Sweet vernal grasses (Anthoxanthum) colonized African mountains along two fronts in the Late Pliocene, followed by secondary contact, polyploidization and local extinction in the Pleistocene

Abstract

High tropical mountains harbour remarkable and fragmented biodiversity thought to a large degree to have been shaped by multiple dispersals of cold-adapted lineages from remote areas. Few dated phylogenetic/phylogeographic analyses are however available. Here, we address the hypotheses that the sub-Saharan African sweet vernal grasses have a dual colonization history and that lineages of independent origins have established secondary contact. We carried out rangewide sampling across the eastern African high mountains, inferred dated phylogenies from nuclear ribosomal and plastid DNA using Bayesian methods, and performed flow cytometry and AFLP (amplified fragment length polymorphism) analyses. We inferred a single Late Pliocene western Eurasian origin of the eastern African taxa, whose high-ploid populations in one mountain group formed a distinct phylogeographic group and carried plastids that diverged from those of the currently allopatric southern African lineage in the Mid- to Late Pleistocene. We show that Anthoxanthum has an intriguing history in sub-Saharan Africa, including Late Pliocene colonization from southeast and north, followed by secondary contact, hybridization, allopolyploidization and local extinction during one of the last glacial cycles. Our results add to a growing body of evidence showing that isolated tropical high mountain habitats have a dynamic recent history involving niche conservatism and recruitment from remote sources, repeated dispersals, diversification, hybridization and local extinction.

Afro-alpine flagships revisited: Parallel adaptation, intermountain admixture and shallow genetic structuring in the giant senecios (Dendrosenecio)

“…Even when the alpine habitat extended 1000 m further down the mountains and covered an area eight times larger than today during the cold and dry glaciations, migration corridors with alpine habitat did not form across the Ugandan lowland gap [30], leaving long-distance dispersal as the only plausible mechanism facilitating gene flow between Mt Muhavura and Mt Elgon/Cherangani Hills (Figs 2 & 3). This is in line with several studies suggesting that LDD events, with high levels of stochasticity, have been a major driver shaping the genetic structuring in afro-alpine plants [72][73][74][75][76]. The two western mountains (Mt Ruwenzori and Mt Muhavura) are ecologically quite different.…”

“…Our findings of low within-population genetic diversity in Dendrosenecio (mean D = 0.081-0.094 for mountain groups; Table 2, S1 Appendix) are in line with recent studies of other afro-alpine plants [72,73,75,76,78,79]. These values are much lower than those reported in a compilation of some 300 studies of vascular plants from other regions [mean D = 0.23, SD 0.08; 80].…”

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

“…Endemic C 3 lineages of high altitudes in the Pooideae ( Agrostis , Anthoxanthum , Festuca and Poa ; less clear in Brachypodium ) appear to have immigrated from Eurasia in the Pliocene or more recently. They clearly form part of a “pan‐temperate element” in the African mountain flora (Gehrke & Linder, ; Minaya et al., ; Tusiime et al., ). Further endemic mountain C 3 grasses in Sartidia (Aristidoideae), Pentameris and Merxmuellera (both Danthonioideae) appear to have their origin in Southern Africa, which is consistent with previous hypotheses (Besnard et al., , for Sartidia ; Linder, Rabosky, Antonelli, Wüest, & Ohlemüller, , for Danthonioideae).…”

History and evolution of the afroalpine flora: in the footsteps of Olov Hedberg

“…In the Late Pliocene, sub-Saharan Africa was colonized by two divergent lineages of Anthoxanthum, one from southeast Asia to southern Africa, and one from western Eurasia to eastern Africa (Pimentel et al 2013). Intriguingly, these currently allopatric lineages were inferred to have met in East Africa during the Pleistocene and produced an allopolyploid (Box 3, Tusiime et al 2017).…”

“…The most clear-cut evidence of extinction was found for Anthoxanthum; the currently allopatric southern and northern lineages once met in East Africa and produced an allopolyploid, but the southern lineage is absent from East Africa today (Fig. 8, Box 3; Tusiime et al 2017). In the Lychnis phylogeny, the species are distinctly differentiated, but the terminal branches within species are typically very short and suggest not only recent colonisation of individual mountains, but also frequent local extinctions (Fig.…”