Abstract
Tropical savannas cover over 20% of land surface. They sustain a high diversity of mammalian herbivores and promote frequent fires, both of which are dependent on the underlying grass composition. These habitats are typically dominated by relatively few taxa, and the evolutionary origins of the dominant grass species are largely unknown. Here, we trace the origins of the genus Themeda, which contains a number of widespread grass species dominating tropical savannas. Complete chloroplast genomes were assembled for seven samples and supplemented with chloroplast and nuclear ITS markers for 71 samples representing 18 of the 27 Themeda species. Phylogenetic analysis supports a South Asian origin for both the genus and the widespread dominant T. triandra. This species emerged ~1.5 Ma from a group that had lived in the savannas of Asia for several million years. It migrated to Australia ~1.3 Ma and to mainland Africa ~0.5 Ma, where it rapidly spread in pre-existing savannas and displaced other species. Themeda quadrivalvis, the second most widespread Themeda species, is nested within T. triandra based on whole chloroplast genomes, and may represent a recent evolution of an annual growth form that is otherwise almost indistinguishable from T. triandra. The recent spread and modern-day dominance of T. triandra highlight the dynamism of tropical grassy biomes over millennial time-scales that has not been appreciated, with dramatic shifts in species dominance in recent evolutionary times. The ensuing species replacements likely had profound effects on fire and herbivore regimes across tropical savannas.
Highlights
Tropical savannas are among Earth’s most productive biomes and cover around 20% of global land area (Lehmann and Parr 2016)
Sanger-sequenced chloroplast data were obtained for a further 62 samples with a mean of 3,572 bp per sample (SD = 1623 bp; Table S1). These were aligned to the reference chloroplast genome, trimmed of the inverted repeat, and the phylogeny inferred from this 125,270 bp alignment included 18 of the 27 Themeda species (Table S1)
A previously published chloroplast genome attributed to Themeda arguens (L.) Hack. may come from an incorrectly identified individual as it is nested within T. triandra (Figure 1) and is subsequently treated as such
Summary
Tropical savannas are among Earth’s most productive biomes and cover around 20% of global land area (Lehmann and Parr 2016). There is evidence for the co-evolution of grazing mammals with the grass species they consume (Bouchenak-Khelladi et al 2009; Sage and Stata 2015), just as there is palaeo-evidence of a global increase in fire with the expansion of C4 grass-dominated savannas (Edwards et al 2010) Today, these tropical grassy biomes both sustain large numbers of grazing mammals and promote frequent fire (Archibald and Hempson 2016), and feedbacks between both fire and herbivory have been demonstrated as central to the modern dynamics of tropical grassy biomes over much of their range (Archibald and Hempson 2016; Lehmann et al 2014). The origins of these co-dependency feedbacks remain, poorly understood
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