Abstract
Nitraria is a halophytic taxon (i.e., adapted to saline environments) that belongs to the plant family Nitrariaceae and is distributed from the Mediterranean, across Asia into the south-eastern tip of Australia. This taxon is thought to have originated in Asia during the Paleogene (66–23 Ma), alongside the proto-Paratethys epicontinental sea. The evolutionary history of Nitraria might hold important clues on the links between climatic and biotic evolution but limited taxonomic documentation of this taxon has thus far hindered this line of research. Here we investigate if the pollen morphology and the chemical composition of the pollen wall are informative of the evolutionary history of Nitraria and could explain if origination along the proto-Paratethys and dispersal to the Tibetan Plateau was simultaneous or a secondary process. To answer these questions, we applied a novel approach consisting of a combination of Fourier Transform Infrared spectroscopy (FTIR), to determine the chemical composition of the pollen wall, and pollen morphological analyses using Light Microscopy (LM) and Scanning Electron Microscopy (SEM). We analysed our data using ordinations (principal components analysis and non-metric multidimensional scaling), and directly mapped it on the Nitrariaceae phylogeny to produce a phylomorphospace and a phylochemospace. Our LM, SEM and FTIR analyses show clear morphological and chemical differences between the sister groups Peganum and Nitraria. Differences in the morphological and chemical characteristics of highland species (Nitraria schoberi, N. sphaerocarpa, N. sibirica and N. tangutorum) and lowland species (Nitraria billardierei and N. retusa) are very subtle, with phylogenetic history appearing to be a more important control on Nitraria pollen than local environmental conditions. Our approach shows a compelling consistency between the chemical and morphological characteristics of the eight studied Nitrariaceae species, and these traits are in agreement with the phylogenetic tree. Taken together, this demonstrates how novel methods for studying fossil pollen can facilitate the evolutionary investigation of living and extinct taxa, and the environments they represent.
Highlights
The steppe biome occurs all around the world, covering large areas in Africa, the Middle East, Australia and Eurasia
Our data compilation of extant records shows that Nitraria is mostly distributed in central Asia, with N. pamirica, N. roborowskii, N. sphaerocarpa, N. sibirica and N. tangutorum occurring only at the Tibetan Plateau and northwest China (Fig. 1)
According to our occurrence data and distributional data from Fang, Wang & Tang (2011), the species N. roborowskii, N. sphaerocarpa, N. sibirica and N. tangutorum are all well represented across the Tibetan Plateau, whereas N. pamirica only occurs in the most northwestern part of China (Fang, Wang & Tang, 2011) and in southwest China (Pan, Shen & Chen, 1999)
Summary
The steppe biome occurs all around the world, covering large areas in Africa, the Middle East, Australia and Eurasia (see ‘Desert and Xeric Shrublands’ and ‘Mediterranean Forests, Woodlands and Scrubs’ distribution in Olson et al, 2001; Fig. 1). The Asian steppes are thought to have formed in response to the India-Asia collision, Tibetan Plateau uplift, the retreat of the epicontinental proto-Paratethys sea (which is called the Paratethys sea from 33.9–28.1 Ma; Popov et al, 2004) and global cooling (e.g., Han et al, 2016; Bosboom et al, 2014; Bougeois et al, 2018). These processes have shaped present-day population structures and distributions, little is known about the evolution of the Tibetan steppe environment. How the steppe-desert evolved through time is insufficiently known and a better understanding of the ancestral steppe taxa may help us to clarify the overall evolution of the Tibetan steppe environment
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