The recent and rapid spread of Themeda triandra

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 modernday 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.

Functional diversification enabled grassy biomes to fill global climate space

“…Perhaps ecological success facilitated diversification, such that large geographical ranges enabled by unique adaptations made the isolation of populations and allopatric speciation more likely (a geographic mechanism). The rapid spread of the cosmopolitan Themeda triandra from Asia to Africa in < 500,000 years supports this idea (52). Resolving the relative role of these mechanisms requires comparative phylogenetic analyses of the relationships among ecology, functional traits, range sizes and diversification rates.…”

“…The biogeographic contingencies described here in crossover temperatures align with emerging evidence that regional evolutionary and environmental histories have been important modifiers of biome-climate relationships (9, 53). However, the rapid rates of dispersal observed in grasses (52), along with their short generation times (49), raises critical questions about whether the biogeographic contingencies observed in woody plants should be mirrored in grassy communities.…”

Spatial, climate, and ploidy factors drive genomic diversity and resilience in the widespread grassThemeda triandra

“…In Australia, diploids and tetraploids are the most common ploidy variants, but triploid, pentaploid, hexaploid and aneuploid individuals have also been identified (Hayman, 1960). Past studies suggest that T. triandra originally evolved in tropical Asia and migrated through coastal corridors to Australia (Hayman, 1960), with Australian lineages diverging 1.37 mya (0.79 - 3.07 mya) (Dunning et al , 2017). However, dating using secondary calibrations, as in (Dunning et al , 2017) can lead to unreliable and overly young estimates of divergence (Schenk, 2016).…”

“…Past studies suggest that T. triandra originally evolved in tropical Asia and migrated through coastal corridors to Australia (Hayman, 1960), with Australian lineages diverging 1.37 mya (0.79 - 3.07 mya) (Dunning et al , 2017). However, dating using secondary calibrations, as in (Dunning et al , 2017) can lead to unreliable and overly young estimates of divergence (Schenk, 2016). Themeda triandra is widely considered a foundation species for three reasons: 1) it defines particular ecosystems (Snyman et al , 2013), 2) it controls the distribution and abundance of associated flora and fauna (Morgan, 1998), and 3) it regulates the core ecosystem processes especially through fire (Morgan & Lunt, 1999).…”

Phylogenomics using low‐depth whole genome sequencing: A case study with the olive tribe

“…However, the reads corresponding to nuclear low‐copy number loci included in these data sets are generally overlooked, although individual markers can be assembled using a reference‐based approach (Besnard et al, , ). In addition, genetic variation spread across the genome can be extracted from these data sets and used for phylogenetic inference or population genetics (Buerkle & Gompert, ; Dunning et al, ; Li, Sidore, Kang, Boehnke, & Abecasis, ; Olofsson et al, ). However, the usefulness of low‐depth sequence data sets to infer phylogenies that cover a large number of taxa with deep divergence times remains unclear, especially for capturing the diversity stored in herbaria and museums.…”