Chasmogamy and entomophily in Burmannia disticha (Burmanniaceae)

Burmannia shows a set of floral traits that suggest elaborate mechanisms of animal-mediated pollen transfer. These include flower coloration, septal nectaries and a long and narrow floral chamber. The stamens are synorganized with the common style restricting the entrance to the floral chamber, sometimes forming a gynostegium. Contrary to this apparent zoophilous floral syndrome, several species of Burmannia were reported to perform self-pollination via cleistogamy. Understanding of reproductive systems in Burmannia is complicated by scarcity of available results of direct observations on pollination process. Here we present data on pollination biology of B. disticha obtained during field investigations in Vietnam followed by laboratory analyses of ecologically important floral traits and the captured flower visitors. We found that the anthetic perianth is open, i.e. the flower is chasmogamous. The flowers are visited by various Diptera, Hymenoptera, Lepidoptera and Orthoptera. Of them, the bumblebees (Bombus burmensis), a bee (Coelioxys sp.) and some lepidopterans were revealed to carry pollen of B. disticha. Based on the amount of carried pollen, insect behavior during the visits and general knowledge on biology of these insect taxa, we concluded that the bumblebees act as the principal pollinators of B. disticha, whereas the lepidopterans are considered as its possible pollinators. We compared the lengths of proboscises of the captured insects to the depth of the floral chamber, and found that only the bumblebees and lepidopterans should be able to reach the nectar. Finally, we estimated the pollen-ovule ratio of B. disticha as 6.84, which is comparable to the ratio known in autogamous angiosperms. Based on its flower organization and pollination mechanism, we consider B. disticha an entomophilous and predominantly xenogamous species. Its gynostegium is likely an adaptation for pollen transfer by insects with long proboscises. At the same time, earlier investigations together with pollen-ovule ratio indicate that B. disticha possesses a labile pollination strategy, and autogamy sometimes occurs. Since Burmannia is one of the few angiosperm genera that comprise both mycoheterotrophic (achlorophyllous) and autotrophic (green) species, our study provides important evidence for reconstructions of ecological and morphological evolutionary pathways in relation to the mode of organic nutrition.

Flowering Biology of Alisma plantago-aquatica (Alismataceae)

Field observations of flowering Alisma plantago-aquatica plants were carried out in Moscow region (Russia). The A. plantago-aquatica flower remains anthetic for a single light day, from 9:00 a.m. to 8:00 p.m. White petals showed a contrast bicolored pattern in UV light, and the pattern probably serves as a nectar clue for pollinators. Flowers were visited by insects in daytime from 11:00 a.m. to 3:00 p.m. Coleopterans (Coccinellidae), dipterans (Drosophilidae, Hybotidae, Muscidae, Sepsidae, and Syrphidae), and hymenopterans (Apidae) were observed as flower visitors. Hoverflies (Syrphidae) and bees (Apidae) were the most frequent visitors. Large amounts of A. plantago-aquatica pollen grains were found on their bodies, and a major role in pollination was consequently assumed for the insects. Based on the original findings and literature data on A. plantago-aquatica reproductive biology in Belgium, Slovakia, and the Czech Republic, hoverflies were identified as the most stable and efficient pollinators of A. plantago-aquatica in various parts of the species range. Bees (Apidae) were recognized as A. plantago-aquatica pollinators for the first time in this work. A flower isolation experiment confirmed that A. plantago-aquatica is a self-compatible plant, but requires insects for the most efficient cross-pollination.

Structure and Development of Flowers and Inflorescences in Burmannia (Burmanniaceae, Dioscoreales)

Species of the genus Burmannia possess distinctive and highly elaborated flowers with prominent floral tubes that often bear large longitudinal wings. Complicated floral structure of Burmannia hampers understanding its floral evolutionary morphology and biology of the genus. In addition, information on structural features believed to be taxonomically important is lacking for some species. Here we provide an investigation of flowers and inflorescences of Burmannia based on a comprehensive sampling that included eight species with various lifestyles (autotrophic, partially mycoheterotrophic and mycoheterotrophic). We describe the diversity of inflorescence architecture in the genus: a basic (most likely, ancestral) inflorescence type is a thyrsoid comprising two cincinni, which is transformed into a botryoid in some species via reduction of the lateral cymes to single flowers. Burmannia oblonga differs from all the other studied species in having an adaxial (vs. transversal) floral prophyll. For the first time, we describe in detail early floral development in Burmannia. We report presence of the inner tepal lobes in B. oblonga, a species with reportedly absent inner tepals; the growth of the inner tepal lobes is arrested after the middle stage of floral development of this species, and therefore they are undetectable in a mature flower. Floral vasculature in Burmannia varies to reflect the variation of the size of the inner tepal lobes; in B. oblonga with the most reduced inner tepals their vascular supply is completely lost. The gynoecium consists of synascidiate, symplicate, and asymplicate zones. The symplicate zone is secondarily trilocular (except for its distal portion in some of the species) without visible traces of postgenital fusion, which prevented earlier researchers to correctly identify the zones within a definitive ovary. The placentas occupy the entire symplicate zone and a short distal portion of the synascidiate zone. Finally, we revealed an unexpected diversity of stamen-style interactions in Burmannia. In all species studied, the stamens are tightly arranged around the common style to occlude the flower entrance. However, in some species the stamens are free from the common style, whereas in the others the stamen connectives are postgenitally fused with the common style, which results in formation of a gynostegium.

Leaf epidermal micromorphology in Aspidistra (Asparagaceae): diversity and taxonomic significance

Micromorphological characters of leaf epidermis were investigated in 69 species of Aspidistra using scanning electron microscopy. Sculpture of epidermis varies from smooth to verrucose and rugose in the genus. The abaxial epidermis of some species bears papillae, whereas the adaxial surface uniformly lacks the papillae. Sculpture type of epidermis and density of papillae are generally found to be stable characters at a species level. The infraspecific variation of epidermis sculpture, where present, ranges from smooth to verrucose or from verrucose to rugose. Micromorphological characters of leaf epidermis are shown to have potential taxonomic significance in Aspidistra; in combination with the type of shoot structure, they allow to subdivide the species into 13 groups. The groups are largely incongruent with floral morphological traits. An identification key to the studied species of Aspidistra based on vegetative characters (gross leaf and shoot morphology and characters of leaf epidermis) is presented.

Pollination of Vietnamese Aspidistra xuansonensis (Asparagaceae) by female Cecidomyiidi flies: larvae of pollinator feed on fertile pollen in anthers of anthetic bisexual flowers

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PREMISE OF THE STUDY

Aspidistra is a species-rich, herbaceous monocot genus of tropical Southeast Asia. Most species are recently discovered and apparently endangered, though virtually nothing is known about their biology. Species of the genus are primarily distinguished using flower morphology, which is enormously diverse. However, the pollination process has not been directly observed in the center of diversity of the genus (N Vietnam and S China). Indirect and partly direct data on the only widely cultivated species of the genus (A. elatior) placed it among angiosperms with the most unusual pollination biology, though these data are highly controversial, suggesting pollen transfer by mollusks, crustaceans, flies, or possibly tiny soil invertebrates such as collembolans.•

METHODS

Pollination of Aspidistra xuansonensis in the center of diversity of the genus was studied using visual observations and videos and light and scanning electron microscopy investigation of flowers and their pollinators. Pollinators and their larvae were molecularly barcoded.•

KEY RESULTS

Aspidistra xuansonensis is pollinated by female cecidomyiid flies (gall midges). They oviposit on anthers, and larvae develop among the pollen mass. Molecular barcoding proved taxonomic identity of the larvae and the flies. The larvae neither damage floral parts nor cause gall formation, but feed on pollen grains by sucking out their content. The larvae move out of the flowers before decomposition starts. Carebara ants steal developing larvae from flowers but do not contribute to pollination.•

CONCLUSIONS

More than one kind of myiophily is present in Aspidistra. Brood site pollination was documented for the first time in Aspidistra. The pollination system of A. xuansonensis differs from other kinds of brood site pollination in the exit of the larvae prior to the decomposition of floral parts.