Floral isolation is the major reproductive barrier between a pair of rewarding orchid sister species

Gymnadenia winkeliana-A New Orchid Species to Romanian Flora

A novel species, Gymnadenia winkeliana, has been identified in the Bucegi Natural Park ROSCI0013, located in the Southern Carpathians of Central Romania. Two moderately sized populations of Gymnadenia winkeliana, totalling 120-140 individuals, were discovered inhabiting the alpine grasslands of the park, situated 2.000 m above sea level. To describe this newly found population as comprehensively as possible, 44 vegetative and floral organs/organ parts were directly studied and measured from living plants. Special attention was focused on the characteristics that proved to have taxonomic significance, particularly those involving distinctive details in the morphology of the leaves, perianth, labellum and gynostemium. A total of 223 characteristics were analysed encompassing the morphology of every organ of the plant, cytology and breeding system. Furthermore, comprehensive taxonomic treatment and description, accompanied by colour photographs illustrating the holotype, are provided. Voucher specimens were deposited at the Herbarium of the University of Agriculture and Veterinary Medicine, Bucharest (USAMVB Herbarium barcode: 40102, NEA); Gymnadenia winkeliana, a (micro)endemic species, is characterized as a putative allogamous, facultatively apomict that significantly differs from other Gymnadenia R.Br. species found in Romania. Notably, it distinguishes itself through its smaller habitus (reaching heights of up to 8-10 cm), its two-coloured, rounded/hemispherical inflorescence displaying a gradient of pink hues in an acropetal fashion (ranging from whitish-pink at the base to vivid-pink at the topmost flowers), and its limited distribution in high-altitude areas, encompassing approximately 8-10 km2 in the central area of the Bucegi Natural Park. This species has been under observation since 2005, with observed population numbers showing a significant increase over time, from ca. 50-55 (counted at the time of its discovery) to 120-140 individuals (counted in June 2023). Additionally, comprehensive information regarding the habitat, ecology, phenology and IUCN conservation assessments of Gymnadenia winkeliana are provided, including maps illustrating its distribution.

Reproductive isolation between two sympatric bat-pollinated Bauhinia (Leguminosae)

Several barriers contribute to reproductive isolation between plant species, which can be classified as pre- or post-pollination. Understanding the strength of these barriers could clarify the factors that maintain reproductive isolation and thus species integrity. In this study, we quantified reproductive isolation between two bat-pollinated co-occurring Bauhinia species (B. acuruana and B. pentandra) with similar flower morphology. Over the course of 18 months, we assessed reproductive isolation between these two Bauhinia species by quantifying the individual strengths and absolute contributions of five pre- and post- pollination barriers. Our data showed that both species are completely isolated in their reproduction by a combination of several barriers. Although they co-occur in a few populations, we found a high degree of geographic isolation between them. And although their flowering periods overlap, there is a significant difference in flowering peaks. Both species have the same pollinating bats, but the interspecific transfer of pollen between the plant species may be reduced due to the different length of the flower stamens, resulting in different pollen deposition on the bats' bodies. We have documented complete incompatibility between taxa and conclude that pre- and post-pollination barriers are important factors in preventing gene flow, even in contact zones between these two species of Bauhinia. We highlight that our work is the first study to use methods to estimate the strength of reproductive isolation barriers between bat-pollinated species.

Post-pollination barriers contribute to coexistence of partially pollinator-sharing Arisaema species (Araceae)

Reproductive isolation plays an important role in maintaining the species integrity of sympatric close relatives. For sympatric Arisaema species, interspecific gene flow is expected to be effectively prevented by pre-pollination barriers, particularly strong pollinator isolation mediated by fungus gnats. However, due to the lack of quantitative studies combining multiple pre- and post-pollination barriers, it is not known whether pre-pollination isolation is complete, and whether post-pollination barriers also contribute to reproductive isolation among some Arisaema species. In this study, we quantified the individual strengths and absolute contributions of four pre- and post-pollination barriers (phenological isolation, pollinator isolation, hybrid fruit formation, and hybrid seed formation) among three sympatric Arisaema species (A. bockii, A. lobatum, and A. erubescens). Although phenological isolation and pollinator isolation reduced the frequencies of interspecific pollen transfer among these species, the partial overlap of flowering times and pollinator assemblages resulted in incomplete pre-pollination isolation. Post-pollination barriers also contributed to reproductive isolation at the hybrid fruit and seed formation stages. We propose that, although pre-pollination barriers are expected to contribute more to total isolation than post-pollination barriers in Arisaema, pre-pollination barriers may not completely prevent interspecific pollen transfer among some Arisaema species. Post-pollination barriers, which are generally ignored, may also have contributed significantly to reproductive isolation in Arisaema.

Pollination mechanism in Serapias with no pollinaria reconfiguration

Orchidaceae, one of the most numerous families in the world's flora, have evolved various pollination strategies to favour cross-pollination, such as deceptive pollination and pollinarium reconfiguration. Among the terrestrial orchids of the Mediterranean, only species belonging to the genus Serapias show a strategy defined as shelter imitation. The floral elements form a tubular structure that insects use during their resting phases. The purpose of this article was to clarify the mechanisms that guarantee pollination with particular attention to the morphological interactions between orchids and pollinators and whether pollinaria reconfiguration is necessary in the promotion of cross-pollination in Serapias. Breeding system experiments and hand-pollination treatments indicated that Serapias was highly self-compatible, shows low value of natural fruit set and is pollinator limited. Time-lapse photos showed that the pollinarium had no refolding of the stipe or caudicle after its removal from the flower. The morphology of the flower determined the attack of the pollinarium on the occiput/vertex of insect. When the insect left the flower, the pollinarium was unable to encounter the stigma. When the insect made a second visit to another flower, the pollen masses of the first pollinarium ended up on the stigma and at the same time, the insect picked up a second pollinarium. Our observations and analyses suggested that morphological interactions between flower and pollinator are crucial to the success of pollination and to prevent self-pollination and thus that pollinarium reconfiguration is unnecessary in shelter deceptive orchids, such as Serapias species, for the promotion of cross-pollination. Serapias represent a case of interactions between plant and pollinator; the formation of the tubular shape of the flower is an essential preadaptation for the development of resting site mimicry originating exclusively in Serapias among Mediterranean orchids.

Fragrance components of Gymnadenia conopsea and Gymnadenia odoratissima collected at several sites in Italy and Germany

The SPME-GC-MS analysis of the aroma components of Gymnadenia conopsea subsp. conopsea, subsp. densiflora, var. alpina and Gymnadenia odoratissima var. odoratissima, var. idae were reported. The main components of in total 78 found in G. conopsea subsp. conopsea were elemicin, cis-9-hexadecenal, hexadecanal, isoelemicin and (Z)-11-hexadecen-1-ol acetate; in subsp. densiflora benzyl benzoate, eugenol and trans-isoeugenol; in var. alpina benzyl benzoate, methyleugenol and elemicin. In the scent of G. odoratissima var. odoratissima were found 2-phenylethyl acetate, eugenol and pentadecane, in var. idae mainly C15-C21 alkanes and C16, C18 carbonic acids and some isoprenoid-derivatives. As all tested Gymnadenia-taxa are allogamous, the differences in scent composition may play a role in pollinator attraction.

Reproductive Isolation Among Three Nocturnal Moth-Pollinated Sympatric Habenaria Species (Orchidaceae)

Comparison and quantification of multiple pre- and post-pollination barriers to interspecific hybridization are important to understand the factors promoting reproductive isolation. Such isolating factors have been studied recently in many flowering plant species which seek after the general roles and relative strengths of different pre- and post-pollination barriers. In this study, we quantified six isolating factors (ecogeographic isolation, phenological isolation, pollinator isolation, pollinia-pistil interactions, fruit production, and seed development) that could possibly be acting as reproductive barriers at different stages among three sympatric Habenaria species (H. limprichtii, H. davidii, and H. delavayi). These three species overlap geographically but occupy different microhabitats varying in soil water content. They were isolated through pollinator interactions both ethologically (pollinator preference) and mechanically (pollinia attachment site), but to a variable degree for different species pairs. Interspecific crosses between H. limprichtii and H. davidii result in high fruit set, and embryo development suggested weak post-pollination barriers, whereas bidirectional crosses of H. delavayi with either of the other two species fail to produce fruits. Our results revealed that pollinators were the most important isolating barrier including both ethological and mechanical mechanisms, to maintain the boundaries among these three sympatric Habenaria species. Our study also highlights the importance of a combination of pre-and post-pollination barriers for species co-existence in Orchidaceae.

Drivers of population divergence and species differentiation in a recent group of indigenous orchids (Vanilla spp.) in Madagascar

With over 25,000 species, orchids are among families with remarkable high rate of diversification. Since Darwin's time, major advances attributed the exceptional diversity of orchids to plant-pollinator interactions. However, unraveling the processes and factors that determine the phenotypic and genotypic variation of natural orchid populations remains a challenge. Here, we assessed genetic population structure and floral differentiation in recently diverged leafless Vanilla species in a world biodiversity hotspot, Madagascar, using seven microsatellite loci and 26 morphometric variables. Additionally, analyses were performed to test for the occurrence of any patterns of isolation by distance, isolation by environment, and isolation by adaptation and to detect possible physical barriers that might have caused genetic discontinuities between populations. Positive inbreeding coefficients detected in 22 populations were probably due to the presence of null alleles, geitonogamy and/or some admixture (sympatric species). In contrast, the only high-altitude population showed an important rate of clonality leading to heterozygote excess. Genetic diversity was maximum in western populations, suggesting a postglacial colonization to the north and south. Clustering analyses identified seven genetic groups characterized by specific floral traits that matched five botanical descriptions in the literature. A contribution of montane refugia and river barriers on population differentiation was detected. We also detected combined effects of IBD/IBE and IBE/IBA on genetic differentiation and suggested this pattern is more likely determined by ecological isolation, although pollinator-mediated divergent selection could not be ruled out for some of the species. Overall, this study provides further insights on speciation in orchids, a group for which Madagascar shows one of the world's highest level of endemism and confirms the importance of the peculiar biogeography of the island in shaping species differentiation.

Flower evolution in the presence of heterospecific gene flow and its contribution to lineage divergence

The success of species depends on their ability to exploit ecological resources in order to optimize their reproduction. However, species are not usually found within single-species ecosystems but in complex communities. Because of their genetic relatedness, closely related lineages tend to cluster within the same ecosystem, rely on the same resources, and be phenotypically similar. In sympatry, they will therefore compete for the same resources and, in the case of flowering plants, exchange their genes through heterospecific pollen transfer. These interactions, nevertheless, pose significant challenges to species co-existence because they can lead to resource limitation and reproductive interference. In such cases, divergent selective pressures on floral traits will favour genotypes that isolate or desynchronize the reproduction of sympatric lineages. The resulting displacement of reproductive characters will, in turn, lead to pre-mating isolation and promote intraspecific divergence, thus initiating or reinforcing the speciation process. In this review, we discuss the current theoretical and empirical knowledge on the influence of heterospecific pollen transfer on flower evolution, highlighting its potential to uncover the ecological and genomic constraints shaping the speciation process.

Asymmetries of reproductive isolation are reflected in directionalities of hybridization: integrative evidence on the complexity of species boundaries

The complex nature of species boundaries has been a central topic in evolutionary biology ever since Darwin. Despite numerous separate studies on reproductive isolation and hybridization, their relationship remains underinvestigated. Are the strengths and asymmetries of reproductive barriers reflected in the extent and directionalities of interspecific genetic exchange? We combined field, experimental, and molecular data to quantify strengths and asymmetries of sympatric reproductive barriers and hybridization between florally heteromorphic primroses. We also assessed whether generalist pollinators discriminate between different floral cues and contribute to reproductive isolation, a long-debated topic. Sympatric reproductive isolation is high but incomplete, and most phenotypic intermediates are genetic F₁ hybrids, whereas backcrosses are rare, revealing low interspecific gene flow. Species integrity rests on multiple barriers, but ethological isolation is among the strongest, demonstrating that even generalist pollinators crucially contribute to the maintenance of species boundaries. Furthermore, reproductive barriers are weaker for Primula veris and short-styled plants, results corroborated by molecular data. Thus, in florally heteromorphic systems, both species- and morph-dependent asymmetries affect permeability of species boundaries. Our study illustrates how the interactions between complex floral syndromes and pollinators shape species boundaries in unique, previously undescribed ways.

The Leafless Vanilla Species-Complex from the South-West Indian Ocean Region: A Taxonomic Puzzle and a Model for Orchid Evolution and Conservation Research

The pantropical genus Vanilla is a member of the Orchidaceae family, one of the largest and most diverse families of angiosperms in the world. It originated in America and differentiated in America, Africa, and Asia. About 132 species of Vanilla have now been accepted since the discovery of the genus. Within the Vanilla genus, leaflessness has appeared at least three independent times, in Africa, Asia, and America as an adaptation to drought conditions. The South-West Indian Ocean (SWIO) region, a biodiversity hotspot for orchids, is home to seven leafless species. Chloroplast markers failed to differentiate species within this recent SWIO clade. Morphological traits used for description are very similar between species and do not provide reliable identification. Moreover, some of the species have overlapping flowering periods and geographic distribution in Madagascar, increasing the probability of finding some sympatric species and, therefore, hybrids. Leafless Vanilla species from the SWIO islands are thus an excellent model for understanding the evolution of orchids. For their conservation, and to confirm the identity of these leafless species, an integrated approach with classical taxonomy using a large number of samples, intense fieldwork on biology and ecology, and molecular studies using variable markers is necessary.

Divergent selection on flowering phenology but not on floral morphology between two closely related orchids

Closely related species often differ in traits that influence reproductive success, suggesting that divergent selection on such traits contribute to the maintenance of species boundaries. Gymnadenia conopsea ss. and Gymnadenia densiflora are two closely related, perennial orchid species that differ in (a) floral traits important for pollination, including flowering phenology, floral display, and spur length, and (b) dominant pollinators. If plant-pollinator interactions contribute to the maintenance of trait differences between these two taxa, we expect current divergent selection on flowering phenology and floral morphology between the two species. We quantified phenotypic selection via female fitness in one year on flowering start, three floral display traits (plant height, number of flowers, and corolla size) and spur length, in six populations of G. conopsea s.s. and in four populations of G. densiflora. There was indication of divergent selection on flowering start in the expected direction, with selection for earlier flowering in two populations of the early-flowering G. conopsea s.s. and for later flowering in one population of the late-flowering G. densiflora. No divergent selection on floral morphology was detected, and there was no significant stabilizing selection on any trait in the two species. The results suggest ongoing adaptive differentiation of flowering phenology, strengthening this premating reproductive barrier between the two species. Synthesis: This study is among the first to test whether divergent selection on floral traits contribute to the maintenance of species differences between closely related plants. Phenological isolation confers a substantial potential for reproductive isolation, and divergent selection on flowering time can thus greatly influence reproductive isolation and adaptive differentiation.

A Phylogenomic Analysis of the Floral Transcriptomes of Sexually Deceptive and Rewarding European Orchids, Ophrys and Gymnadenia

The orchids (Orchidaceae) constitute one of the largest and most diverse families of flowering plants. They have evolved a great variety of adaptations to achieve pollination by a diverse group of pollinators. Many orchids reward their pollinators, typically with nectar, but the family is also well-known for employing deceptive pollination strategies in which there is no reward for the pollinator, in the most extreme case by mimicking sexual signals of pollinators. In the European flora, two examples of these different pollination strategies are the sexually deceptive genus Ophrys and the rewarding genus Gymnadenia, which differ in their level of pollinator specialization; Ophrys is typically pollinated by pseudo-copulation of males of a single insect species, whilst Gymnadenia attracts a broad range of floral visitors. Here, we present and describe the annotated floral transcriptome of Ophrys iricolor, an Andrena-pollinated representative of the genus Ophrys that is widespread throughout the Aegean. Furthermore, we present additional floral transcriptomes of both sexually deceptive and rewarding orchids, specifically the deceptive Ophrys insectifera, Ophrys aymoninii, and an updated floral transcriptome of Ophrys sphegodes, as well as the floral transcriptomes of the rewarding orchids Gymnadenia conopsea, Gymnadenia densiflora, Gymnadenia odoratissima, and Gymnadenia rhellicani (syn. Nigritella rhellicani). Comparisons of these novel floral transcriptomes reveal few annotation differences between deceptive and rewarding orchids. Since together, these transcriptomes provide a representative sample of the genus-wide taxonomic diversity within Ophrys and Gymnadenia (Orchidoideae: Orchidinae), we employ a phylogenomic approach to address open questions of phylogenetic relationships within the genera. Specifically, this includes the controversial placement of O. insectifera within the Ophrys phylogeny and the placement of "Nigritella"-type morphologies within the phylogeny of Gymnadenia. Whereas in Gymnadenia, several conflicting topologies are supported by a similar number of gene trees, a majority of Ophrys gene topologies clearly supports a placement of O. insectifera as sister to a clade containing O. sphegodes.

Reproductive isolation in alpine gingers: How do coexisting Roscoea (R. purpurea and R. tumjensis) conserve species integrity?

Multiple barriers may contribute to reproductive isolation between closely related species. Understanding the relative strength of these barriers can illuminate the ecological factors that currently maintain species integrity and how these factors originally promoted speciation. Two Himalayan alpine gingers, Roscoea purpurea and R. tumjensis, occur sympatrically in central Nepal and have such similar morphology that it is not clear whether or how they maintain a distinct identity. Our quantitative measurements of the components of reproductive isolation show that they are, in fact, completely isolated by a combination of phenological displacement of flowering, earlier for R. tumjensis and later for R. purpurea, and complete fidelity of visitation by different pollinator species, bumblebees for R. tumjensis and a long-tongued fly for R. purpurea. Furthermore, the nectar of R. tumjensis flowers is available to the shorter tongued bumblebees while R. purpurea nectar is less accessible, requiring deep probing from long-tongued flies. Although flowering phenology is a strong current barrier that seemingly obviates any need for pollinator discrimination, this current pattern need not reflect selective forces occurring at the initial divergence of R. tumjensis. There has been considerable pollinator switching during the radiation of the Himalayan Roscoea, and the association of flowering time with type of pollinator in these sympatric species may have originated among the earliest or latest flowering individuals or populations of an ancestor to exploit either bumblebee activity early in the breeding season or long-tongued fly abundance later in the season. These two sympatric Roscoea species add to accumulating evidence of the primacy of prezygotic pollination traits in speciation among angiosperms even in the absence of postzygotic incompatibility.

Restriction-site associated DNA sequencing supports a sister group relationship of Nigritella and Gymnadenia (Orchidaceae)

The orchid genus Nigritella is closely related to Gymnadenia and has from time to time been merged with the latter. Although Nigritella is morphologically distinct, it has been suggested that the separating characters are easily modifiable and subject to rapid evolutionary change. So far, molecular phylogenetic studies have either given support for the inclusion of Nigritella in Gymnadenia, or for their separation as different genera. To resolve this issue, we analysed data obtained from Restriction-site associated DNA sequencing, RADseq, which provides a large number of SNPs distributed across the entire genome. To analyse samples of different ploidies, we take an analytical approach of building a reduced genomic reference based on de novo RADseq loci reconstructed from diploid accessions only, which we further use to map and call variants across both diploid and polyploid accessions. We found that Nigritella is distinct from Gymnadenia forming a well-supported separate clade, and that genetic diversity within Gymnadenia is high. Within Gymnadenia, taxa characterized by an ITS-E ribotype (G. conopsea s.str. (early flowering) and G. odoratissima), are divergent from taxa characterized by ITS-L ribotype (G. frivaldii, G. densiflora and late flowering G. conopsea). Gymnigritella runei is confirmed to have an allopolyploid origin from diploid Gymnadenia conopsea and tetraploid N. nigra ssp. nigra on the basis of RADseq data. Within Nigritella the aggregation of polyploid members into three clear-cut groups as suggested by allozyme and nuclear microsatellite data was further supported.

The independent and combined effects of floral traits distinguishing two pollination ecotypes of a moth-pollinated orchid

Identifying traits and agents of selection involved in local adaptation is important for understanding population divergence. In southern Sweden, the moth-pollinated orchid Platanthera bifolia occurs as a woodland and a grassland ecotype that differ in dominating pollinators. The woodland ecotype is taller (expected to influence pollinator attraction) and produces flowers with longer spurs (expected to influence efficiency of pollen transfer) compared to the grassland ecotype. We examined whether plant height and spur length affect pollination and reproductive success in a woodland population, and whether effects are non-additive, as expected for traits influencing two multiplicative components of pollen transfer. We reduced plant height and spur length to match trait values observed in the grassland ecotype and determined the effects on pollen removal, pollen receipt, and fruit production. In addition, to examine the effects of naturally occurring variation, we quantified pollinator-mediated selection through pollen removal and seed production in the same population. Reductions of plant height and spur length decreased pollen removal, number of flowers receiving pollen, mean pollen receipt per pollinated flower, and fruit production per plant, but no significant interaction effect was detected. The selection analysis demonstrated pollinator-mediated selection for taller plants via female fitness. However, there was no current selection mediated by pollinators on spur length, and pollen removal was not related to plant height or spur length. The results show that, although both traits are important for pollination success and female fitness in the woodland habitat, only plant height was sufficiently variable in the study population for current pollinator-mediated selection to be detected. More generally, the results illustrate how a combination of experimental approaches can be used to identify both traits and agents of selection.

Orchid diversity: Spatial and climatic patterns from herbarium records

AIM

We test for spatial and climatic patterns of diversification in the Orchidaceae, an angiosperm family characterized by high levels of species diversity and rarity. Globally, does orchid diversity correlate with land area? In Australia, does diversity correlate with herbarium collecting effort, range size, or climate niche breadth? Where are Australia's orchids distributed spatially, in protected areas, and in climate space?

LOCATION

Global, then Australia.

METHODS

We compared orchid diversity with land area for continents and recognized orchid diversity hotspots. Then, we used cleaned herbarium records to compare collecting effort (for Australian Orchidaceae vs. all other plant families, and also among orchid genera). Spatial and climate distributions were mapped to determine orchids' coverage in the protected area network, range sizes, and niche breadths.

RESULTS

Globally, orchid diversity does not correlate with land area (depauperate regions are the subantarctic: 10 species, and northern North America: 394 species). Australian herbarium records and collecting effort generally reflect orchid species diversity (1,583 spp.), range sizes, and niche breadths. Orchids are restricted to 13% of Australia's landmass with 211 species absent from any protected areas. Species richness is the greatest in three biomes with high general biodiversity: Temperate (especially southwest and southeast Australia), Tropical, and Subtropical (coastal northern Queensland). Absence from the Desert is consistent with our realized climate niche-orchids avoid high temperature/low rainfall environments. Orchids have narrower range sizes than nonorchid species. Highly diverse orchid genera have narrower rainfall breadths than less diverse genera.

MAIN CONCLUSIONS

Herbarium data are adequate for testing hypotheses about Australian orchids. Distribution is likely driven by environmental factors. In contrast, diversification did not correlate with increases in range size, rainfall, or temperature breadths, suggesting speciation does not occur via invasion and local adaptation to new habitats. Instead, diversification may rely on access to extensive obligate symbioses with mycorrhizae and/or pollinators.

Impact of pre- and post-pollination barriers on pollen transfer and reproductive isolation among three sympatric Pedicularis (Orobanchaceae) species

Pedicularis is the largest genus in the Orobanchaceae (>300) with many species co-occurring and co-blooming in subalpine to alpine meadows in the Himalayas. Although it is well known that different Pedicularis species place pollen on different parts of the same bumblebee's body, thus reducing interspecific pollen transfer, it is not known whether post-pollination components also contribute to reproductive isolation (RI). In this study, we quantified the individual strengths and absolute contributions of six pre- and post-pollination components of RI between three sympatric species in two pairs; Pedicularis gruina × Pedicularis tenuisecta (gru × ten) and Pedicularis comptoniifolia × Pedicularis tenuisecta (com × ten). All three Pedicularis species shared the same Bombus species. Individual foragers showed a high, but incomplete, floral constancy for each species. Therefore, pre-pollination barriers were potentially 'leaky' as Bombus species showed a low but consistent frequency of interspecific visitation. The RI strength of pre-pollination was lower in com × ten than in gru × ten. In contrast, post-pollination barriers completely blocked gene flow between both sets of species pairs. Two post-pollination recognition sites were identified. Late acting rejection of interspecific pollen tube growth occurred in com♀ × ten♂, while seeds produced in bi-directional crosses of gru × ten failed to germinate. We propose that, although floral isolation based on pollen placement on pollinators in the genus Pedicularis is crucial to avoid interspecific pollen transfer, the importance of this mode of interspecific isolation may be exaggerated. Post-pollination barriers may play even larger roles for currently established populations of co-blooming and sympatric species in this huge genus in the Himalayas.

Contrasting lengths of Pelargonium floral nectar tubes result from late differences in rate and duration of growth

BACKGROUND AND AIMS

Much of morphological evolution in flowers has arisen from pollinator-mediated selection, often manifest as a match between the length of the pollinator's proboscis and the depth of tubular corollas or spurs. We investigate development, growth and homology of the unique nectar tube of Pelargonium, frequently described as 'a spur adnate to the pedicel'.

METHODS

We focused on two species. The nectar tube of P. ionidiflorum is three times longer than that of P. odoratissimum. Light and scanning electron microscopy were carried out, and daily growth measurements were used to compare nectar tube development and vascular patterns.

KEY RESULTS

Nectar tubes in both species are initiated centripetally to the dorsal sepal in a space created by lateral displacement of two antepetalous stamens. The cavity deepens through subsequent intercalary growth of the receptacle that proceeds at the same rate in both species until tubes reach approx. 10 mm in length. Differences in final nectar tube lengths arise via an increase in the rate and duration of growth of the receptacle that begins just before anthesis (floral opening) and continues for several days past anthesis in P. ionidiflorum but does not occur in P. odoratissimum. Epidermal cells of the dorsal surface of the nectar tube in P. ionidiflorum are approx. 1.6 times longer than those in P. odoratissimum. Histological sections show no evidence that the nectar tube is a spur that became evolutionarily fused to the pedicel.

CONCLUSIONS

Nectar tubes in Pelargonium are localized cavities that form in the receptacle via intercalary growth. Differences in the rate and duration of growth just prior to and following anthesis underlie differences in final tube lengths. Because differences in cell lengths do not fully account for differences in nectar tube lengths, evolutionary diversification must involve changes in both cell cycle and cell expansion.

Multiple factors contribute to reproductive isolation between two co-existing Habenaria species (Orchidaceae)

Reproductive isolation is a key feature that forms barriers to gene flow between distinct plants. In orchids, prezygotic reproductive isolation has been considered to be strong, because their associations with highly specific pollinators. In this study, the reproductive ecology and reproductive isolation of two sympatric Habenaria species, H. davidii and H. fordii, was investigated by floral phenology and morphology, hand-pollination experiments and visitor observation in southwest China. The two species were dependent on insects for pollination and completely self-compatible. A number of factors have been identified to limit gene flow between the two species and achieved full reproductive isolation. Ecogeographic isolation was a weak barrier. H. fordii and H. davidii had completely overlapped flowering periods, and floral morphology plays an important role in floral isolation. The two species shared the same hawkmoth pollinator, Cechenena lineosa, but the pollinaria of the two orchids were attached on different body parts of pollinators. Prezygotic isolation was not complete, but the interspecific pollination treatments of each species resulted in no seed sets, indicating that unlike many other orchid species, in which the postzygotic reproductive isolation is very weak or complete absence, the post-zygotic isolation strongly acted in the stage of seed production between two species. The results illustrate the reproductive isolation between two species involves multiple plant life-history stages and a variety of reproductive barriers can contribute to overall isolation.

Functional differentiation in pollination processes among floral traits in Serapias species (Orchidaceae)

Floral displays, influencing attractiveness to insects, increase the number of pollinator visits and the efficiency of each visit in terms of pollen exchange and thus affect the plant reproductive success. Here, we conducted an in situ manipulation experiment to investigate whether the floral modifications affect reproductive success in natural orchid populations of Serapias lingua and Serapias vomeracea. We estimated male and female reproductive success of three treatment groups, disassembly of floral tube, cutting of lip, and painting of the callus surface, in terms of pollinaria removed/deposited and fruit production. Results revealed that phenotypic modification had opposite effects on reproductive success of two examine species. Indeed, reproductive success was significantly increased by the detached of the petals and sepals, and decreased, due to callus painting and lip removal, in S. lingua. On the contrary, unmanipulated plants of S. vomeracea showed significantly higher value of pollinaria removed and deposited and fruit set than manipulated ones. The differences between S. lingua and S. vomeracea agree to the different pollination strategy of examined species. S. vomeracea shows shelter imitation strategy, and thus, the disassembly of tunnel-like corolla does not allow the insects to use the flower as a refuge, while S. lingua is a sexually deceptive orchid and therefore the opening of the flower made more visible callus (visible at a greater distance) increasing the pollinators attraction. This study provides evidence that pollinators were largely sensitive to the experimental modification of the flower phenotype, which is consistent with the presence of significant selection on individual floral characters. Our experimental investigations of the effects of variation in display on pollinator visitation provide insights into the evolution of floral morphology in orchid with shelter imitation strategy.

Floral scent and species divergence in a pair of sexually deceptive orchids

Speciation is typically accompanied by the formation of isolation barriers between lineages. Commonly, reproductive barriers are separated into pre‐ and post‐zygotic mechanisms that can evolve with different speed. In this study, we measured the strength of different reproductive barriers in two closely related, sympatric orchids of the Ophrys insectifera group, namely Ophrys insectifera and Ophrys aymoninii to infer possible mechanisms of speciation. We quantified pre‐ and post‐pollination barriers through observation of pollen flow, by performing artificial inter‐ and intraspecific crosses and analyzing scent bouquets. Additionally, we investigated differences in mycorrhizal fungi as a potential extrinsic factor of post‐zygotic isolation. Our results show that floral isolation mediated by the attraction of different pollinators acts apparently as the sole reproductive barrier between the two orchid species, with later‐acting intrinsic barriers seemingly absent. Also, the two orchids share most of their fungal mycorrhizal partners in sympatry, suggesting little or no importance of mycorrhizal symbiosis in reproductive isolation. Key traits underlying floral isolation were two alkenes and wax ester, present predominantly in the floral scent of O. aymoninii. These compounds, when applied to flowers of O. insectifera, triggered attraction and a copulation attempt of the bee pollinator of O. aymoninii and thus led to the (partial) breakdown of floral isolation. Based on our results, we suggest that adaptation to different pollinators, mediated by floral scent, underlies species isolation in this plant group. Pollinator switches may be promoted by low pollination success of individuals in dense patches of plants, an assumption that we also confirmed in our study.