Pseudopollen in Dendrobium unicum Seidenf. (Orchidaceae): Reward or Deception?

BACKGROUND AND AIMS: In 1987, Kjellsson and Rasmussen described the labellar trichomes of Dendrobium unicum Seidenf. and proposed that these hairs function as pseudopollen. Pseudopollen is a mealy material that superficially resembles pollen, is usually laden with food substances and is formed when labellar hairs either fragment into individual cells or become detached from the labellum. However, the trichomes of D. unicum are very different from pseudopollen-forming hairs found in other orchid genera such as Maxillaria and Polystachya. Moreover, Kjellsson and Rasmussen were unable to demonstrate the presence of food substances within these trichomes and argued that even in the absence of food substances, the hairs, in that they superficially resemble pollen, can still attract insects by deceit. The aim of this paper is to investigate whether the labellar trichomes of D. unicum contain food reserves and thus reward potential pollinators or whether they are devoid of foods and attract insects solely by mimicry. METHODS: Light microscopy, histochemistry and transmission electron microscopy. KEY RESULTS: Dendrobium unicum produces pseudopollen. Pseudopollen here, however, differs from that previously described for other orchid genera in that the pseudopollen-forming trichomes consist of a stalk cell and a 'head' of component cells that separate at maturity, in contrast to Maxillaria and some Polystachya spp. where pseudopollen is formed by the fragmentation of moniliform hairs. Moreover, the pseudopollen of Maxillaria and Polystachya largely contains protein, whereas in D. unicum the main food substance is starch. CONCLUSIONS: Flowers of D. unicum, rather than attracting insects solely by deceit may also reward potential pollinators.     

Do floral and ecogeographic isolation allow the co‐occurrence of two ecotypes of Anacamptis papilionacea (Orchidaceae)?

Ecotypes are relatively frequent in flowering plants and considered central in ecological speciation as local adaptation can promote the insurgence of reproductive isolation. Without geographic isolation, gene flow usually homogenizes the allopatrically generated phenotypic and ecological divergences, unless other forms of reproductive isolation keep them separated. Here, we investigated two orchid ecotypes with marked phenotypic floral divergence that coexist in contact zones. We found that the two ecotypes show different ecological habitat preferences with one being more climatically restricted than the other. The ecotypes remain clearly morphologically differentiated both in allopatry and in sympatry and differed in diverse floral traits. Despite only slightly different flowering times, the two ecotypes achieved floral isolation thanks to different pollination strategies. We found that both ecotypes attract a wide range of insects, but the ratio of male/female attracted by the two ecotypes was significantly different, with one ecotype mainly attracts male pollinators, while the other mainly attracts female pollinators. As a potential consequence, the two ecotypes show different pollen transfer efficiency. Experimental plots with pollen staining showed a higher proportion of intra‐ than interecotype movements confirming floral isolation between ecotypes in sympatry while crossing experiments excluded evident postmating barriers. Even if not completely halting the interecotypes pollen flow in sympatry, such incipient switch in pollination strategy between ecotypes may represent a first step on the path toward evolution of sexual mimicry in Orchidinae.     

The evolution of floral nectaries in Disa (Orchidaceae: Disinae): recapitulation or diversifying innovation?

Background and Aims

The Orchidaceae have a history of recurring convergent evolution in floral function as nectar production has evolved repeatedly from an ancestral nectarless state. However, orchids exhibit considerable diversity in nectary type, position and morphology, indicating that this convergence arose from alternative adaptive solutions. Using the genus Disa, this study asks whether repeated evolution of floral nectaries involved recapitulation of the same nectary type or diversifying innovation. Epidermis morphology of closely related nectar-producing and nectarless species is also compared in order to identify histological changes that accompanied the gain or loss of nectar production.

Methods

The micromorphology of nectaries and positionally equivalent tissues in nectarless species was examined with light and scanning electron microscopy. This information was subjected to phylogenetic analyses to reconstruct nectary evolution and compare characteristics of nectar-producing and nectarless species.

Key Results

Two nectary types evolved in Disa. Nectar exudation by modified stomata in floral spurs evolved twice, whereas exudation by a secretory epidermis evolved six times in different perianth segments. The spur epidermis of nectarless species exhibited considerable micromorphological variation, including strongly textured surfaces and non-secreting stomata in some species. Epidermis morphology of nectar-producing species did not differ consistently from that of rewardless species at the magnifications used in this study, suggesting that transitions from rewardlessness to nectar production are not necessarily accompanied by visible morphological changes but only require sub-cellular modification.

Conclusions

Independent nectary evolution in Disa involved both repeated recapitulation of secretory epidermis, which is present in the sister genus Brownleea, and innovation of stomatal nectaries. These contrasting nectary types and positional diversity within types imply weak genetic, developmental or physiological constraints in ancestral, nectarless Disa. Such functional convergence generated by morphologically diverse solutions probably also underlies the extensive diversity of nectary types and positions in the Orchidaceae.     

Changes in Haemoproteus sex ratios: fertility insurance or differential sex lifespan?

There is little direct evidence of the fitness effects of changes in malaria gametocyte sex ratio. Gametocyte sex ratios in haemospororin parasites (phylum Apicomplexa) are usually female skewed. However, in some cases and especially in Haemoproteus parasites, less female-biased and even male-biased sex ratios are encountered. The 'fertility insurance hypothesis' tries to explain these biases as an evolutionary strategy to facilitate gamete encounter. Thus, the hypothesis predicts that, if there is a reduction in gametocyte density (intensity of infection) or other factors preventing gametes from meeting, a change to a higher proportion of male gametocytes may be favoured. By contrast, a change in sex ratio may be caused by other non-adaptive mechanisms, for example differential survival of the gametocytes of each sex. We study within-host changes in Haemoproteus majoris sex ratios following an experimental reduction in the density of the parasites in the blood in a breeding population of blue tits (Parus caeruleus). Medication with the antimalarial drug primaquine induced a significant reduction in Haemoproteus gametocyte infection intensity in two different breeding seasons and under two different doses of medication. Sex ratios became male skewed following the experimental treatment in agreement with the predictions of the 'fertility insurance' hypothesis. Also in support of the hypothesis, a significant change towards male-biased sex ratios emerged for non-medicated birds in one year, probably owing to the natural immune reduction of the density of the parasites in the blood. The alternative possibility that changes are caused by different lifespans of gametocytes is not supported by changes in sex ratios in control hosts, where new production and release of gametocytes occur.     

Cyclooxygenase-2 inhibitors: promise or peril?

The discovery of two isoforms of the cyclooxygenase enzyme, COX-1 and COX-2, and the development of COX-2-specific inhibitors as anti-inflammatories and analgesics have offered great promise that the therapeutic benefits of NSAIDs could be optimized through inhibition of COX-2, while minimizing their adverse side effect profile associated with inhibition of COX-1. While COX-2 specific inhibitors have proven to be efficacious in a variety of inflammatory conditions, exposure of large numbers of patients to these drugs in postmarketing studies have uncovered potential safety concerns that raise questions about the benefit/risk ratio of COX-2-specific NSAIDs compared to conventional NSAIDs. This article reviews the efficacy and safety profiles of COX-2-specific inhibitors, comparing them with conventional NSDAIDs.     

Ants (Formicidae) as food for birds in southern Africa: opportunism or survival?

Ants (Hymenoptera: Formicidae) are eaten by a number of bird species in southern Africa. Our database contained 545 species (excluding waterbirds and raptors), of which 179 species have been observed feeding on ants, or had ants in their stomachs. Ants are eaten by birds in all ecosystems, but the consumption of ants is disproportionately high in species that occur in arid ecosystems. The number of bird species eating ants increases in proportion to the number of bird species in any ecosystem, but it increases disproportionately with decreasing mean annual rainfall. There was a small, but not significant, difference in the proportion of ant-eating bird species between 502 resident species, of which 154 (30.6%) eat ants, and 47 nomadic species, of which 18 (38.2%) eat ants. Foraging behaviour and substrate, i.e. whether foraging in the air, from a perch or on the ground, significantly influenced the probability that ants would be included in the diet.     

Exine Micromorphology of Orchidinae (Orchidoideae,Orchidaceae): Phylogenetic Constraints or Ecological Influences?

BACKGROUND AND AIMS: Pollen characters have been widely used in defining evolutionary trends in orchids. In recent years, information on pollination biology and phylogenetic patterns within Orchidinae has become available. Hence, the aim of the presented work is to re-evaluate exine micromorphology of Orchidinae in light of recent phylogenetic studies and to test whether pollen micromorphology strictly depends on phylogenetic relationships among species or whether it is influenced by the marked differences in pollination ecology also reported among closely related species. METHODS: Pollen sculpturing of 45 species of Orchidinae and related taxa was investigated using scanning electron microscopy. To cover potential intraspecific variation, several accessions of the same species were examined. KEY RESULTS: Orchidinae show remarkable variation in exine sculpturing, with a different level of variation within species groups. In some genera, such as Serapias (rugulate) and Ophrys (psilate to verrucate), intrageneric uniformity corresponds well to a common pollination strategy and close relationships among species. However, little exine variability (psilate-scabrate and scabrate-rugulate) was also found in the genus Anacamptis in spite of striking differences in floral architecture and pollination strategies. A larger variety of exine conditions was found in genera Dactylorhiza (psilate, psilate-scabrate and reticulate) and Orchis s.s. (psilate, reticulate, perforate-rugulate and baculate) where no unequivocal correspondence can be found to either phylogenetic patterns or pollination strategies. CONCLUSIONS: Changes in pollen characteristics do not consistently reflect shifts in pollination strategy. A unique trend of exine evolution within Orchidinae is difficult to trace. However, the clades comprising Anacamptis, Neotinea, Ophrys and Serapias show psilate to rugulate or scabrate pollen, while that of the clade comprising Chamorchis, Dactylorhiza, Gymnadenia, Orchis s.s., Platanthera, Pseudorchis and Traunsteinera ranges from psilate to reticulate. Comparison of the data with exine micromorphology from members of the tribe Orchidieae and related tribes suggests a possible general trend from reticulate to psilate.     

Apoptosis in the treatment of cancer: a promise kept?

A common feature of cancer cells is their ability to evade apoptosis as a result of alterations that block cell death signaling pathways. The extensive research efforts that elucidated these signaling pathways over the past decade have set the stage for the development of therapeutic agents that either kill cancer cells selectively or reset their apoptotic threshold. Over the past two years a number of these agents have been evaluated in preclinical and clinical trials. The results of these studies suggest that it might soon be possible to modulate apoptosis in cancer cells for therapeutic benefit.     

The effect of temperature on sex ratio in the isopod Porcellionides pruinosus: Environmental sex determination or a by-product of cytoplasmic sex determination?

The sex ratios of the progenies of woodlice Porcellionides pruinosus (Crustacea, Isopoda) raised at different temperatures were studied. Females from three French populations sampled in the wild produced highly female-biased broods at 20°C and male-biased broods above 30°C. The effect of high temperature was not due to selective mortality of females. Sex determination was thus sensitive to temperature in P. pruinosus. We also found an interpopulation variability of sex ratio thermosensitivity and a weak inheritance of male-biased sex ratios at high temperatures. Samples taken from a wild population throughout the year showed that while the thermal conditions required for changes in the sex ratio occurred, there was no significant variation in the sex ratio. On the other hand, almost all the females and many males in the four populations studied harboured intracytoplasmic bacteria. These maternally inherited symbionts belong to the genus Wolbachia and are known to possess a feminizing effect. While in other arthropods Wolbachia are destroyed at high temperatures, the symbionts of P. pruinosus were detected by a PCR procedure whatever the rearing temperatures. In light of these results, we propose that the thermosensitivity of sex determination in P. pruinosus could reflect the removal of the cytoplasmic effect on sex determination rather than environmental sex determination sensu stricto. The reduction in the amount of bacteria (but not their entire elimination), or the inhibition of bacterial metabolism, may be responsible for sex ratio variations relating to temperature. The incomplete inheritance of male-biased sex ratios at high temperatures might reflect a selection of thermo-tolerant bacterial strains.     

Basking aggregations in the adder (Vipera berus): attraction to conspecific cues or to scarce suitable microhabitats?

Journal of Ethology - Snakes live a predominantly solitary and secretive life, and are most often considered to be “non-social”. We here provide quantitative information on the...     

Do pollinators determine hybridization patterns in sympatricSatyrium (Orchidaceae) species?

The breakdown of species integrity was studied in three sympatricSatyrium (Orchidaceae) species at a site in the Western Cape province of South Africa. Hybrids with intermediate morphology were identified using multivariate analysis.Satyrium erectum ×S. bicorne andS. erectum ×S. coriifolium hybrids were common, while only oneS. bicorne ×S. coriifolium hybrid was found (a new record). Reciprocal artificial crossing experiments confirmed that genetic compatibility exists between all three species. However, the species have different pollinators: moths inS. bicorne, bees inS. erectum and sunbirds inS. coriifolium. Pollinators, rather than genetic factors, therefore, appear to be responsible for reproductive isolation between these species. Breakdown in pollinator specificity results in interspecific pollen transfer and the formation of hybrids. Overlap in floral signals, including nocturnal scent in bothS. erectum andS. bicorne, and bright coloration in bothS. erectum andS. coriifolium, facilitates the formation of hybrids between these species pairs. The brown, scented flowers ofS. bicorne, on the other hand, share no obvious floral signals with the bright orange, unscented flowers ofS. coriifolium, accounting for the rarity of hybrids between this species pair. Differences in floral morphology also result in mechanical isolation between these two species. Since the three species are sympatric in a very small part of their geographic ranges, it is unlikely that avoidance of hybridization has been an important selective factor in the evolution of specialized pollination systems.     

Mammalian sex chromosomes: design or accident?

Mammalian sex chromosomes evolved (and are still evolving) from a homomorphic pair by the progressive loss of active genes from the Y chromosome. Among the changes that have accompanied this differentiation, it is difficult to determine causes, effects and correlates. Comparative studies suggest that the choice of a gene, and thus a chromosome pair, to control the sex-determining pathway may be quite arbitrary, and that sex chromosomes and sex-determining genes are more likely to be the products of random changes than the products of selection for function.     

Seed development: With or without sex?

In Arabidopsis, mutations in the genes FIE, FIS2 or MEDEA disrupt the normal dependence of seed development on fertilization. The products of these genes are similar to Polycomb group proteins, suggesting possible mechanisms for determining whether seeds can be produced sexually or asexually.     

Male-biased sex ratios in mature damselfly populations: real or artefact?

1. There is ongoing controversy about whether biased sex ratios in diploid insect populations are real or an artefact caused by different behaviours and/or different catchability of the sexes. This was tested by monitoring two field and three semi-natural populations of the damselfly Lestes sponsa. 2. Capture–mark–recapture data showed that population size estimates were about twice as large for males as for females at both field sites. Independent estimates of the sex ratios based on total numbers of males and females captured supported the male bias. 3. Males had higher recapture probabilities than females due to longer times between successive visits in females. Because the same pattern was found in the semi-natural populations, the longer intervals in females are no artefact due to their lower detectability. 4. Theoretical models show that the strong temporary emigration of females tends, if anything, to overestimate female population sizes and that the heterogeneity of recapture probabilities observed in males tends to underestimate male population sizes. Hence, behavioural differences between the sexes do not cause an artificially male-biased sex ratio. 5. Spatial data show that operational sex ratios are male biased at the pond but become female biased in the plots further away from the shoreline; however because of the decrease in densities away from the shoreline, this does not result in a global even sex ratio. 6. Spatial data, temporary emigration patterns, and independent estimates suggest strongly that the male-biased sex ratios in mature damselfly populations are real.