Angiosperm trees generally form tension wood, a special type of secondary xylem, in response to a gravitational stimulus. Nugroho et al. find that pre-treatment with paclobutrazole and uniconazole-P, inhibitors of the synthesis of gibberellin, to inclined Acacia mangium seedlings inhibits negative gravitropism of the stems. The inhibitors suppress increases in the thickness of gelatinous layers and the elongation of gelatinous fibres in the tension wood. In contrast, pre-treatment with gibberellin stimulates the elongation of these fibres. The results suggest that gibberellin is important for the development of gelatinous fibres, and therefore in gravitropism.
Many abiotic variables affect plants, e.g. levels of light, carbon dioxide and water. One of the most important of those non-biotic factors is temperature. Now, given its importance you could be forgiven for assuming that it is recorded accurately and correctly. Unfortunately, that isn’t always the case. Take for instance the temperature of the meristem (symbolised as Tmeristem), which is important in driving plant development. For such a crucial aspect of plant biology studies have largely relied on measuring the temperature of the air surrounding the plant (Tair). Tair is measured because it is assumed to represent the meristem temperature because plants are poikilotherms (organisms whose ‘internal temperature varies considerably … Usually the variation is a consequence of variation in the ambient environmental temperature’). Whilst that assumption may seem reasonable – and it does save the would-be investigator the trouble of penetrating the umpteen layers of developing leaves, etc, that may sheathe the apical meristem, it is nonetheless an assumption. And the veracity of assumptions must be tested, which is what Andreas Savvides et al. did. Guess what they found! That’s right: Tmeristem differed from Tair – ranging between –2.6 and 3.8 °C in tomato, and –4.1 and 3.0 °C in cucumber(!). As the team conclude, ‘for properly linking growth and development of plants to temperature… Tmeristem should be used instead of Tair’.
If you’re now intrigued by detecting temperatures within cells, you might like to explore the nanoscale thermometer developed by G. Kucsko et al. Using ‘quantum manipulation of nitrogen vacancy (NV) colour centres in diamond nanocrystals’ it can detect temperature variations as small as 44 mK(!) and can measure the local thermal environment at length scales as low as 200 nm(!!). Or, if you want a more biological approach, check out the genetically encoded sensor that fuses green fluorescent protein to a thermosensing protein derived from Salmonella, as showcased by Shigeki Kiyonaka et al. Although proof of this particular principle was demonstrated with thermogenesis in the iconic mitochondria of brown adipocytes (and the somewhat less iconic endoplasmic reticulum of myotubes), the team envisage it could be used to investigate this phenomenon in other living cells. Maybe even within the cone cells of tropical cycads that undergo impressive increases in temperature, where Tcone can be markedly greater that Tair. In view of concerns about global temperature changes and effects of temperature on regulation of such economically important processes as flowering, accurate temperature information in planta – and an appreciation of the temperature that plants are actually responding to – is likely to become increasingly important.
[For a useful set of slides summarising Savvides et al.’s work, visit slideshare.net. For a less physics-oriented interpretation of the Nature nanoscale thermometry article try the accompanying ‘News and Views’ item by Konstantin Sokolov – Ed.].
The Orchidaceae have a history of recurring convergent evolution in floral function as nectar production has evolved repeatedly from an ancestral nectarless state. Hobbhahn et al. study the South African orchid genus Disa and find that independent nectary evolution has 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. With its morphologically diverse solutions to the problem of nectar production, Disa is a good example illustrating the contribution of functional convergence to phenotypic diversification, which probably also underlies the extensive diversity of nectary types and positions in the orchid family.
Olives have a long and complex history. The origins of the Mediterranean cultivated olive (Olea europaea subsp. europaea) are hotly debated, but it is usually accepted that its domestication started in the Levant based on archaeological, historical and molecular evidence. Multiple local selections of cultivars has been suggested by genetic analyses, followed by secondary diversification of the crop followed the oleiculture diffusion over the whole Mediterranean basin. The contribution of western wild olives in this diversification process remains poorly understood.
A recent paper in Annals of Botany describes patterns of genetic differentiation in Mediterranean and Saharan olives, and tests for admixture between these taxa. Based on the results, the human-meditated diffusion of the oleiculture over the Mediterranean basin and the contribution of O. europaea subsp. laperrinei to the cultivated olive diversification are discussed. Although its genetic contribution is limited, it is clear from this work that Laperrine’s olive has been involved in the diversification of cultivated olives.
Besnard G., El Bakkali A., Haouane H., Baali-Cherif D., Moukhli A. & Khadari B. (2013). Population genetics of Mediterranean and Saharan olives: geographic patterns of differentiation and evidence for early generations of admixture, Annals of Botany, 112 (7) 1293-1302 DOI: 10.1093/aob/mct196
It’s time for the last paper of the pollinator-driven speciation week. The previous posts have examined how pollinators can select and cause diversity among plants but, if they’re all the same species, could pollinators simply mix all the genes back up again?
Floral odour chemistry defines species boundaries and underpins strong reproductive isolation in sexually deceptive orchids by Rod Peakall and Michael R. Whitehead tackles the final part of the process of speciation. Once you have differences between plants, how do barriers to cross-pollination arise? Peakall and Whitehead examined orchids in the genus Chiloglottis, which appeared in the blog last month.
Chiloglottis orchids are found in eastern Australia. The flower doesn’t provide food, instead is appears to offer another reward. Part of the flower looks like a female wasp, and it has the scent to match. Any male attempting to mate will be disappointed not simply because the female is a fraud, but also because the orchid tags the wasp with pollinaria. These get carried to the next destination, which might well be another Chilogottis orchid.
Chilogottis appeals to thynnine wasps, but these aren’t all one species. Each wasp will be looking for specific mate. The way the wasp finds the mate is through the scent. Could variations in the chemistry of the flowers isolate species by attracting one kind of wasp, but not another?
The study initially showed this was plausible. Some of the orchids flowered around the same time of year in the same locations as their sister species, so clearly there was some barrier that wasn’t caused by geography. In addition, there was a lot of morphological overlap between many of the flowers, so there was no mechanical reason why they should be isolated.
It was also possible to cross-breed the flowers when pollinated by hand, and these produced viable seeds so this adds to the puzzle of what the barrier is.
The big observable difference what the chemical cocktails that the plants put out as their floral scent. If you categorise the flowers this way, then there are genetic differences. Peakall and Whitehead argue that what the Chilogottis orchids represent are a number of plants in the process of divergence. This helps give an overview of the divergence process. For example if you visited a forest, you could see trees in all states from saplings to fallen trunks and work out the life cycle of a tree without waiting hundreds of years. In a similar way, being able to identify different diverging branches of orchids at different stages means you get an overview of speciation without having to wait generations for the final result.
To some extent the idea of pollinator-driven speciation could be a puzzle. For many angiosperms, it’s pollinators that keep the species together, exchanging pollen from the flowers of one plant to the other. Peakall and Whitehead’s paper show how the Grant-Stebbins model works, with the shift in pollinators leading to the speciation of the orchids. What might look like a paradox is soluble after all.
Our Grant-Stebbins week continues. Today it’s a question of looking at the geographical context of floral adaptations. Why might you find a plant here and not there? Is it the pollinators that cause it?
Matching floral and pollinator traits through guild convergence and pollinator ecotype formation by Ethan Newman, John Manning and Bruce Anderson examines seventeen members of a pollination guild. These are plants that might look similar and appeal to similar pollinators, but are not closely related. These plants are all pollinated by Prosoeca longipennis a fly with a long proboscis, though the longipennis actually refers to long wings.
How long a P. longipennis proboscis is depends on which flies you observe. It’s found across a wide range of South Africa. The flowers that attract P. longipennis all have similar attributes. They have long corollas, tubes of petals that made the head of the flower. They have similar colours and no scent. Newman et al. also note another feature that makes them specific to P. longipennis – they flower in the autumn.
Observations in the field were able to confirm the existence of a guild and add more members to them. It was then a matter of examining different sites and seeing how they correlated with the local flies. What they found was there was a strong correlation between the tube length of a flower and the average length of the fly’s proboscis at each site. If a site had significantly longer P. longipennis, then the flowers would also have significantly longer tubes.
However, not all of the plants relied on P. longipennis. Some could be found at sites where the flies didn’t visit. Here the same plants would have morphologically different floral displays, showing there was a correlation between the flowers and the local pollinators.
What Newman et al. show with their work is that a variety of plants with different evolutionary histories are all hitting on the same solution. The common factor is they’re all trying to attract P. longipennis to pollinate them. At the same time they also show that while corolla size matters, and colour might matter, the size of the reward doesn’t. Being able to put together a description of the guild means that they can make a prediction. They say that from what they have observed, P. longipennis should visit Watsonia plants, in particular W. galpinii
This kind of prediction reminds me of the Angraecum sesquipedale prediction that Darwin made. em>Angraecum sesquipedale is an orchid that has an amazingly long tube. Darwin predicted that a moth with an equally amazingly long tongue would be found that pollinated it. He didn’t live to see his prediction proved right. Hopefully Newman, Manning and Anderson will have their prediction confirmed a lot sooner. If they’re right they will not just know what fly pollinates the flower. If they know the location they’ll have a good idea of how long its proboscis is too.
Nepenthes (Nepenthaceae, approx. 120 species) are carnivorous pitcher plants with a centre of diversity comprising the Philippines, Borneo, Sumatra and Sulawesi. They rely on captured prey to augment nutrition from nutrient-poor substrates. Nepenthes pitchers use three main mechanisms for capturing prey: epicuticular waxes inside the pitcher; a wettable peristome (a collar-shaped structure around the opening); and viscoelastic fluid. It has been suggested that the Nepenthaceae provide an example of adaptive radiation, based on pitcher specialization for nutrient capture.
Previous studies have provided evidence suggesting that the first mechanism may be more suited to seasonal climates, whereas the latter two might be more suited to perhumid (very wet) environments. A recent paper in Annals of Botany tests this idea using climate envelope modelling. A total of 94 species, comprising 1978 populations, were grouped by prey capture mechanism (large peristome, small peristome, waxy, waxless, viscoelastic, non-viscoelastic, ‘wet’ syndrome and ‘dry’ syndrome). Nineteen bioclimatic variables were used to model habitat suitability.
Prey capture groups putatively associated with perhumid conditions (large peristome, waxless, viscoelastic and ‘wet’ syndrome) had more restricted areas of probable habitat suitability than those associated putatively with less humid conditions (small peristome, waxy, non-viscoelastic and ‘dry’ syndrome). Overall, the viscoelastic group showed the most restricted area of modelled suitable habitat.
This is the first study to demonstrate that the prey capture mechanism in a carnivorous plant is constrained by climate. Nepenthes species employing peristome-based and viscoelastic fluid-based capture are largely restricted to perhumid regions; in contrast, a wax-based mechanism allows successful capture in both perhumid and more seasonal areas. With a geographical range extending from Madagascar to New Caledonia, Nepenthaceae is a successful and diverse carnivorous family. Nepenthes can be found from sea level to >3000 m elevation, on a variety of substrates. They occur in both perhumid and seasonal tropical environments, and the range of pitcher morphologies is striking, even to the casual observer. This diversity of pitcher morphology is mirrored by the range of nitrogen sources exploited, including bat excreta, leaf litter and arthropods. Yet, despite this wide range of pitcher structure and function, this paper demonstrates a consistent, large-scale pattern defining the relationship between bioclimatic variables and prey capture mechanism.
Moran J.A., Gray L.K., Clarke C. & Chin L. (2013). Capture mechanism in Palaeotropical pitcher plants (Nepenthaceae) is constrained by climate, Annals of Botany, 112 (7) 1279-1291 DOI: 10.1093/aob/mct195
Pollinator-driven speciation week continues, with Floral adaptation to local pollinator guilds in a terrestrial orchid, a paper by Sun, Gross and Schiestl. The star is the orchid Gymnadenia odoratissima, and orchid found in lowland temperate Europe and up in the mountains. It flowers between June and mid-August. Unlike the orchids yesterday, these flowers offer a food reward as well as a strong scent.
The study examined a number of lowland and mountain orchids in Switzerland. A team of patient observers with nets watched to see what visited the orchids in all the locations, during the day and night. In the lowland orchids they found visitors to b butterflies/moths and beetles. In the mountains there were also Diptera, flies. There wasn’t much overlap between the species of pollinators.
The next stage was to start transferring populations of orchids. The obvious way to move the orchids was to change their altitude, to move the lowland plants to the mountains and bring the highland plants down. Sun et al. also moved some samples along the same altitude. This helped provide another check on how the orchids coped with being moved. What they found was that the mountain orchids were fairly successful in the lowlands, but the lowland plants did comparatively badly in the mountains.
However, the transferred mountain orchids did much worse in the lowlands than if they’d stayed put. Sun et al. point out one of the things the lowlands lack are Empidid flies. Fly pollination is an important factor for plants at higher altitudes. The lowland plants weren’t much less successful when they moved. It suggests that the mountain plants have adapted their flowers to take advantage of the flies in a way that the lowland plants haven’t. For example the chemicals in the scents of the flowers are different between the lowland and mountain plants. The mountain flowers also tended to be paler, and this is attractive to moths, who are more common in the uplands than the lowlands. There are more butterflies in the lowlands, who pollinate by day and so darker flowers may be comparatively more attractive here.
What the experiments show is that the changes in floral display aren’t simply products of altitude. It seems that G. odoratissima is altering its flowers in reaction to the local pollinator guilds. You can pick up this paper from Annals of Botany.
In contrast to our broad understanding of population and phylogenetic dynamics, we do not know when diploid taxa are sufficiently different to spawn allopolyploids with two very distinctive genomes (strict allopolyploids), i.e. when their diploid interspecific hybrids would be sterile or nearly so. Levin reviews and integrates published information and determines that despite limitations in methodology and sampling, the estimated times to hybrid sterility are somewhat congruent across disparate lineages. Whereas the waiting time for hybrid sterility is roughly 4–5 million years, that for cross-incompatibility is roughly 8–10 million years, sometimes considerably more. Strict allopolyploids may be formed in the intervening time window. The progenitors of several allopolyploids diverged between 4 and 6 million years before allopolyploid synthesis, as expected. This is the first study to propose a general temporal framework for strict allopolyploidy.
Following from yesterday, do pollinators act as selectors for evolution? A pollinator shift explains floral divergence in an orchid species complex in South Africa by Peter and Johnson tests this idea.
The orchid in question is Eulophia parviflora. This is a deceptive orchid found in Africa, and deceptive means it doesn’t offer a reward to pollinators, it merely looks like it does. The aim is to entice insects in when they look for food and hit them with their pollinaria to carry to other orchids. To do this they need to look and smell convincing, but they also need to make things as easy as possible for the pollinators. The orchid’s problem is that there are so many insects that it could build its flowers in all sorts of ways.
This is indeed what happens.
Peter and Johnson identified two forms of Eulophia parviflora. In the image above, the one on the left is the short-spur morph. This grows tall from the ground with plenty of flowers. The one on the right is the long-spur version. This opens when the stalk is barely out of the ground. They look different and they smell different, but they’re both E. parviflora. So what is it that makes the same plant grow long or short spurs?
The answer seems to be the pollinators. The short-spur plants are pollinated by the beetle Cyrtothyrea marginalis who can get in close to the orchid. The long-spur orchid is pollinated the bee Amegilla fallax. However, simply watching and seeing that the plant has two forms pollinated by two different creatures isn’t enough. There might be some other cause, like local climate that explains the spurs and the presence or lack of an insect. So Peter and Johnson have done some experiments.
Are bees deliberately picking long-spurs in flowers? If they are then that would show the bees are selecting flowers and helping drive the morphological change. The experiment is simple. Reduce the size of the spurs in some of the long-spur flowers. If the spurs matter, then the bees will pick the long-spur plants and ignore the short spur plants. Sure enough, the bees went for the long-spur flowers.
Another experiment was to see how the scent attracted insects. They tried it with both beetles and bees, but found the bees weren’t cooperating, so there were just results from beetles. The experiment is simple and elegant. You have a Y shape. At the top of each arm of the Y you have a fan pushing out the scent of a flower. Put a beetle at the bottom and where does it go to? In this case, it picks the scent of the short-spur plant.
In fact the paper notes the experiment wasn’t quite as simple as I made out. It wasn’t just the scent that attracted beetles, they’d also pick a tunnel depending on the position of the sun, so they found they had to calibrate the tunnels properly before they could sensibly test the beetles.
Peter and Johnson also show that the two forms of the plant are not just diverging in shape but also in time. It makes sense to flower when the pollinators are about. The short-spur flower doesn’t start till after the winter frosts in October (remember South Africa is in the southern hemisphere). This is when the beetles emerge. In contrast the long-spur flower can get going sooner in July when A. fallax starts getting active.
The isolation in time for exchanging pollen, and the specificity of the pollinators means that the pollinators seem to be definitely acting as selectors for the plants. Peter and Johnson say that the two forms might already be considered two sister species given the genetic differences.