Self-pollination is often regarded as an evolutionary dead end, yet many selfers seem capable of retaining high adaptive potential. Andersson and Ofori perform experimental crosses within an initially self-sterile population of Crepis tectorum to produce an outbred and inbred progeny population, and find that a shift to selfing promotes adaptive potential for leaf morphology by increasing the overall genetic variance and by exposing potentially advantageous recessive alleles to selection. The results point to a positive role for inbreeding in phenotypic evolution, at least during or immediately after a rapid shift in mating system.
It is widely acknowledged that eukaryotic cells (you know, the ones with a membrane-bound nucleus and a variety of other membrane-bound organelles (cf. prokaryotes)) came to be so complex by a series of ‘mergers and acquisitions’ that saw a prokaryote-like cell internalise other, smaller ‘cells’ to gain organelles such as mitochondria and chloroplasts. That is the essence of the Serial Endosymbiotic Hypothesis/Theory. But have you ever wondered how long ago such events took place? Well, Patrick Shih and Nicholas Matzke have done so on our behalf .
Using ‘cross-calibrated phylogenetic dating of duplicated ATPase proteins’ (which are retained by mitochondria and chloroplasts and involved in energy production in both), the duo’s results suggest that primary plastid endosymbiosis (which eventually gave us plant cells) occurred approximately 900 Mya (millions of years ago), whereas mitochondrial endosymbiosis occurred around 1200 Mya. Interestingly, both authors contributed equally to this work, and both were PhD students at the time! I’d so like one of the authors to have done the mitochondria work, and the other to have been ‘responsible’ for chloroplasts; that would make for a pleasingly symmetrical, modern-day parallel to the 19th century’s Cell Theory, largely attributed to Schleiden (‘botanist’) and Schwann (‘zoologist’). Way to go, gentlemen!
[Please don’t construe Mr Cuttings’ comments about putative parallels with Schleiden and Schwann to mean that only animal cells have mitochondria, and only plant cells have chloroplasts; plant cells can contain both (yes, so they are better than animals…)! – Ed.]
When it comes to carnivorous plants it’s Venus Flytraps that get the most attention, with their snapping jaws. Bladderworts have stunningly fast traps. Sundews glisten and coil around their prey. Pitcher plants like the Nepenthaceae in contrast don’t seem to do much. It looks like they’re just sitting there, waiting for gravity to do the work, almost like couch potatoes. In fact there’s a lot going on, as a paper from next month’s Annals of Botany shows.
Jonathan A. Moran, Laura K. Gray, Charles Clarke and Lijin Chin have written a paper Capture mechanism in Palaeotropical pitcher plants (Nepenthaceae) is constrained by climate (you can read it for free) that not only looks at what pitcher plants do, but also where they do what do. And they’ve looked at a lot of plants – almost 2000 populations of over 90 different species. So what is it that pitcher plants are doing?
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, DOI: 10.1093/aob/mct195
Angiosperms, the flowering plants, are the astonishingly diverse. But what drives the selection pressures to create this diversity? One explanation is the Grant-Stebbins model. This model looks at pollination as a selection process. Many flowers need pollinators. If there are no pollinators, the plants don’t get pollinated and they have no offspring. It means that plants have to live where the ranges of their pollinators are. Plants that evolve to pull in more pollinators will have more success. Flowers are therefore constantly evolving to attract visitors.
It’s an interesting idea, but how do you test it?
Newman, Anderson and Johnson investigated the South African plant Disa ferruginea for their paper ‘Flower colour adaptation in a mimetic orchid‘. This is a clever piece of work based on a mimetic orchid. Disa ferruginea doesn’t waste time producing a reward for visiting insects, but this means there’s no obvious reason for a pollinator to want to visit. Instead D. ferruginea looks like the flowers of other nectar producing plants. What makes D. ferruginea odd is that it doesn’t always look like the same plant. In the west it has red flowers. In the east it’s orange.
Newman et al. thought this might be that pollinators were selecting the plants that looked most like the nectar bearing species, so they conducted an experiment. They swapped some orchids, so some orange orchids were found in the west, and some red orchids were moved to the east. Then they watched to see what happened.
The only insect that pollinates D. ferruginea is a butterfly Aeropetes tulbaghia. Sure enough they found that in the west they tended to ignore the orange plants and stick with the red orchids. In the east they skipped over the red orchids but stuck with the orange flowers. In both sites it was the orchids that looked most like the reward-bearing plants that attracted pollinators, so here was proof that pollinator selection was happening.
I suppose the follow-up experiment would be to relocate butterflies from one range to another, but I’ve no idea how you’d manage to fit them with radio transmitters to track them.
This isn’t the only test of Grant-Stebbins.
Ellis and Johnson (the same Johnson mentioned above) have published a paper ‘The Evolution of Floral Variation Without Pollinator Shifts in Gorteria Diffusa (Asteraceae)‘. Gorteria Diffusa is a South African daisy that’s usually pollinated by the bee fly Megapalpus capensis. Ellis and Johnson found fourteen different varieties of G. Diffusa. Each of the forms seems to be inherited, not a plastic adaptation to the environment, but they couldn’t find fourteen different pollination scenarios. If pollination isn’t the selective force, what is going on? Ellis and Johnson point to other work that argues that there’s more to plant survival than pollination. For example once you have the seeds you have to ensure that at least some of them get past predators to germinate. Also, while you want a flower to look tasty to a visiting pollinator, do you really want to attract larger herbivores that will simply eat the flower?
It’s clearly a problem that needs more research, and there’s a whole special issue on pollinator-driven speciation on the way from Annals of Botany. The earliest papers are available as advanced access to subscribers including ‘Do pollinator distributions underlie the evolution of pollination ecotypes in the Cape shrub Erica plukenetii?‘ by Van der Niet, Pirie, Shuttleworh, Johnson and Midgley. Yes, the same Johnson.
Erica plukenetii is a shrub that grows waist-high, if your waist is typically 90cm above the ground. You can find it on the slopes of South African mountains, but you’ll find it with more than one pollinator. The typical E. plukenetii has a corolla (set of petals) of medium length. These are pollinated by the Orange-breasted sunbird (Anthobaphes violacea). A sunbird is a big like a hummingbird in that it’s a small nectar-feeding bird. A difference is that sunbirds tend to perch to feed instead of hovering.
The Orange-breasted sunbird is not the only pollinator of E. plukenetii. In the north of their range the plants are pollinated by Malachite sunbirds (Nectarinia famosa). Malachite sunbirds have a longer bill and the E. plukenetii in this region have longer corollas. In the centre of their range there’s also a shorter corolla variety of E. plukenetii and this is not bird pollinated. Instead it’s pollinated by a moth. These means there are three ways to pollinate E. plukenetii and they seem to have developed from the middle form. How does this fit with the Grant-Stebbins model?
Van der Niet et al. argue that the longer corolla plant fits the Grant-Stebbins model very well. As you go north Orange-breasted sunbirds become rarer and Malachite sunbirds more common, so the flowers better suited to the Malachite sunbirds will produce more offspring. Here pollinator selection makes sense. What about the short-corolla plants?
These are in the middle of the range, but here there are already plenty of Orange-breasted sunbirds, so they didn’t need to change to attract them. In fact, they’ve evolved to move away from the pollinators. This contradicts the Grant-Stebbins model. Van der Niet et al. suggest that other pressures must have an influence, and compare the bird pollinated E. plukenetii, found on mountainsides with the moth-pollinated E. plukenetii, found on flatter land. The moth-pollinated plants have slender branches that don’t support birds well. If this is necessary to colonise the flats, then it’s the habitat that changes the pollinator for the plant, instead of the pollinator defining the plant’s habitat.
It seems likely that it’s not just pollinators that select plants, but that plants can also select pollinators. One example is the paper ‘Domestication of cardamom (Elettaria cardamomum) in Western Ghats, India: divergence in productive traits and a shift in major pollinators‘ by Kuriakose, Sinu and Shivanna. Domesticating crops brings about a lot of changes. In the case of cardamom one change is there are flowers around for a lot longer than in the wild. For wild cardamom the pollinators tend to be solitary bees. For domesticated cardamom the flowers attracted social bees, the Purple sunbird and the Little Spiderhunter, a bird which – despite its name – is fond of nectar. The change in floral display seems to attract an entirely different kind of pollinator. Wild and cultivated cardamom don’t seem to share pollinators, despite being compatible with each other.
What seems to be happening is that where selection can occur, it will occur. While there are many scenarios where that can happen, like herbivory or habitat, competition for pollinators is in some cases a major factor in driving the evolution of plants.
The Annals of Botany special issue on pollinator-driven speciation is due out early 2014, with subscribers getting early access to some papers now. The issue will become free-access in 2015.
Ellis A.G. & Johnson S.D. (2009). The evolution of floral variation without pollinator shifts in Gorteria diffusa (Asteraceae), American Journal of Botany, 96 (4) 793-801. DOI: 10.3732/ajb.0800222 (free access)
Kuriakose G., Sinu P.A. & Shivanna K.R. (2008). Domestication of cardamom (Elettaria cardamomum) in Western Ghats, India: divergence in productive traits and a shift in major pollinators, Annals of Botany, 103 (5) 727-733. DOI: 10.1093/aob/mcn262
Newman E., Anderson B. & Johnson S.D. (2012). Flower colour adaptation in a mimetic orchid, Proceedings of the Royal Society B: Biological Sciences, 279 (1737) 2309-2313. DOI: 10.1098/rspb.2011.2375 (free access)
Van der Niet T., Pirie M.D., Shuttleworth A., Johnson S.D. & Midgley J.J. Do pollinator distributions underlie the evolution of pollination ecotypes in the Cape shrub Erica plukenetii?, Annals of Botany, DOI: 10.1093/aob/mct193 (subscription access till 2015)
I’ve been catching up with blog posts from Botany 2013.
One session that grabbed a lot of attention was “Yes, Bobby, Evolution is Real!“, a session that tackled encroaching attempts to inject selected religious viewpoints over others in US science classes. The session’s title was pointed, given that New Orleans is in Louisiana, where the state’s governor is Bobby Jindal. It was described by the Huffington Post as The Day That Botany Took on Bobby Jindal by Just Being Itself. Uh,oh! Botanists laugh at LA legislators who don’t like evolution, says the Phytophactor. Honest Ab notes that beyond that one session there was Evolution throughout the Botany 2013 Meetings, because that’s how plants get things done.
A search of the #BOTANY2013 hashtag on Twitter might still prove useful though as new tweets keep coming and some look very helpful.
Hypericum perforatum (St. John’s wort) is a widespread Eurasian perennial plant species with remarkable variation in its morphology, ploidy and breeding system. Koch et al. analyse extensive field collections and demonstrate that H. perforatum is not of hybrid origin, and for the first time document wild diploid populations. They find various cryptic gene pools and demonstrate past and contemporary gene flow within and between gene pools, and with the sister species H. maculatum. Cytogenetic analyses highlight that these processes are highly influenced by the reproductive system in both species, with a switch to predominantly apomictic reproduction in polyploids, irrespective of their evolutionary origin.
The suprafamilial phylogenetic structure of Malpighiales, one of the largest orders of flowering plants, has been continuously improved over the past decade and the relationships of some subgroups are radically different from pre-molecular classifications. Endress et al. provide a first attempt at a floral structural characterization of the currently resolved suprafamilial clades based on comparative studies and use of primary literature. They find that most new suprafamilial clades are well supported by floral structural features, and inner morphological structures of the gynoecium and ovules appear to be especially suitable for characterizing suprafamilial clades within Malpighiales.
There are numerous examples in nature of distantly-related organisms converging on similar shapes that have proved useful to each. This convergent evolution can generate strikingly similar but independently evolved forms such as the streamlined bodies of dolphins and ichthyosaurs (a group of extinct marine reptiles); the wing shapes of birds and bats and the similar body shapes of the placental wolf and the thylacine (a recently extinct wolf-like marsupial). Such similarities have long been assumed to result from exposure to similar environmental conditions and selection pressures.
A recent AoB paper considered the similarities between the habitats of American and African succulent plants. The spurges, milkweeds and iceplants of Africa and the ancestrally distant cacti of America are outwardly very similar in appearance, and these resemblances have been explained by similarities in local climate. This study aimed to quantify the environmental spaces in which the two groups of plants exist, and therefore show whether similarity in form is indeed a corollary of similarity in habitat.
The selected study sites were hotspots of succulent abundance and diversity on each continent. In these dry, warm areas, storage of water and prevention of water loss are priorities for plants and so spherical or globular growth forms have evolved in each group. The authors analysed local climate data and employed GIS and niche equivalence modelling to compare the American and African succulent hotspot sites. What they found were surprisingly many differences in variables such as rainfall and temperature between the sites, and these differences outnumbered any similarities.
The authors concluded that the resemblance between the succulents on each continent may be explained by factors that were not included in their climate analyses, such as soil type, distance from the sea, and possibly important contributions of water to the plants by fog and dew. They also point out that the “similarities” (to the human eye) between these groups may be quite subjective, and that more robust measurements of similarity may be required when claiming convergence of growth forms.
Alvarado-Cárdenas, L. O., Martínez-Meyer, E., Feria, T. P., Eguiarte, L. E., Hernández, H. M., Midgley, G., & Olson, M. E. (2013). “To converge or not to converge in environmental space: testing for similar environments between analogous succulent plants of North America and Africa”. Annals of Botany, 111(6), 1125-1138. DOI:10.1093/aob/mct078
It can take a close eye to spot subtle but important differences. This is even more difficult when one organism is trying to look like another. But Fritz Müller had a very keen eye, making him The King of Observers. The English version is below the French.
Fritz Muller, de son vrai nom Johann Friedrich Theodor Fritz Müller, est un naturaliste allemand né près d’Erfurt en 1821. Fils de pasteur, il commence des études en pharmacie qu’il abandonne rapidement pour s’orienter vers les mathématiques et les sciences naturelles. Il obtient en 1844 le diplôme de Docteur, avec une thèse sur les sangsues. Il décide en 1845 d’étudier la médecine, son rêve étant de s’embarquer en tant que médecin sur les bateaux en partance pour les régions tropicales lointaines, dont la faune sauvage l’intéresse énormément. Mais ce libéral estime que les lois de la nature et la mystique chrétienne sont inconciliables : il décide d’abjurer sa religion pour devenir athée, ce qui, à cette époque, le conduit à ne pouvoir exercer la médecine et le contraint à fermer son cabinet.&
En 1852, son rêve chevillé au corps, et accompagné de sa femme, sa fille et son frère, il décide de s’expatrier à Blumenau. Il s’agit d’une ville allemande fondée au sud-est du Brésil, sur le fleuve Itajai-Açu, à mi-chemin entre Rio de Janeiro et la frontière uruguayenne. Il s’y installe comme professeur d’histoire naturelle. Puis, il enseigne les mathématiques à Desterro, ville côtière, avant que les jésuites ne prennent le contrôle du collège et l’obligent à quitter l’établissement, au vu de ses positions à l’égard de la religion. Il obtient en 1876 un poste de naturaliste itinérant rattaché au Museum d’Histoire Naturelle de Rio, plus proche de ses passions, mais qu’il doit quitter en 1891 pour avoir refusé de s’installer à Rio même. Il vit ses dernières années à Blumenau, où il est frappé à jamais par la mort de sa femme et de sa fille, puis par le suicide de sa sœur demeurant à Berlin. Il y décède le 21 mai 1897 à 76 ans.&
Parasitic plants – angiosperms that directly attach to another plant via a haustorium, a modified root that forms a morphological and physiological link between the parasite and host – tend to get a bad press. And it’s little surprise with the antics of such villains as Striga, the ‘violet vampire’, which greatly reduces the production of staple foods and commercial crops such as maize, sorghum, millet, rice, sugarcane and cowpea in many African countries, and can cause up to 100% crop loss. Slightly less devastating and livelihood-threatening is Rhinanthus minor – ‘yellow rattle’ – a hemiparasite on grasses, which is found in Europe, Russia, western Siberia, northern USA and Canada.
Whilst it is generally recognised that such plants have major negative impacts on plant community structure via influence on host productivity and competitive ability, James Fisher et al. show that nutrient-rich leaf litter from R. minor has a positive effect on plant community structure: ‘critically, in the case of grass and total community biomass, this partially negates biomass reductions caused directly by parasitism’. From sub-terranean to supra-terrestrial community impacts now, with another hemiparasite – mistletoes – and work by David Watson and Matthew Herring. Having already been established as ‘keystone resources’ – species providing important resources for a broad range of taxa and determining local diversities in these habitats – Watson and Herring experimentally investigated the role of Australian mistletoes such as Amyema miquelii (Loranthaceae, bog mistletoe) in eucalyptus woodland. After 3 years, sites from which mistletoe was removed lost, on average, a fifth of their total species’ richness, 26.5% of woodland-dependent bird species and more than one-third of their woodland-dependent residents.
The researchers, from The Institute for Land, Water and Society at Australia’s Charles Sturt University suggest that ‘nutrient enrichment via litter-fall is the main mechanism whereby the mistletoe promotes species’ richness, driving small-scale heterogeneity in productivity and food availability for woodland animals’. They further propose that this explanation applies to other parasitic plants with high turnover of enriched leaves, and that the community-scale influence of these plants is most apparent in low-productivity systems. I wonder if they had R. minor in mind? Prescience is, after all, a virtue…
[In the interests of fairness, it should be stated that Fisher et al. do cite Watson and Herring’s paper – Ed.]