A sophisticated case of division of labour in the trimorphic stamens of the Cassia fistula (Leguminosae) flower

Buzz-pollinated pollen flowers have pollen as the primary resource for pollinators and must deal with a conflict between the exploitation of pollen grains by bees and pollination success. It has been hypothesized that heterostemony allows division of labour between stamens as a solution to the pollen dilemma. To test the division of labour hypothesis, we chose Cassia fistula, which has a trimorphic androecium and analysed androecium development, pollen grain release mechanisms and visitor behaviour. We explored the reflectance of floral organs and carried out an exclusion experiment to test the attractiveness of each stamen morph to the bee species. Finally, we explored the structural, ultrastructural and functional variation between the pollen grains, including pollen viability across stamen morphs. The differences among the three stamen morphs, which is developed from two whorls of the stamen, are the first evidence of the division of labour in our study system. Large Bombus and Xylocopa bees actively and exclusively exploited the pollen grains from the central poricidal anthers generating pollen deposition on their bodies. The reflectance pattern of floral organs indicated a targeting of these large bees to the central anthers, corroborated by the anther manipulative experiment where only the exclusion of the anthers positioned in the flower centre, especially the intermediate stamens, reduced bee visits. Both results revealed a division of labour, in which the intermediate stamen morph was responsible for both floral attractiveness and pollen resources. Only the largest stamen morph produced germinable pollen grains, highlighting their role as pollinating stamens. The smallest stamen morph has a less clear function, likely representing an economy in pollen production for feeding function. Our findings suggest that the evolution of the trimorphic androecium is associated with division of labour in large pollen flowers and can represent a strong strategy for circumventing the pollen dilemma, optimizing the feeding function by reducing pollen grain investment from central anthers.

Morphofunctional characteristics of the tomato male gametophyte under conditions of viral pathogenesis

Relevance. The action of viral phytopathogens changes the quality of the male gametophyte, which ultimately leads to a decrease in plant productivity. So, information on the features of the variability of morphofunctional characteristics of the microgametophyte becomes topical. Taking into account the expression of a part of the plant genome in pollen, the male gametophyte can be used as a reliable system for assessing the response of genotypes to the action of viral pathogens. Presuming that the problems of variability of characteristics of the male gametophyte in conditions of viral pathogenesis have not been enough studied, as well as the main factors determining the variability have not been identified, the aim of the research was to study the effect of viral phytopathogens on the male gametophyte characters in tomato.Material and methods. Hybrids F1 and varieties of tomato were used in the experiments. Plants were grown in a greenhouse. They were inoculated with tobacco mosaic virus (TMV) or tomato aspermia virus (TAV) in the 4-5 leaf phase. To determine the quality of pollen, flowers of control and infected plants were collected; pollen was isolated and sown on a nutrient medium. The preparations were analyzed under microscope. The viability of the pollen and the length of the pollen tubes were assessed, and the ratio of these indicators was calculated.Results. Infection of plants with viruses changes some functional characteristics of the male gametophyte. The responses manifested as stimulation, inhibition, or neutral effect. The main sources of variability in characters of pollen were the genotype and viral agents. In viral pathogenesis, unequal indicators of pollen grains were found in terms of the rate of germination and growth of pollen tubes (PT). Analysis of the heritability coefficients of the ratio of pollen viability and PT length (experiment / control) revealed positive dominance and overdominance in 70% of cases. Genotypes have been identified that combine a high level of viability when infected with different viruses. The prospect of their use in further research is proposed.

Bee-Mediated Selection Favors Floral Sex Specialization in a Heterantherous Species: Strategies to Solve the Pollen Dilemma

Animal-pollinated plants show a broad variation in floral morphology traits and gametophyte production within populations. Thus, floral traits related to plant reproduction and sexuality are usually exposed to pollinator-mediated selection. Such selective pressures may be even stronger in heterantherous and pollen flowers, in which pollen contributes to both bee feeding and pollination, overcoming the “pollen dilemma” or the inability to perform both functions simultaneously. We describe the phenotypic gender and sexual organ morphology of flowers in two populations of Macairea radula (Melastomataceae), a heterantherous and buzz-pollinated species with pollen flowers. We estimated selection gradients on these traits through female and male fitness components. Both populations showed sizeable phenotypic gender variation, from strict hermaphrodites to increased femaleness or maleness. We found a continuous variation in style and stamen size, and this variation was correlated with corresponding shape values of both sexual organs. We detected bee-mediated selection towards short and long styles through seed number and towards intermediate degrees of heteranthery through pollen removal in one population, and selection towards increased maleness through pollen dispersal in both populations. Our results suggest that bee-mediated selection favors floral sex specialization and stylar dimorphism in M. radula, optimizing reproductive success and solving the pollen dilemma.

True UV color vision in a butterfly with two UV opsins

ABSTRACTTrue color vision in animals is achieved when wavelength discrimination occurs based on chromatic content of the stimuli, regardless of intensity. In order to successfully discriminate between multiple wavelengths, animals must use at least two photoreceptor types with different spectral sensitivity peaks.Heliconius butterflies have duplicate UV opsin genes, which encode two kinds of photoreceptors with peak sensitivities in the ultraviolet and violet, respectively. In H. erato, the ultraviolet photoreceptor is only expressed in females.Evidence from intracellular recordings suggests female H. erato may be able to discriminate between UV wavelengths, however, this has yet to be tested experimentally.Using an arena with a controlled light setting, we tested the ability of H. erato, and two species lacking the violet receptor, H. melpomene and outgroup Eueides isabella, to discriminate between two ultraviolet wavelengths, 380 and 390 nm, as well as two blue wavelengths, 400 and 436 nm, after being trained to associate each stimulus with a food reward. Wavelength stimuli were presented in varying intensities to rule out brightness as a cue.We found that H. erato females were the only butterflies capable of color vision in the UV range; the other butterflies had an intensity-dependent preference for UV stimuli. Across species, both sexes showed color vision in the blue-range.Models of H. erato color vision suggest that females have an advantage over males in discriminating the inner UV-yellow corolla of Psiguria pollen flowers from the surrounding outer orange petals, while previous models (McCulloch et al. 2017) suggested that H. erato males have an advantage over females in discriminating Heliconius 3-hyroxykynurenine (3-OHK) yellow wing coloration from non-3-OHK yellow wing coloration found in mimics.These results provide some of the first behavioral evidence for UV color discrimination in Heliconius females in the context of foraging, lending support to the hypothesis (Briscoe et al. 2010) that the duplicated UV opsin genes function together in UV color vision. Taken together, the sexually dimorphic visual system of H. erato appears to have been shaped by both sexual selection and sex-specific natural selection.

Towards a new understanding of the division of labour in heterantherous flowers: the case of Pterolepis glomerata (Melastomataceae)

Pollen-flowers with heteromorphic stamens have been shown to promote an intrafloral division of labour as a solution to fitness costs arising from pollen consumption by bees, known as the pollen dilemma. Usually, the division is based on morphological differences in anther and pollen traits that correlate with stamen function: pollinating anthers are larger and contain more and higher-quality pollen grains than feeding anthers. Here, we present a new strategy based on a high investment in reward production and thus attraction, in the heterantherous Pterolepis glomerata, to overcome short flower longevity and maintain reproductive success. In P. glomerata small feeding anthers not only produced more pollen grains and more grains with cytoplasmic content, but also released more pollen than pollinating anthers after a single visit. This pattern was consistent until the end of floral anthesis, showing the existence of pollen-dosing mechanisms. Bees equally visited flowers with yellow feeding anthers and pollinating anthers with yellow connective appendages, indicating a visual similarity, as predicted by bee vision modelling. Our results demonstrate that the division of labour might have different outcomes. Instead of the classical expectation of more investment in reproductive pollen in pollinating stamens, P. glomerata invested more in attraction and reward in feeding stamens.

Ecological aspects of the floral structure and flowering in Pulsatilla species

In terms of flowering ecology, <em>Pulsatilla</em> flowers are classified as “pollen flowers” producing inconsiderable amounts of nectar. The aim of this study was to assess the length of the flowering period in <em>Pulsatilla slavica</em> and <em>P. vulgaris</em> and to investigate the structure of the epidermis of the perianth and generative elements of their flowers. Special focus was placed on the structure of hairs and the distribution of stomata. The weight of nectar released by the flowers of the two <em>Pulsatilla</em> species and the content of sugars in the nectar was also evaluated. In SE Poland, both species flowered for similar periods between the second half of April and the first half of May. The flower life-span of both was determined to be 9–14 days. The lower part of each sepal was observed to be covered by long hairs having cellulose-pectin cell walls of varying thickness. Hairs present on the pistil style are thinner; they may provide some protection against cold and can play a role of a secondary pollen presenter for insects. The bowl-shaped structure of the perianth and the nature of the adaxial surface of the sepal epidermis may facilitate reflection of sunlight into the inner parts of the flower, which may contribute to an elevation of the intraflower temperature. This is particularly important for the functionality of the ovary. The surface of the hairs was seen to be covered by a cuticle ensuring water impermeability. Flowers are visited by honeybees, bumblebees, butterflies, and ants, for which nectar and pollen are the main attractants. Ants, which are regarded as illegitimate flower visitors, were found to cause damage to the androecium. The number of fruits produced in the flowers of both <em>Pulsatilla</em> species was lower than 50% of the number of pistils.

The flower morphology in three Convallariaceae species with various attractive traits

<p>The general morphology and micromorphology of the flower in <em>Polygonatum multiflorum</em>, <em>Maianthemum bifolium</em>, and <em>Convallaria majalis</em> were studied using light microscopy methods. Among the studied species, <em>P. multiflorum</em> and <em>C. majalis</em> have syntepalous and trimerous flowers, and in <em>M. bifolium</em> flowers are the most reduced: they are dimerous, pentacyclic, and with free tepals. Only in <em>P. multiflorum</em> stamens are considerably adnate to the floral tube. The gynoecium of <em>P. multiflorum</em> consists of synascidiate, hemisymplicate, and asymplicate zones. In the gynoecium of <em>M. bifolium</em> and<em> C. majalis</em>, synascidiate, symplicate, and asymplicate vertical zones were revealed. In <em>P. multiflorum</em> and <em>M. bifolium</em>, the style is composed of postgenitally connated carpels, while in <em>C. majalis</em> the style is formed by congenitally fused carpels (symplicate gynoecium zone). A common pattern of the venation of the floral parts was revealed in all the species.</p><p>The external flower morphology and the gynoecium inner structure are different in all three species, providing adaptations for the pollination mode. Attractive elements observed in the flower of <em>P. multiflorum</em> are the long septal nectary in the ovary and epidermal trichomes on the inner perigonium surface and on the filaments. In <em>M. bifolium</em>, a rudimentary external septal nectary was observed for the first time. No nectaries or other morphologically distinct secretory structures were found in the <em>C. majalis</em> flower, allowing considering the <em>C. majalis</em> flowers as pollen flowers.</p>