Using phylogenetic methods, our research investigates the mechanisms that led to the functional diversity of plants. We combine analyses of gene sequences, genomes, transcriptomes, ecological, physiological and morphological traits to address questions of importance for evolutionary biology in general:
These questions are addressed using comparative approaches, working both at large taxonomic scales and within some selected species. As a study system, we use mainly the evolution of C4 and CAM photosynthesis in grasses and other groups of plants. While transcriptomes and genomes are compared across distantly related families, the origins and consequences of variation in photosynthetic traits is also investigated within the grass species Alloteropsis semialata, the only taxon known to include both C3 and C4 individuals.
C4 photosynthesis is an adaptation that boosts productivity in tropical conditions [1,11]. A number of anatomical and biochemical components must act together to generate a C4 physiology, yet this complex trait is a textbook example of convergent evolution, having evolved more than 60 times independently in flowering plants . We are interested in understanding the factors that eased C4 evolution in some lineages, as well as the effects of the transition from C3 to C4 on the evolutionary trajectories of the plants and their ecological sorting.
The multiple origins of C4 photosynthesis all happened in the last 30 million years [5,6], which supports the hypothesis that declines of atmospheric CO2 during the Oligocene promoted C4 evolution . Phylogenetic models have shown that C4 origins were more likely in those groups possessing leaf anatomies that already fitted some of the C4 requirements [7,14]. In addition, independent C4 origins have recurrently used those genes that were ancestrally highly expressed, so that possessing such genes might have facilitated C4 evolution [8,9,18].
The effects of C4 evolution depend on the organisms. While C4 boosts growth , it also affects the ecological sorting. Across angiosperms, the ecological niche of C4 taxa is correlated to that of their C3 ancestors, indicating significant phylogenetic effects . Despite this effect of evolutionary history, C4 lineages tend to occur in warmer climates than their non-C4 relatives . Within grasses, phylogenetic analyses confirmed that C4 originated mainly in tropical conditions, but surprisingly, the C4 trait increased the rate of migration to colder climates . Therefore, C4 evolution can be said to broaden the ecological niche [11,15], in addition to increasing diversification rates .
Relationships among species can be depicted using phylogenetic trees, but reconstructing the history of transitions among character states can be complicated when relying solely on species trees. To gain further insights into the history of modifications underlying adaptive transitions, we decompose complex traits into their constituents and track each of them independently using combinations of species and gene trees. This innovative approach revealed that the complex C4 trait emerged repeatedly within the small grass genus Alloteropsis via a combination of recurrent co-option of pre-extisting components and hybridization . Expanding this approach to other groups, we further showed that genetic modifications associated to some C4 taxa evolved after the C4 physiology itself, so that the number of origins differs among C4 components [3,12].
Reticulate evolution underlying multiple photosynthetic transitions in Alloteropsis; from Dunning et al. 
The origins of C4 photosynthesis are traditionally investigated at large evolutionary scales, through comparisons of species differing in their photosynthetic type. Such research cannot elucidate the microevolutionary processes underlying photosynthetic transitions, and we therefore conduct intraspecific analyses, using as a study system the grass Alloteropsis semialata. This species contains C3 and C4 populations, as well as intermediates . Phylogenetic analyses of chloroplast genomes indicate that the species originated in Central/Eastern Africa . While the intermediates remained ecologically and geographically restricted, the C4 lineage rapidly spread through geographical and ecological spaces, confirming that C4 acts as a niche broadener .
Analyses of nuclear genomes showed that Alloteropsis semialata underwent recurrent events of hybridization during its history . This study system allowed us to track the spread of adaptive loci, showing that C4 components can be acquired independently in isolated populations, and then combined through secondary gene flow . We are currently expanding these investigations to a large sample of populations from across the species range, to reconstruct the history of C4 mutations, and the mechanisms driving their fixation and subsquent spread.
The Alloteropsis semialata system allows comparing recently diverged C4 and non-C4 accessions, as well as C4 populations that evolved independently after the transition to C4. Capitalizing on this system, we recently showed that gene duplication facilitated physiological innovation, by providing rapid increases in gene expression levels . We now use the same study to identify the anatomical, physiological and genomic traits that differ systematically among photosynthetic types.
Nuclear history of Alloteropsis semialata, showing the distribution of laterally acquired genes (LGT), from Olofsson et al. 
The advent of next generation sequencing allows generating genome data for large numbers of species. While initially used for model species, high-throughput sequencing can also be applied to samples stored in historical collections, such as those found in herbarium and museums. In collaboration with colleagues from Toulouse, we use high-throughput sequencing of historical samples to explore the genomic diversity stored in historical collections. We are able to reconstruct relationships among numerous taxa based on both organellar and nuclear genomes, but can also perform comparative genomics on rare species that would be difficult to collect in the wild [2,3,13,19].
Colin Osborne, Sheffield Link
Guillaume Besnard, Toulouse Link
1. Atkinson RRL, Mockford EJ, Bennett C, Christin PA, Spriggs EL, Freckleton RP, Thompson K, Rees M, Osborne CP. 2016. C4 photosynthesis boosts growth by altering physiology, allocation and size. Nature Plants 2: 16038 Link
2. Besnard G, Christin PA, Male PJG, Lhuillier E, Lauzeral C, Coissac E, Vorontsova M. 2014. From museums to genomics: old herbarium specimen sheds light on a C3 to C4 transition. Journal of Experimental Botany 65:6711-6721 Link
3. Besnard G, Bianconi ME, Hackel J, Manzi S, Vorontsova MS, Christin PA. 2018. Herbarium genomics retraces the origins of C4-specific carbonic anhydrase in grasses. Botany Letters
4. Bianconi ME, Dunning LT, Moreno-Villena JJ, Osborne CP, Christin PA. 2018. Gene duplication and dosage effects during the early emergence of C4 photosynthesis. Journal of Experimental Botany 8:1967-1980 Link
5. Christin PA, Besnard G, Samaritani E, Duvall MR, Hodkinson TR, Savolainen V, Salamin N. Oligocene CO2 decline promoted C4 photosynthesis in grasses. Current Biology 18: 37-43 Link
6. Christin PA, Osborne CP, Sage RF, Arakaki M, Edwards EJ. 2011. C4 eudicots are not younger than C4 monocots. Journal of Experimental Botany 62: 3171-3181 Link
7. Christin PA, Osborne CP, Chatelet DS, Columbus JT, Besnard G, Hodkinson TR, Garrison LM, Vorontsova MS, Edwards EJ. 2013. Anatomical enablers and the evolution of C4 photosynthesis in grasses. Proceedings of the National Academy of Sciences (PNAS) 110: 1381-1386 Link
8. Christin PA, Boxall SF, Gregory R, Edwards EJ, Hartwell J, Osborne CP. 2013. Parallel recruitment of multiple genes into C4 photosynthesis. Genome Biology and Evolution 5: 2174-2187 Link
9. Christin PA, Arakaki M, Osborne CP, Edwards EJ. 2015. Genomic enablers underlying the clustered evolutionary origins of C4 photosynthesis in angiosperms. Molecular Biology and Evolution 32:846-858 Link
10. Christin PA, Osborne CP. 2013. The recurrent assembly of C4 photosynthesis, an evolutionary tale. hotosynthesis Research 117: 163-175 Link
11. Christin PA, Osborne CP. 2014. The evolutionary ecology of C4 plants. New Phytologist 204: 765-781 Link
12. Dunning LT, Lundgren MR, Moreno-Villena JJ, Namaganda M, Edwards EJ, Nosil P, Osborne CP, Christin PA. 2017. Introgression and repeated co-option facilitated the recurrent emergence of C4 photosynthesis among close relatives. Evolution 71:1541-1555 Link
13. Dunning LT, Liabot AL, Olofsson JK, Smith EK, Vorontsova MS, Besnard G, Simpson KJ, Lundgren MR, Addicott E, Handasyde T, Gallagher RV, Chu Y, Pennington RT, Christin PA, Lehmann CER. 2017. The recent and rapid spread of Themeda triandra. Botany Letters 164:327-337 Link
14. Lundgren MR, Osborne CP, Christin PA. 2014. Deconstructing Kranz anatomy to understand C4 evolution. Journal of Experimental Botany 3357-3369 Link
15. Lundgren MR, Besnard G, Ripley BS, Lehmann CER, Chatelet DS, Kynast RG, Namaganda M, Vorontsova MS, Hall RC, Elia J, Osborne CP, Christin PA. 2015. Photosynthetic innovation broadens the niche within a single species. Ecology Letters 18:1021-1029 Link
16. Lundgren MR, Christin PA, Gonzalez Escobar E, Ripley BS, Besnard G, Long CM, Hattersley PW, Ellis RP, Leegood RC, Osborne CP. 2016. Evolutionary implications of C3-C4 intermediates in the grass Alloteropsis semialata. Plant, Cell and Environment 39: 1874-1885 Link
17. Lundgren MR, Christin PA. 2017. Despite phylogenetic effects, C3-C4 lineages bridge the ecological gap to C4 photosynthesis. Journal Experimental Botany 68:241-254 Link
18. Moreno-Villena JJ, Dunning LT, Osborne CP, Christin PA. 2018. Highly expressed genes are preferentially co-opted for C4 photosynthesis. Molecular Biology and Evolution 35:94-102 Link
19. Olofsson JK, Bianconi M, Besnard G, Dunning LT, Lundgren MR, Holota H, Vorontsova MS, Hidalgo O, Leitch IJ, Nosil P, Osborne CP, Christin PA. 2016. Genome biogeography reveals the intraspecific spread of adaptive mutations for a complex trait. Molecular Ecology 25: 6107-6123 Link
20. Sage RF, Christin PA, Edwards EJ. 2011. The C4 plant lineages of planet Earth. Journal of Experimental Botany 62: 3155-3169 Link
21. Spriggs E, Christin PA, Edwards EJ. 2014. C4 photosynthesis promoted species diversification during the Miocene grassland expansion. Plos One 9: e97722 Link
22. Watcharamonkol T, Christin PA, Osborne CP. 2018. C4 photosynthesis evolved in warm climates but promoted migration to colder ones. Ecology Letters 21:376-383 Link