Scaevola plumieri is an important pioneer on many tropical and subtropical sand dunes, forming a large perennial subterranean plant with only the tips of the branches emerging above accreting sand. In South Africa it is the dominant pioneer on sandy beaches along the east coast, less abundant on the south coast and absent from the southwest and west coasts. Transpiration rates (E) of S. plumieri are predictably related to atmospheric vapour pressure deficit under a wide range of conditions and can therefore be predicted from measurement of ambient temperature and relative humidity. Scaling measurements of E at the leaf level to the canopy level has been demonstrated previously. Using a geographic information system, digital maps of regional climatic variables were used to calculate digital maps of potential transpiration from mean monthly temperature and relative humidity values, effectively scaling canopy level transpiration rates to a regional level. Monthly potential transpiration was subtracted from the monthly median rainfall to produce a map of mean monthly water balance. Seasonal growth was correlated with seasonal water balance. Localities along the coast with water deficits in summer corresponded with the recorded absence of S. plumieri, which grows and reproduces most actively in the summer months. This suggests that reduced water availability during the summer growth period limits the distribution of S. plumieri along the southwest coast, where water deficits develop in summer. Temperature is also important in limiting the distribution of S. plumieri on the southwest coast of South Africa through its effects on the growth and phenology of the plant.
Introduction The terrestrial orchid genus Nervilia is diagnosed by its hysteranthous pattern of emergence but is nested among leafless myco-heterotrophic lineages in the lower Epidendroideae. Comprising ca. 80 species distributed across Africa, Asia and Oceania, the genus remains poorly known and plagued by vague and overlapping species circumscriptions, especially within each of a series of taxonomically intractable species complexes. Prior small-scale, exploratory molecular phylogenetic analyses have revealed the existence of cryptic species, but little is otherwise understood of origin, the scale and timing of its biogeographic spread, or the palaeoclimatic factors that have shaped its ecology and given rise to contemporary patterns of occurrence. Methods Here, we sample widely throughout the generic range, including 45 named taxa and multiple accessions referable to several widespread ‘macrospecies’, as well as material of equivocal identity and probable undescribed status, for the first time enabling an evaluation of taxonomic boundaries at both species and sectional level. Using nuclear ( ITS ) and plastid ( matK , trnL-F ) sequence data, we conduct phylogenetic (maximum parsimony and Bayesian inference) and ancestral area analysis to infer relationships and resolve probable origin and colonisation routes. Results The genus is strongly supported as monophyletic, as are each of its three sections. However, the number of flowers in the inflorescence and other floral characters are poor indicators of sectional affinity. Dated ancestral area analysis supports an origin in Africa in the Early Oligocene, with spread eastwards to Asia occurring in the Late Miocene, plausibly via the Gomphotherium land bridge at a time when it supported woodland and savanna ecosystems. Discussion Taxonomic radiation in Asia within the last 8 million years ties in with dramatic Himalayan-Tibetan Plateau uplift and associated intensification of the Asia monsoon. Multiple long-range migrations appear to have occurred thereafter, as the genus colonised Malesia and Oceania from the Pliocene onwards. The bulk of contemporary species diversity is relatively recent, potentially explaining the ubiquity of cryptic speciation, which leaves numerous species overlooked and unnamed. Widespread disjunct species pairs hint at high mobility across continents, extinction and a history of climate-induced vicariance. Persistent taxonomic challenges are highlighted.
Autonomous self-pollination is surprisingly common among orchids and is thought to provide reproductive assurance when pollinators are scarce. During investigations of the reproductive biology of the orchid genus Eulophia, consistently high rates of capsule set were observed in Eulophia clavicornis var. clavicornis, E. c. var. inaequalis, E. c. var. nutans and E. tenella. A breeding system experiment showed that E. c. var. nutans is capable of autonomous self-pollination. Emasculated flowers of this taxon did not set fruit, suggesting that agamospermy is unlikely. The likely mechanism of autonomous self-pollination in these taxa was identified as the partial or complete absence of rostellum tissue, allowing contact between pollinia and stigmatic fluid, and thus for pollen tubes to grow from in situ pollinia to the ovules. In some individuals, basal flowers on an inflorescence possess intact rostellae and functional pollinaria, whereas distal flowers lack pollinia. Neither of these two flower classes set capsules. A few individuals of the otherwise outcrossing E. zeyheriana, which normally have well-developed rostellae, show evidence of autonomous self-pollination resulting from interrupted rostellae in these plants. Other outcrossing Eulophia species (E. speciosa and E. streptopetala) sometimes show high levels of fruit set, seemingly without insect visitation. However, investigations showed that these are pseudo-fruits lacking seeds and are a result of insect parasitism. Therefore, high levels of fruit set alone should not be used to infer autonomous self-pollination in orchids.
The shape of flowers frequently corresponds to the morphology of pollinators but some floral traits may also function to prevent non-pollinating flower visitors from stealing flower rewards. Despite the presence of such structures few studies have demonstrated their efficacy in limiting the nectar intake by nectar thieves. The flowers of Strelitzia reginae are regularly visited by sunbirds that do not effect pollination and act solely as nectar thieves. In this species, the nectary is covered by the convoluted bases of the petals ("nectar barriers"). In this study we investigate how non-pollinating sunbirds interact with these nectar barriers and whether nectar barriers play a role in limiting the amount of nectar sunbirds can steal. We quantified the volume of nectar that sunbirds consume while visiting flowers where nectar barriers were present and in flowers where these were experimentally removed. We found that sunbirds consume a median of 106.8 µl of nectar when visiting flowers with nectar barriers present and consumed a significantly greater volume of nectar (median = 158.03 µl) in flowers without nectar barriers. These results suggest that the convoluted petals that cover the nectary of S. reginae may function to reduce nectar theft but are likely to be more effective against insect nectar thieves. This is one of the first studies to quantitatively demonstrate the role of flower features that may function to limit nectar theft.
The Putranjivaceae is an enigmatic family, notable for being the only lineage outside the Capparales to possess the glucosinolate biochemical pathway, which forms the basis of an induced chemical defense system against herbivores (the “mustard oil bomb”). We investigated the pollination biology and floral scent chemistry of Drypetes natalensis (Putranjivaceae), a dioecious subcanopy tree with flowers borne on the stem (cauliflory). Flowering male trees were more abundant than female ones and produced about 10‐fold more flowers. Flowers of both sexes produce copious amounts of nectar on disc‐like nectaries accessible to short‐tongued insects. The main flower visitors observed were cetoniid beetles, bees, and vespid wasps. Pollen load analysis indicated that these insects exhibit a high degree of fidelity to D. natalensis flowers. Insects effectively transfer pollen from male to female plants resulting in about 31% of female flowers developing fruits with viable seeds. Cetoniid beetles showed significant orientation toward the scent of D. natalensis flowers in a Y‐maze olfactometer. The scents of male and female flowers are similar in chemical composition and dominated by fatty acid derivatives and isothiocyanates from the glucosinolate pathway. The apparent constitutive emission of isothiocyanates raises interesting new questions about their functional role in flowers.
Abstract It has been suggested that plants that are good colonizers will generally have either an ability to self‐fertilize or a generalist pollination system. This prediction is based on the idea that these reproductive traits should confer resistance to Allee effects in founder populations and was tested using Gomphocarpus physocarpus (Asclepiadoideae: Apocynaceae), a species native to South Africa that is invasive in other parts of the world. We found no significant relationships between the size of G. physocarpus populations and various measures of pollination success (pollen deposition, pollen removal and pollen transfer efficiency) and fruit set. A breeding system experiment showed that plants in a South African population are genetically self‐incompatible and thus obligate outcrossers. Outcrossing is further enhanced by mechanical reconfiguration of removed pollinaria before the pollinia can be deposited. Self‐pollination is reduced when such reconfiguration exceeds the average duration of pollinator visits to a plant. Observations suggest that a wide variety of wasp species in the genera Belonogaster and Polistes (Vespidae) are the primary pollinators. We conclude that efficient pollination of plants in small founding populations, resulting from their generalist wasp‐pollination system, contributes in part to the colonizing success of G. physocarpus . The presence of similar wasps in other parts of the world has evidently facilitated the expansion of the range of this milkweed.
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