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 C4 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 C3 and C4 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 C3 and C4 grasses. Results: We recovered 69 lineages endemic to or with an estimated origin in the Madagascar region, 25 of them C3 and 44 C4. 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 C3 and C4 taxa. Two large C3 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 C4 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. C3 grasses are characterized by two large presumed in situ radiations of shade grasses in the Paniceae and bamboos. Endemic C4 lineages result from twice as many immigration events, resulting in smaller clades. Ages of C4 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.