Evolutionary history of four Gymnocalycium species of the Sierras Pampeanas at intra- and interspecific level

Investigators: Rahul Raveendran Nair, Diego Ezequiel Gurvich, and Alicia Noemi Sérsic

Institution: Instituto Multidisciplinario de Biología Vegetal, IMBIV, FCEFyN-CONICET, Universidad Nacional de Córdoba, Argentina

Past evolutionary processes induced complex ecological changes leading to the formation of extant patterns of biological diversity at various spatio-temporal scales. Among the past evolutionary processes, quaternary climatic oscillations played a major role in speciation, delimiting species distributions, and formation of biotic communities. Besides the role of climate fluctuations in species evolution and dispersal, multi-millennial scale climate changes that occurred in the past significantly contributed to the evolution of various ecosystems. Arid and semi-arid environments form the most broadly spanned terrestrial ecosystem on Earth. Global aridification, occurred during late Pliocene/Miocene, was one of the major reasons behind the speciation and radiation of many species within the succulent plant families such as Cactaceae, Aizoaceae, or Agavaceae. Arid and semi-arid conditions induce drought-stress and thereby offer favorable conditions for succulent plants to undergo high diversification and proliferation. 

Gymnocalycium Pfeiff. ex Mittler (Cactaceae) is a genus of globose cacti containing ~50 species with an estimated divergence time of 6.12 Mya (late Miocene). It is currently distributed across southern Bolivia, southwestern and northern Paraguay, southern Brazil, Uruguay, and central Argentina. Most of the Gymnocalycium species (30) are distributed along the Sierras Pampeanas which offer unique habitats; actually, most species are endemic, with small ranges. The Sierras Pampeanas experienced various geological transformations ranging from the incursion of Paleozoic volcanic rocks during the Miocene to the recent compressional tectonics. Subduction of the Nazca plate beneath the South American plate produced an eastward deformation around 33º S, causing the exhumation and final uplift, at different geological times scales, of individual Pampean mountain ranges between 8-2.6 Mya. The Sierras Pampeanas constitute a collection of discontinuous mountain ranges running through the provinces of San Luis, San Juan, Córdoba, La Rioja, Catamarca, Santiago del Estero and Tucumán. The uplifted mountain terrains of Sierras Pampeanas may be considered as biogeographic islands, separated by valleys, and lowlands, favoring species isolation, resulting in the prevention of gene flow. Additionally, the Andean orogeny impeded the entry of humid Pacific winds, giving rise to the Arid Diagonal, possibly augmenting further species diversification, and distributional shifts. To date, biogeographic patterns of plants in the Sierras Pampeanas remain poorly known with an exception of the detailed exploration of spatial genetic structure of Sophora linearifolia, distributed in central highlands of Argentina.

We selected four Gymnocalycium species (G. andreae, G. baldianum, G. monvillei and G. pugionacanthum) (Figure 1). The selected Gymnocalycium membersexhibit specific habitat preferences across the ranges of Sierras Pampeanas (Figure 2). Gymnocalycium andreae is distributed at an elevation range of 1800—2300 m in the Sierras of Córdoba and San Luis provinces, G. baldianum inhabits similar environments as G. andreae, but occupying the Sierras Pampeanas of Catamarca (Ambato and Ancasti); G. monvillei grows on rocky outcrops of the Sierras de Córdoba and San Luis, with a  distributional range covering a wide extent of altitude (880— 2200 m). G. pugionacanthum inhabits Sierras of Catamarca within an altitudinal range of 1000 —1900 m. These proposed study sites have three major features favoring speciation and diversification: (1) island mountain systems, (2) occurrence of uplifts in different geological time scales, and (3) altitudinal gradient-based climatic niches.

Illustration for Nair research report, Gymnocalycium
Distribution map for Nair research report, Gymnocalycium

Further, historical processes that govern the process of speciation and diversification of G. andreae, G. baldianum, G. monvillei and G. pugionacanthum distributed in these restricted environments have not been investigated so far except the attempts to decipher the inter-specific phylogenetic relationship within the genus Gymnocalycium.

Therefore, in this project, we test the hypothesis that Miocene geotectonic processes mainly affected interspecific diversification of the genus Gymnocalycium, while climatic and altitudinal factors influenced lineage diversification within each Gymnocalycium species. Thus, we expect that a direct effect of the orogeny as a barrier for gene flow during the time lapse of the exhumation of the Sierras Pampeanas, between 8—2.6 Mya, affected diversification at interspecific level, we expect that diversification times (stem age) of each species would be concordant with the uplift time of the corresponding mountain range; and finally, we expect that climate fluctuations, occurred in the Pleistocene would be drivers of intraspecific lineage diversification, and that species survived at distinct climatic niches, characteristic of each biogeographic island.

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