Grass diversification in Madagascar: In situ radiation of two large C3 shade clades and support for a Miocene to Pliocene origin of C4 grassy biomes

Aim: Grasses (Poaceae) are found in all major habitats of Madagascar and have a particular importance in C-4 grasslands, whose origins are controversial. We aimed to estimate the number, age and origins of endemic grass lineages in the Madagascar region, and to compare the diversification of C-3 and C-4 taxa. Location: Madagascar and the surrounding Indian Ocean islands, integrated within a global dataset. Methods: We estimated 11 time-calibrated molecular phylogenies including 73% of Madagascar's known grass flora (65% of endemics), using two calibration scenarios. Integrating the available sequences from worldwide grass species, a total of 1928 accessions were analysed. We tested range evolution models, estimated ancestral ranges, and compared the patterns of lineage accumulation between endemic C-3 and C-4 grasses. Results: We recovered 69 lineages endemic to or with an estimated origin in the Madagascar region, 25 of them C-3 and 44 C-4. Range evolution analysis suggests widespread distance-scaling of dispersal and strongest historical links to Africa. Extant grass diversity largely accumulated since the Miocene, with parallel increases in C-3 and C-4 taxa. Two large C-3 groups in the Forest shade clade (Paniceae: Boivinellinae) and the bamboos (subtribe Hickeliinae) have an estimated origin in the Madagascar region. Divergences and crown ages of endemic C-4 lineages largely coincide with the Miocene grassland expansion. Main conclusions: Madagascar's extant grass flora is the result of multiple overseas dispersals, predominantly from Africa, and diversified from the Miocene onwards. C-3 grasses are characterized by two large presumed in situ radiations of shade grasses in the Paniceae and bamboos. Endemic C-4 lineages result from twice as many immigration events, resulting in smaller clades. Ages of C-4 lineages are consistent with a Pliocene or Late Miocene origin of grasslands in Madagascar, but estimating the nature and expanse of such early grasslands will require further research.