C4 photosynthesis promoted species diversification during the Miocene grassland expansion

Abstract

Identifying how organismal attributes and environmental change affect lineage diversification is essential to our understanding of biodiversity. With the largest phylogeny yet compiled for grasses, we present an example of a key physiological innovation that promoted high diversification rates. C4 photosynthesis, a complex suite of traits that improves photosynthetic efficiency under conditions of drought, high temperatures, and low atmospheric CO2, has evolved repeatedly in one lineage of grasses and was consistently associated with elevated diversification rates. In most cases there was a significant lag time between the origin of the pathway and subsequent radiations, suggesting that the 'C4 effect' is complex and derives from the interplay of the C4 syndrome with other factors. We also identified comparable radiations occurring during the same time period in C3 Pooid grasses, a diverse, cold-adapted grassland lineage that has never evolved C4 photosynthesis. The mid to late Miocene was an especially important period of both C3 and C4 grass diversification, coincident with the global development of extensive, open biomes in both warm and cool climates. As is likely true for most "key innovations", the C4 effect is context dependent and only relevant within a particular organismal background and when particular ecological opportunities became available.

Figure 1. Poaceae phylogeny with 3595 taxa.

C₄ lineages are mapped in blue. Red labels indicate the PACMAD clade, yellow labels indicate the BEP clade, and grey labels indicate the early diverging Poaceae lineages. Lineage names are abbreviated as: O.P. Outlying Panicoideae, Ehrh. Ehrhartoideae, Ar.M. Arundinoideae+Micrairoideae, and Arist. Aristidoideae.

Figure 2. Histograms of BiSSE model inferences based on 100 replicate PACMAD trees.

Each tree had 1774 taxa, and the missing diversity was represented as a proportion (sampling frequency). Black bars indicate C₄ rates, white bars indicate C₃ rates. The panels show: a. Net diversification rates derived from a 6-parameter model, b. Chi Squared values derived from ANOVA comparison of a 6-parameter model and a 4-parameter (equal diversification) model for each tree. The red lines indicate significance values of .05, .01, and .005.

Figure 3. Simplified representation of shifts in diversification rates across Poaceae based on calibration #1.

Darker shades of grey indicate higher rates of diversification. Red triangles indicate the approximate phylogenetic placement of C₄ lineages. The left point of each triangle corresponds to the stem age of the inferred shift. The transition from dark green to yellow across the bottom indicates the average timing of the rise of open, grassland habitats on different continents . Rate shifts correspond to Table S5 and are labeled as follows: 1) background diversification rate, 2) BEP+PACMAD 3)Bambusoideae+Pooideae, 4) early diverging Pooideae, 5)Phaneospermateae, 6) Perrierbambus+Bonia clade 7) Poeae 2 clade, 8) Poa+Alopecurus clade, 9) Agrostis+Calamagrostis clade, 10) Festuca, 11) Core Panicineae, 12) Andropogoneae+Paspaleae, 13) Sorghum+Andropogon clade, 14) Axonopus+Paspalum clade, 15) Poecilostachys, 16) Eragrostis clade, 17) Spartina clade, 18) Tripogon (Table S5).

Figure 4. Timing of shifts in diversification rate across both dating hypotheses.

Both grey and black rectangles indicate shifts in diversification rate bounded by the estimated stem and crown node ages for the branch where the shift occurred. Error bars are determined by the 95% confidence interval for each age estimate. Each shift is numbered are corresponds to the shifts in both Figure 3 and Table S5. The black rectangles indicate accelerations that occurred within C₄ clades or immediately before C₄ origins. Orange dots indicate the crown node ages for each of the estimated 24 origins of C₄ photosynthesis in Poaceae. The red diamonds are the origins that are associated with subsequent rate shifts. The blue area indicates the time when grasslands are estimated to have arisen on various continents –, , , and the green area is the time when C₃ grasslands were replaced by C₄ grasses –, . Overlap between the two is indicated by diagonal hatches.