Mutualistic ants contribute to tank-bromeliad nutrition

Background and Aims

Epiphytism imposes physiological constraints resulting from the lack of access to the nutrient sources available to ground-rooted plants. A conspicuous adaptation in response to that lack is the phytotelm (plant-held waters) of tank-bromeliad species that are often nutrient-rich. Associations with terrestrial invertebrates also result in higher plant nutrient acquisition. Assuming that tank-bromeliads rely on reservoir-assisted nutrition, it was hypothesized that the dual association with mutualistic ants and the phytotelm food web provides greater nutritional benefits to the plant compared with those bromeliads involved in only one of these two associations.

Methods

Quantitative (water volume, amount of fine particulate organic matter, predator/prey ratio, algal density) and qualitative variables (ant-association and photosynthetic pathways) were compared for eight tank- and one tankless-bromeliad morphospecies from French Guiana. An analysis was also made of which of these variables affect nitrogen acquisition (leaf N and δ¹⁵N).

Key Results

All variables were significantly different between tank-bromeliad species. Leaf N concentrations and leaf δ¹⁵N were both positively correlated with the presence of mutualistic ants. The amount of fine particulate organic matter and predator/prey ratio had a positive and negative effect on leaf δ¹⁵N, respectively. Water volume was positively correlated with leaf N concentration whereas algal density was negatively correlated. Finally, the photosynthetic pathway (C₃ vs. CAM) was positively correlated with leaf N concentration with a slightly higher N concentration for C₃-Tillandsioideae compared with CAM-Bromelioideae.

Conclusions

The study suggests that some of the differences in N nutrition between bromeliad species can be explained by the presence of mutualistic ants. From a nutritional standpoint, it is more advantageous for a bromeliad to use myrmecotrophy via its roots than to use carnivory via its tank. The results highlight a gap in our knowledge of the reciprocal interactions between bromeliads and the various trophic levels (from bacteria to large metazoan predators) that intervene in reservoir-assisted nutrition.     

Spider-fed bromeliads: seasonal and interspecific variation in plant performance

Background and Aims

Several animals that live on bromeliads can contribute to plant nutrition through nitrogen provisioning (digestive mutualism). The bromeliad-living spider Psecas chapoda (Salticidae) inhabits and breeds on Bromelia balansae in regions of South America, but in specific regions can also appear on Ananas comosus (pineapple) plantations and Aechmea distichantha.

Methods

Using isotopic and physiological methods in greenhouse experiments, the role of labelled (¹⁵N) spider faeces and Drosophila melanogaster flies in the nutrition and growth of each host plant was evaluated, as well as seasonal variation in the importance of this digestive mutualism.

Key Results

Spiders contributed 0·6 ± 0·2 % (mean ± s.e.; dry season) to 2·7 ± 1 % (wet season) to the total nitrogen in B. balansae, 2·4 ± 0·4 % (dry) to 4·1 ± 0·3 % (wet) in An. comosus and 3·8 ± 0·4 % (dry) to 5 ± 1 % (wet) in Ae. distichantha. In contrast, flies did not contribute to the nutrition of these bromeliads. Chlorophylls and carotenoid concentrations did not differ among treatments. Plants that received faeces had higher soluble protein concentrations and leaf growth (RGR) only during the wet season.

Conclusions

These results indicate that the mutualism between spiders and bromeliads is seasonally restricted, generating a conditional outcome. There was interspecific variation in nutrient uptake, probably related to each species'' performance and photosynthetic pathways. Whereas B. balansae seems to use nitrogen for growth, Ae. distichantha apparently stores nitrogen for stressful nutritional conditions. Bromeliads absorbed more nitrogen coming from spider faeces than from flies, reinforcing the beneficial role played by predators in these digestive mutualisms.     

Variation in the functioning of autonomous self-pollination, pollinator services and floral traits in three Centaurium species

Background and Aims

Reproductive assurance through autonomous selfing is thought to be one of the main advantages of self-fertilization in plants. Floral mechanisms that ensure autonomous seed set are therefore more likely to occur in species that grow in habitats where pollination is scarce and/or unpredictable.

Methods

Emasculation and pollen supplementation experiments were conducted under laboratory conditions to investigate the capacity for, and timing of autonomous selfing in three closely related Centaurium species (Centaurium erythraea, C. littorale and C. pulchellum). In addition, observations of flower visitors were combined with emasculation and pollen addition experiments in natural populations to investigate the degree of pollinator limitation and pollination failure and to assess the extent to which autonomous selfing conferred reproductive assurance.

Results

All three species were capable of autonomous selfing, although this capacity differed significantly between species (index of autonomous selfing 0·55 ± 0·06, 0·68 ± 0·09 and 0·92 ± 0·03 for C. erythraea, C. littorale and C. pulchellum, respectively). The efficiency and timing of autogamous selfing was primarily associated with differences in the degree of herkogamy and dichogamy. The number of floral visitors showed significant interspecific differences, with 1·6 ± 0·6, 5·4 ± 0·6 and 14·5 ± 2·1 floral visitors within a 2 × 2 m² plot per 20-min observation period, for C. pulchellum, C. littorale and C. erythraea, respectively. Concomitantly, pollinator failure was highest in C. pulchellum and lowest in C. erythraea. Nonetheless, all three study species showed very low levels of pollen limitation (index of pollen limitation 0·14 ± 0·03, 0·11 ± 0·03 and 0·09 ± 0·02 for C. erythraea, C. littorale and C. pulchellum, respectively), indicating that autonomous selfing may guarantee reproductive assurance.

Conclusions

These findings show that limited availability of pollinators may select for floral traits and plant mating strategies that lead to a system of reproductive assurance via autonomous selfing.     

The presence of co-flowering species facilitates reproductive success of Pedicularis monbeigiana (Orobanchaceae) through variation in bumble-bee foraging behaviour

Background and Aims The presence of co-flowering species can alter pollinator foraging behaviour and, in turn, positively or negatively affect the reproductive success of the focal species. Such interactions were investigated between a focal species, Pedicularis monbeigiana, and a co-flowering species, Vicia dichroantha, which was mediated by behaviour alteration of the shared bumble-bee pollinator. Methods Floral display size and floral colour change of P. monbeigiana were compared between pure (P. monbeigiana only) and mixed (P. monbeigiana and V. dichroantha) plots in two populations. Pollinator visitation rates, interspecific floral switching and successive within-plant pollinator visits were recorded. In addition, supplemental pollination at plant level was performed, and the fruit set and seed set were analysed in pure and mixed plots with different densities of P. monbeigiana. Key Results Pollinator visitation rates were dramatically higher in mixed plots than in pure plots. The higher pollinator visitation rates were recorded in both low- and high-density plots. In particular, successive flower visits within an individual plant were significantly lower in mixed plots. Supplemental pollination significantly increased fruit set and seed set of individuals in pure plots, while it only marginally increased seed set per fruit of plants in mixed plots. Conclusions The presence of V. dichroantha can facilitate pollination and increase female reproductive success of P. monbeigiana via both quantity (mitigating pollinator limitation) and quality (reducing geitonogamy) effects. This study suggests that successive pollinator movements among flowers within a plant, as well as pollinator visitation rates and interspecific flower switching, may be important determinants of the direction and mechanisms of interaction between species.     

Functional evidence for physiological mechanisms to circumvent neurotoxicity of cardenolides in an adapted and a non-adapted hawk-moth species

Because cardenolides specifically inhibit the Na⁺K⁺-ATPase, insects feeding on cardenolide-containing plants need to circumvent this toxic effect. Some insects such as the monarch butterfly rely on target site insensitivity, yet other cardenolide-adapted lepidopterans such as the oleander hawk-moth, Daphnis nerii, possess highly sensitive Na⁺K⁺-ATPases. Nevertheless, larvae of this species and the related Manduca sexta are insensitive to injected cardenolides. By radioactive-binding assays with nerve cords of both species, we demonstrate that the perineurium surrounding the nervous tissue functions as a diffusion barrier for a polar cardenolide (ouabain). By contrast, for non-polar cardenolides such as digoxin an active efflux carrier limits the access to the nerve cord. This barrier can be abolished by metabolic inhibitors and by verapamil, a specific inhibitor of P-glycoproteins (PGPs). This supports that a PGP-like transporter is involved in the active cardenolide-barrier of the perineurium. Tissue specific RT-PCR demonstrated expression of three PGP-like genes in hornworm nerve cords, and immunohistochemistry further corroborated PGP expression in the perineurium. Our results thus suggest that the lepidopteran perineurium serves as a diffusion barrier for polar cardenolides and provides an active barrier for non-polar cardenolides. This may explain the high in vivo resistance to cardenolides observed in some lepidopteran larvae, despite their highly sensitive Na⁺K⁺-ATPases.     

First record of bat-pollination in the species-rich genus Tillandsia (Bromeliaceae)

Background and Aims

Bromeliaceae is a species-rich neotropical plant family that uses a variety of pollinators, principally vertebrates. Tillandsia is the most diverse genus, and includes more than one-third of all bromeliad species. Within this genus, the majority of species rely on diurnal pollination by hummingbirds; however, the flowers of some Tillandsia species show some characteristics typical for pollination by nocturnal animals, particularly bats and moths. In this study an examination is made of the floral and reproductive biology of the epiphytic bromeliad Tillandsia macropetala in a fragment of humid montane forest in central Veracruz, Mexico.

Methods

The reproductive system of the species, duration of anthesis, production of nectar and floral scent, as well as diurnal and nocturnal floral visitors and their effectiveness in pollination were determined.

Key Results

Tillandsia macropetala is a self-compatible species that achieves a higher fruit production through outcrossing. Nectar production is restricted to the night, and only nocturnal visits result in the development of fruits. The most frequent visitor (75 % of visits) and the only pollinator of this bromeliad (in 96 % of visits) was the nectarivorous bat Anoura geoffroyi (Phyllostomidae: Glossophaginae).

Conclusions

This is the first report of chiropterophily within the genus Tillandsia. The results on the pollination biology of this bromeliad suggest an ongoing evolutionary switch from pollination by birds or moths to bats.     

Mycoheterotrophy evolved from mixotrophic ancestors: evidence in Cymbidium (Orchidaceae)

Background and Aims

Nutritional changes associated with the evolution of achlorophyllous, mycoheterotrophic plants have not previously been inferred with robust phylogenetic hypotheses. Variations in heterotrophy in accordance with the evolution of leaflessness were examined using a chlorophyllous–achlorophyllous species pair in Cymbidium (Orchidaceae), within a well studied phylogenetic background.

Methods

To estimate the level of mycoheterotrophy in chlorophyllous and achlorophyllous Cymbidium, natural ¹³C and ¹⁵N contents (a proxy for the level of heterotrophy) were measured in four Cymbidium species and co-existing autotrophic and mycoheterotrophic plants and ectomycorrhizal fungi from two Japanese sites.

Key Results

δ¹³C and δ¹⁵N values of the achlorophyllous C. macrorhizon and C. aberrans indicated that they are full mycoheterotrophs. δ¹³C and δ¹⁵N values of the chlorophyllous C. lancifolium and C. goeringii were intermediate between those of reference autotrophic and mycoheterotrophic plants; thus, they probably gain 30–50 % of their carbon resources from fungi. These data suggest that some chlorophyllous Cymbidium exhibit partial mycoheterotrophy (= mixotrophy).

Conclusions

It is demonstrated for the first time that mycoheterotrophy evolved after the establishment of mixotrophy rather than through direct shifts from autotrophy to mycoheterotrophy. This may be one of the principal patterns in the evolution of mycoheterotrophy. The results also suggest that the establishment of symbiosis with ectomycorrhizal fungi in the lineage leading to mixotrophic Cymbidium served as pre-adaptation to the evolution of the mycoheterotrophic species. Similar processes of nutritional innovations probably occurred in several independent orchid groups, allowing niche expansion and radiation in Orchidaceae, probably the largest plant family.     

Extinction and ecological retreat in a community of primates

The lemurs of Madagascar represent a prodigious adaptive radiation. At least 17 species ranging from 11 to 160 kg have become extinct during the past 2000 years. The effect of this loss on contemporary lemurs is unknown. The concept of competitive release favours the expansion of living species into vacant niches. Alternatively, factors that triggered the extinction of some species could have also reduced community-wide niche breadth. Here, we use radiocarbon and stable isotope data to examine temporal shifts in the niches of extant lemur species following the extinction of eight large-bodied species. We focus on southwestern Madagascar and report profound isotopic shifts, both from the time when now-extinct lemurs abounded and from the time immediately following their decline to the present. Unexpectedly, the past environments exploited by lemurs were drier than the protected (albeit often degraded) riparian habitats assumed to be ideal for lemurs today. Neither competitive release nor niche contraction can explain these observed trends. We develop an alternative hypothesis: ecological retreat, which suggests that factors surrounding extinction may force surviving species into marginal or previously unfilled niches.     

Stable isotopes, ecological integration and environmental change: wolves record atmospheric carbon isotope trend better than tree rings

Large-scale patterns of isotope ratios are detectable in the tissues of organisms, but the variability in these patterns often obscures detection of environmental trends. We show that plants and animals at lower trophic levels are relatively poor indicators of the temporal trend in atmospheric carbon isotope ratios (delta13C) when compared with animals at higher trophic levels. First, we tested how differences in atmospheric delta13C values were transferred across three trophic levels. Second, we compared contemporary delta13C trends (1961-2004) in atmospheric CO2 to delta13C patterns in a tree species (jack pine, Pinus banksiana), large herbivore (moose, Alces alces) and large carnivore (grey wolf, Canis lupus) from North America. Third, we compared palaeontological (approx. 30000 to 12000 14C years before present) atmospheric CO2 trends to delta13C patterns in a tree species (Pinus flexilis, Juniperus sp.), a megaherbivore (bison, Bison antiquus) and a large carnivore (dire wolf, Canis dirus) from the La Brea tar pits (southern California, USA) and Great Basin (western USA). Contrary to previous expectations, we found that the environmental isotope pattern is better represented with increasing trophic level. Our results indicate that museum specimens of large carnivores would best reflect large-scale spatial and temporal patterns of carbon isotopes in the palaeontological record because top predators can act as ecological integrators of environmental change.     

Relationships between functional traits and inorganic nitrogen acquisition among eight contrasting European grass species

Backgrounds and Aims Leaf functional traits have been used as a basis to categoize plants across a range of resource-use specialization, from those that conserve available resources to those that exploit them. However, the extent to which the leaf functional traits used to define the resource-use strategies are related to root traits and are good indicators of the ability of the roots to take up nitrogen (N) are poorly known. This is an important question because interspecific differences in N uptake have been proposed as one mechanism by which species’ coexistence may be determined. This study therefore investigated the relationships between functional traits and N uptake ability for grass species across a range of conservative to exploitative resource-use strategies.Methods Root uptake of NH4+ and NO3–, and leaf and root functional traits were measured for eight grass species sampled at three grassland sites across Europe, in France, Austria and the UK. Species were grown in hydroponics to determine functional traits and kinetic uptake parameters (I_max and K_m) under standardized conditions.Key Results Species with high specific leaf area (SLA) and shoot N content, and low leaf and root dry matter content (LDMC and RDMC, respectively), which are traits associated with the exploitative syndrome, had higher uptake and affinity for both N forms. No trade-off was observed in uptake between the two forms of N, and all species expressed a higher preference for NH4+.Conclusions The results support the use of leaf traits, and especially SLA and LDMC, as indicators of the N uptake ability across a broad range of grass species. The difficulties associated with assessing root properties are also highlighted, as root traits were only weakly correlated with leaf traits, and only RDMC and, to a lesser extent, root N content were related to leaf traits.     

Sensitivity of growth and biomass allocation patterns to increasing nitrogen: a comparison between ephemerals and annuals in the Gurbantunggut Desert,north-western China

Background and Aims

Biomass accumulation and allocation patterns are critical to quantifying ecosystem dynamics. However, these patterns differ among species, and they can change in response to nutrient availability even among genetically related individuals. In order to understand this complexity further, this study examined three ephemeral species (with very short vegetative growth periods) and three annual species (with significantly longer vegetative growth periods) in the Gurbantunggut Desert, north-western China, to determine their responses to different nitrogen (N) supplements under natural conditions.

Methods

Nitrogen was added to the soil at rates of 0, 0·5, 1·0, 3·0, 6·0 and 24·0 g N m⁻² year⁻¹. Plants were sampled at various intervals to measure relative growth rate and shoot and root dry mass.

Key Results

Compared with annuals, ephemerals grew more rapidly, increased shoot and root biomass with increasing N application rates and significantly decreased root/shoot ratios. Nevertheless, changes in the biomass allocation of some species (i.e. Erodium oxyrrhynchum) in response to the N treatment were largely a consequence of changes in overall plant size, which was inconsistent with an optimal partitioning model. An isometric log shoot vs. log root scaling relationship for the final biomass harvest was observed for each species and all annuals, while pooled data of three ephemerals showed an allometric scaling relationship.

Conclusions

These results indicate that ephemerals and annuals differ observably in their biomass allocation patterns in response to soil N supplements, although an isometric log shoot vs. log root scaling relationship was maintained across all species. These findings highlight that different life history strategies behave differently in response to N application even when interspecific scaling relationships remain nearly isometric.     

The role of multiple partners in a digestive mutualism with a protocarnivorous plant

Background and aims

The protocarnivorous plant Paepalanthus bromelioides (Eriocaulaceae) is similar to bromeliads in that this plant has a rosette-like structure that allows rainwater to accumulate in leaf axils (i.e. phytotelmata). Although the rosettes of P. bromelioides are commonly inhabited by predators (e.g. spiders), their roots are wrapped by a cylindrical termite mound that grows beneath the rosette. In this study it is predicted that these plants can derive nutrients from recycling processes carried out by termites and from predation events that take place inside the rosette. It is also predicted that bacteria living in phytotelmata can accelerate nutrient cycling derived from predators.

Methods

The predictions were tested by surveying plants and animals, and also by performing field experiments in rocky fields from Serra do Cipó, Brazil, using natural abundance and enriched isotopes of ¹⁵N. Laboratory bioassays were also conducted to test proteolytic activities of bacteria from P. bromelioides rosettes.

Key Results

Analyses of ¹⁵N in natural nitrogen abundances showed that the isotopic signature of P. bromelioides is similar to that of carnivorous plants and higher than that of non-carnivorous plants in the study area. Linear mixing models showed that predatory activities on the rosettes (i.e. spider faeces and prey carcass) resulted in overall nitrogen contributions of 26·5 % (a top-down flux). Although nitrogen flux was not detected from termites to plants via decomposition of labelled cardboard, the data on ¹⁵N in natural nitrogen abundance indicated that 67 % of nitrogen from P. bromelioides is derived from termites (a bottom-up flux). Bacteria did not affect nutrient cycling or nitrogen uptake from prey carcasses and spider faeces.

Conclusions

The results suggest that P. bromelioides derive nitrogen from associated predators and termites, despite differences in nitrogen cycling velocities, which seem to have been higher in nitrogen derived from predators (leaves) than from termites (roots). This is the first study that demonstrates partitioning effects from multiple partners in a digestion-based mutualism. Despite most of the nitrogen being absorbed through their roots (via termites), P. bromelioides has all the attributes necessary to be considered as a carnivorous plant in the context of digestive mutualism.     

Early Zn2+-induced effects on membrane potential account for primary heavy metal susceptibility in tolerant and sensitive Arabidopsis species

Background and Aims

Uptake of heavy metals by plant root cells depends on electro-physiological parameters of the plasma membrane. In this study, responses of the plasma membrane in root cells were analysed where early reactions to the metal ion-induced stress are localized. Three different Arabidopsis species with diverse strategies of their adaptation to heavy metals were compared: sensitive Arabidopsis thaliana and tolerant A. halleri and A. arenosa.

Methods

Plants of A. thaliana Col-0 ecotype and plants of A. arenosa and A. halleri originating from natural metallicolous populations were exposed to high concentrations of Zn²⁺. Plants were tested for root growth rate, cellular tolerance, plant morphology and cell death in the root apex. In addition, the membrane potential (E_M) of mature cortical root cells and changes in the pH of the liquid culture media were measured.

Key Results

Primary roots of A. halleri and A. arenosa plants grew significantly better at increased Zn²⁺ concentrations than A. thaliana plants. Elevated Zn²⁺ concentrations in the culture medium induced rapid changes in E_M. The reaction was species-specific and concentration-dependent. Arabidopsis halleri revealed the highest insensitivity of the plasma membrane and the highest survival rate under prolonged treatment with extra-high concentrations. Plants were able to effectively adjust the pH in the control, but much less at Zn²⁺-induced lower pH.

Conclusions

The results indicate a similar mode of early reaction to Zn²⁺, but with different extent in tolerant and sensitive species of Arabidopsis. The sensitivity of A. thaliana and a high tolerance of A. halleri and A. arenosa were demonstrated. Plasma membrane depolarization was lowest in the hyperaccumulator A. halleri and highest in A. thaliana. This indicates that rapid membrane voltage changes are an excellent tool to monitor the effects of heavy metals.     

Ribosomal DNA organization before and after magnification in Drosophila melanogaster

In all eukaryotes, the ribosomal RNA genes are stably inherited redundant elements. In Drosophila melanogaster, the presence of a Ybb(-) chromosome in males, or the maternal presence of the Ribosomal exchange (Rex) element, induces magnification: a heritable increase of rDNA copy number. To date, several alternative classes of mechanisms have been proposed for magnification: in situ replication or extra-chromosomal replication, either of which might act on short or extended strings of rDNA units, or unequal sister chromatid exchange. To eliminate some of these hypotheses, none of which has been clearly proven, we examined molecular-variant composition and compared genetic maps of the rDNA in the bb(2) mutant and in some magnified bb(+) alleles. The genetic markers used are molecular-length variants of IGS sequences and of R1 and R2 mobile elements present in many 28S sequences. Direct comparison of PCR products does not reveal any particularly intensified electrophoretic bands in magnified alleles compared to the nonmagnified bb(2) allele. Hence, the increase of rDNA copy number is diluted among multiple variants. We can therefore reject mechanisms of magnification based on multiple rounds of replication of short strings. Moreover, we find no changes of marker order when pre- and postmagnification maps are compared. Thus, we can further restrict the possible mechanisms to two: replication in situ of an extended string of rDNA units or unequal exchange between sister chromatids.     

Herbivory-induced mortality increases with radial growth in an invasive riparian phreatophyte

Background and Aims

Under equal conditions, plants that allocate a larger proportion of resources to growth must do so at the expense of investing fewer resources to storage. The critical balance between growth and storage leads to the hypothesis that in high-resource environments, plants that express high growth rates are more susceptible to episodic disturbance than plants that express lower growth rates.

Methods

This hypothesis was tested by measuring the radial growth, basal area increment (BAI) and carbon isotope ratios (δ¹³C) in tree-ring α-cellulose of 62 mature tamarisk trees (Tamarix spp.) occurring at three sites in the western USA (n = 31 live and 31 killed trees across all sites, respectively). All of the trees had been subjected to periods of complete foliage loss by episodic herbivory over three or more consecutive growing seasons by the tamarisk leaf beetle (Diorhabda carinulata), resulting in approx. 50 % mortality at each site.

Key Results

Mean annual BAI (measured from annual ring widths) in the 10 years prior to the onset of herbivory was on average 45 % higher in killed trees compared with live trees (P < 0·0001). Killed trees that had higher growth rates also expressed higher (less negative) δ¹³C ratios compared with live trees. In fact, at one site near Moab, UT, the mean annual BAI was 100 % higher in killed trees despite having about a 0·5 ‰ higher δ¹³C relative to live trees (P = 0·0008). Patterns of δ¹³C suggest that the intrinsic water-use efficiency was higher in killed than surviving trees, possibly as a consequence of lower whole-canopy stomatal conductance relative to live trees.

Conclusions

The results show that a likely trade-off occurs between radial growth and survival from foliage herbivory in Tamarix spp. that currently dominates riparian areas throughout the western USA and northern Mexico. Thus, herbivory by D. carinulata may reduce the overall net primary productivity of surviving Tamarix trees and may result in a reduction in genetic variability in this dominant invasive tree species if these allocation patterns are adaptive.     

Identification and characterization of improved nitrogen efficiency in interspecific hybridized new-type Brassica napus

Background and Aims

Oilseed rape (Brassica napus) is an important oil crop worldwide. The aim of this study was to identify the variation in nitrogen (N) efficiency of new-type B. napus (genome A^rA^rC^cC^c) genotypes, and to characterize some critical physiological and molecular mechanisms in response to N limitation.

Methods

Two genotypes with contrasting N efficiency (D4-15 and D1-1) were identified from 150 new-type B. napus lines, and hydroponic and pot experiments were conducted. Root morphology, plant biomass, N uptake parameters and seed yield of D4-15 and D1-1 were investigated. Two traditional B. napus (genome AⁿAⁿCⁿCⁿ) genotypes, QY10 and NY7, were also cultivated. Introgression of exotic genomic components in D4-15 and D1-1 was evaluated with molecular markers.

Key Results

Large genetic variation existed among traits contributing to the N efficiency of new-type B. napus. Under low N levels at the seedling stage, the N-efficient new-type D4-15 showed higher values than the N-inefficient D1-1 line and the traditional B. napus QY10 and NY7 genotypes with respect to several traits, including root and shoot biomass, root morphology, N accumulation, N utilization efficiency (NutE), N uptake efficiency (NupE), activities of nitrate reductase (NR) and glutamine synthetase (GS), and expression levels of N transporter genes and genes that are involved in N assimilation. Higher yield was produced by the N-efficient D4-15 line compared with the N-inefficient D1-1 at maturity. More exotic genome components were introgressed into the genome of D4-15 (64·97 %) compared with D1-1 (32·23 %).

Conclusions

The N-efficient new-type B. napus identified in this research had higher N efficiency (and tolerance to low-N stress) than traditional B. napus cultivars, and thus could have important potential for use in breeding N-efficient B. napus cultivars in the field.     

Floral odour chemistry defines species boundaries and underpins strong reproductive isolation in sexually deceptive orchids

Background and Aims

The events leading to speciation are best investigated in systems where speciation is ongoing or incomplete, such as incipient species. By examining reproductive barriers among incipient sister taxa and their congeners we can gain valuable insights into the relative timing and importance of the various barriers involved in the speciation process. The aim of this study was to identify the reproductive barriers among sexually deceptive orchid taxa in the genus Chiloglottis.

Methods

The study targeted four closely related taxa with varying degrees of geographic overlap. Chemical, morphological and genetic evidence was combined to explore the basis of reproductive isolation. Of primary interest was the degree of genetic differentiation among taxa at both nuclear and chloroplast DNA markers. To objectively test whether or not species boundaries are defined by the chemistry that controls pollinator specificity, genetic analysis was restricted to samples of known odour chemistry.

Key Results

Floral odour chemical analysis was performed for 600+ flowers. The three sympatric taxa were defined by their specific chiloglottones, the semiochemicals responsible for pollinator attraction, and were found to be fully cross-compatible. Multivariate morphometric analysis could not reliably distinguish among the four taxa. Although varying from very low to moderate, significant levels of genetic differentiation were detected among all pairwise combinations of taxa at both nuclear and chloroplast loci. However, the levels of genetic differentiation were lower than expected for mature species. Critically, a lack of chloroplast DNA haplotype sharing among the morphologically indistinguishable and most closely related taxon pair confirmed that chemistry alone can define taxon boundaries.

Conclusions

The results confirmed that pollinator isolation, mediated by specific pollinator attraction, underpins strong reproductive isolation in these taxa. A combination of large effective population sizes, initial neutral mutations in the genes controlling floral scent, and a pool of available pollinators likely drives diversity in this system.