpDNA (two intergenic spacers of trnL (UAA)-trnF (GAA) and psbA-trnH and three introns, i.e., atpF, trnG (UCC) and trnL (UAA)), and nrDNA (ITS and ETS regions). Our phylogenetic analysis revealed two monophyletic groups; one consisted of seven taxa of Tylophora and Vincetoxicum inamoenum, Vincetoxicum magnificum and Vincetoxicum macrophyllum (Clade I) and the other consisted of 17 accessions of Vincetoxicum (Clade II). The monophyly of the genus Vincetoxicum was not supported. Although many nucleotide substitutions were observed in Clade I, the genetic differentiation within Clade II was small. Low genetic diversification but considerable morphological divergence suggests that the species in Clade II had undergone rapid diversification. Although most species in Clade I have tiny flowers, those in Clade II have larger and more nectariferous ones. Thus, we hypothesized that the rapid morphological radiation in Clade II may have been due to the gaining of floral characters such as large flowers and large amounts of nectar corresponding to diverse pollinators.
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