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Lateral transmission of genes for C4 photosynthesis

The origin of complex traits, such as eyes and wings, has been studied by biologists since Darwin. In all cases looked-at so far, new traits have arisen through the modification of genes inherited from ancestors, and we’ve long understood how these evolutionary adaptations are passed from parents to offspring. But in a new paper published this month in Current Biology, we’ve discovered an exception to this rule, showing that adaptive genes required for a complex trait can be transmitted between distant plant cousins without direct contact between the species.

We looked at the evolution of genes involved in the C4 photosynthetic pathway, a well-studied complex trait that boosts productivity in hot environments. Our study organism was Alloteropsis semialata, an African grass that’s a favourite of ours for work on C4 evolution and ecology. For the first time, our work has revealed that crucial elements of this pathway have been transmitted between distantly related grass species. This unexpected discovery of adaptation by “lateral gene transfer” adds to growing evidence that the evolutionary tree of life is better described as a tangled bush. Crucially, it shows that genetic adaptations can cross the species boundary and spread among unrelated plants.

Lateral gene transfer. Credit: Les Watson and Wikipedia Commons

Lateral gene transfer among grass species: photo credits Les Watson and Wikipedia Commons.

Dr Hui Liu!

Congratulations to lab member Hui Liu who successfully defended her PhD thesis today. The celebrations have begun….

Hui’s thesis examined phylogenetic patterns in the ecology, morphology and physiology of grasses. Look out for papers on these topics from Hui in the coming months. The first is due out in Oecologia.

New hypothesis on C4 evolution

Colin has published a new hypothesis about the evolution of C4 photosynthesis in a paper with Lawren Sack from UCLA appearing in this month’s Philosophical Transactions of The Royal Society. In it, we argue that plant hydraulics should be considered an important aspect of C4 plant evolution (link to paper).  An interaction with CO2 arises because atmospheric CO2 starvation forces plants to open stomata wider, causing greater loss of water.  C4 plants reduce this problem with a CO2-concentrating mechanism that boosts photosynthesis without the need for high water loss. The paper was reported today by the online magazine Planet Earth: read more here. The paper is part of a theme issue of Phil. Trans. B looking at the roles played by atmospheric CO2 in plant evolution (link to journal).

PEPG mini-symposium

Several members of the group attended the first annual BES Plant Environmental Physiology Group mini-symposium in London. After a long day of interesting talks at Charles Darwin House, the discussion continued in a local pub.

Expedition to Madagascar

Russell Hall just got back from a plant collecting expedition to Madagascar to seek out grasses in the Forest Shade Clade – a group of around 150 species, including both C3 and C4 lineages. The main goal was to collect fresh material for DNA extraction, as part of the effort to construct a densely sampled phylogeny of the Forest Shade Clade of grasses. The phylogeny will be used to explore the history and causes of photosynthesis evolution in this clade using comparative methods.

Madagascar was targeted for collection as a third of the species in the clade grow in Madagascar, and a fifth of them are endemic to Madagascar.

A brief account of the expedition is given on the eMonocot project website.

Evolution: there and back again?

The C4 pathway is a turbo-charger for photosynthesis that has evolved from the ancestral C3 type of photosynthesis many times in nature. But just how many times remains an open question. This is important because scientists are currently working hard to bioengineer highly productive C4 rice crops in the laboratory. Learning that the pathway is easy to evolve would bring great hope to this enterprise.

Origins of the C4 pathway are inferred by looking at its distribution across the evolutionary tree of life. Purely C4 lineages interspersed by C3 lineages can be interpreted as independent evolutionary origins of the pathway. However, evolutionary theory tells us that a complex trait like C4 photosynthesis should be easier to lose than it is to acquire. If this is true, it would mean the pathway could actually have evolved very few times, way back in time, with many lineages subsequently reverting back to the ancestral C3 type of photosynthesis.

We tackle this problem in a paper published this month in Trends in Ecology and Evolution, arguing that assembly and deconstruction of the C4 pathway are equally difficult. There is no reason to expect evolutionary loss of C4 photosynthesis to be more common than evolutionary origins. Current genetic evidence backs this claim; reversal back to the C3 pathway seems to be extremely rare and may not even occur at all. However, the piecewise evolutionary assembly of C4 photosynthesis seems to involve a huge number of steps, and could well have been reversed many times before it was completed. In order to detect these reversals, we will need a much better understanding of the genetics underpinning the pathway.

Download the paper here

Grassland origins

The origin of C4 photosynthesis is the most important evolutionary innovation in plants since the evolution of flowers. It has dramatic effects on plant performance, and turbo-charges growth in hot sunny environments. Today, grasses using the C4 photosynthetic pathway dominate tropical savannas and warm-climate grasslands across large areas of the Earth’s surface. The geological record tells us this ecological dominance arose suddenly around 8-3 million years ago. Why?

For a long time, the C4 pathway itself has been considered a crucial part of the answer, the argument being that this new form of photosynthesis gave a performance advantage to the grasses that used it. In a new paper published in Science, we suggest that the drivers of C4 grassland expansion were significantly more complex. It is time to start looking beyond the C4 pathway to other traits that could allow species to dominate grassland ecosystems.

In our paper, we propose a new way forward, in which grass evolutionary biology is synthesized with grassland ecosystem science. We think this will help to increase our knowledge of the evolution of plant traits that promote dominance in grassland systems, and better our understanding of the key question: just how important was C4 photosynthesis in transforming ecosystems across large regions of the Earth’s surface?

Download the paper here

Efficiency savings

At high temperatures, the C4 pathway greatly increases the efficiency of photosynthesis over the C3 type. This means that C4 leaves can use less water and nitrogen than their C3 counterparts, without cost to photosynthesis. That’s the theory at least and, until now, it’s been backed by experimental data.

We challenge this view in a new paper, which reports an experimental comparison of more than 30 grass species from closely related C3 and C4 groups. We find no evidence of a C3 / C4 difference in nitrogen content. This result raises the intriguing possibility that evolutionary history may be more critical for plant nitrogen usage than photosynthetic pathway. But what about water? Just as expected, the C4 species used less than their C3 relatives, providing significant improvements in plant water status.

Download the paper here

Turbo-charging photosynthesis

Half of all grass species use C4 photosynthesis, a ‘turbo-charger’ that boosts growth in hot climates, and allows these plants to dominate hot, dry ecosystems like the world’s tropical savannas.  For decades, scientists have debated how this turbo-charger evolved – especially how environmental factors drove the evolutionary process.

Based on one of the latest DNA-based reconstructions of grass evolution, our work suggests that the C4 pathway evolved in sunny, open habitats away from trees.  Overall, drought does not seem to have driven the origins of C4 photosynthesis. Instead, the high prevalence of modern C4 grasses in dry environments seems to have arisen as a result of the C4 photosynthetic pathway.

Download the paper here

Read the Nature News report here


Evolution ‘in reverse’?

C4 photosynthesis has evolved many times from the ancestral C3 pathway. However, whether a C4 plant can revert back to the C3 type or not remains an important unresolved question.

We addressed this problem by studying the grass genus Alloteropsis, which is unique among plants in containing a species with both C3 and C4 variants. Our molecular phylogeny is the first to suggest a recent evolutionary reversion from C4 back to C3 photosynthesis.

Read more about this work

Read more about this work

Download the paper here