Opposite trends in the genus Monsonia (Geraniaceae): specialization in the African deserts and range expansions throughout eastern Africa

Sara García-Aloy,Gudrun Kadereit,Marisa Alarcón,Juan José Aldasoro,Ana María Millanes,Daniel Vitales,Cristina Roquet,Isabel Sanmartín,Pablo Vargas

doi:10.1038/s41598-017-09834-6

Abstract

The African Austro-temperate Flora stands out by its important species richness. A distinctive element of this flora is Monsonia (Geraniaceae), mostly found in the Namib-Karoo but also in the Natal-Drakensberg, the Somalian Zambezian and the Saharo-Arabian regions. Here, we reconstruct the evolution and biogeographic history of Monsonia based on nuclear and plastid markers, and examine the role of morphological and niche evolution in its diversification using species distribution modeling and macroevolutionary models. Our results indicate that Monsonia first diversified in the Early Miocene c.21 Ma, coinciding with the start of desertification in southwestern Africa. An important diversification occurred c. 4–6 Ma, after a general cooling trend in western South Africa and the rising of the Eastern African Mountains. The resulting two main lineages of Monsonia are constituted by: (1) Namib-Karoo succulents, and (2) herbs of the Natal-Drakensberg plus three species that further colonised steppes in north and eastern Africa. The highest diversity of Monsonia is found in the Namib-Karoo coastal belt, within a mosaic-like habitat structure. Diversification was likely driven by biome shifts and key innovations such as water-storing succulent stems and anemochorous fruits. In contrast, and unlike other arid-adapted taxa, all species of Monsonia share a C3 metabolism.

Highlights

The African Austro-temperate Flora stands out by its important species richness
The Namib Desert stretches from southern Angola through Namibia into South Africa with a variable width (80–200 km) and gradual rise from the coast to the Namib Escarpment
We investigate whether the appearance of key innovations related to succulence, dispersal and metabolism type (C3, CAM) drove the diversification of Monsonia, enabling the colonization of novel biomes, such as arid environments that became dominant in the Late Miocene

Summary

Introduction

The African Austro-temperate Flora stands out by its important species richness. A distinctive element of this flora is Monsonia (Geraniaceae), mostly found in the Namib-Karoo and in the NatalDrakensberg, the Somalian Zambezian and the Saharo-Arabian regions. The Namib Desert stretches from southern Angola through Namibia into South Africa (i.e. the Succulent Karoo) with a variable width (80–200 km) and gradual rise from the coast to the Namib Escarpment These climate changes generated a wealth of new habitats across southwestern Africa, and triggered speciation in drought-tolerant geophytes, succulents, and species with CAM/C4 metabolism[1, 14, 15]. The uplift of the East African Rift System during the Plio-Pleistocene underlies diversification rate shifts in other groups[11, 15] Between these mountain ranges and the West-Central Guineo-Congolian region is the “African Arid Corridor” (AAC) or the “Arid Track”, a corridor that connects South Africa with other regions, such as the Great Rift, the North African savannas and the Sahara Desert[17]

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Conclusion