Abstract
Here we conduct research to understand the evolutionary history of a shrubby species known as Agarito (Berberis trifoliolata), an endemic species to the Chihuahuan Desert. We identify genetic signatures based on plastid DNA and AFLP markers and perform niche modelling and spatial connectivity analyses as well as niche modelling based on records in packrats to elucidate whether orogenic events such as mountain range uplift in the Miocene or the contraction/expansion dynamics of vegetation in response to climate oscillations in the Pliocene/Pleistocene had an effect on evolutionary processes in Agarito. Our results of current niche modelling and palaeomodelling showed that the area currently occupied by Berberis trifoliolata is substantially larger than it was during the Last Interglacial period and the Last Glacial Maximum. Agarito was probably confined to small areas in the Northeastern and gradually expanded its distribution just after the Last Glacial Maximum when the weather in the Chihuahuan Desert and adjacent regions became progressively warmer and drier. The most contracted range was predicted for the Interglacial period. Populations remained in stable areas during the Last Glacial Maximum and expanded at the beginning of the Holocene. Most genetic variation occured in populations from the Sierra Madre Oriental. Two groups of haplotypes were identified: the Mexican Plateau populations and certain Northeastern populations. Haplogroups were spatially connected during the Last Glacial Maximum and separated during interglacial periods. The most important prediction of packrat middens palaeomodelling lies in the Mexican Plateau, a finding congruent with current and past niche modelling predictions for agarito and genetic results. Our results corroborate that these climate changes in the Pliocene/Pleistocene affected the evolutionary history of agarito. The journey of agarito in the Chihuahuan Desert has been dynamic, expanding and contracting its distribution range and currently occupying the largest area in its history.
Highlights
We identify genetic signatures, based on chloroplast DNA and AFLPs along with ecological niche modelling and spatial connectivity analyses using the Chihuahuan Desert shrub Berberis trifoliolata as a model to: (1) understand whether the extent of its distribution varied in different periods and whether these changes are linked to differences in population genetics; (2) recognize whether Miocene orogeny and/or Pleistocene/Pliocene climate fluctuations influenced the evolutionary processes in Agarito; (3) assess how historical climate change has influenced the spatial connectivity of natural populations
Our AFLP results showed a genetic break between the Mexican Plateau and Northeastern populations, it was not identified by our cpDNA markers
Differences in genetic structure revealed by the AFLP and plastid DNA of populations has been often reported in several plants and attributed mostly to gene flow by pollination (e.g. [88], [89], [90])
Summary
The origin and evolution of the biota of the five North American deserts (Great Basin, Mojave, Colorado Plateau, Sonoran and Chihuahuan) have been of great interest to scientists since the last century [1], [2], [3], [4], [5], [6], [7], and their evolutionary processes have been associated with historical orogenic and climate events [8], [9], [10], [11], [12].In the deserts of North America the uplift of mountain chains occurred in different stages from the middle of the Miocene to as recently as the Pliocene, a series of events known as the Neogene uplift (~15–2 Ma), while climate fluctuations are more recent, through the Pleistocene [1], [2], [4], [13], [6], [7], [10]. A largescale aridification event began after the end of the Last Glacial Maximum in northern Mexico and adjacent southwestern United States, which allowed the xeric flora to establish [21], [22], [23], [13], [21], [24]. This dry xeric vegetation was abundant until 8000 years ago [25], [26], and subsequent changes to semi-desert grassland and eventually to desert shrubby vegetation have been documented [27], [21], [23], [13] [20]. Vicariance events related to Miocene orogenic activity (e.g. [8], [31], [9], [32]), and changes in genetic and geographic structure influenced by climate fluctuations during the Pleistocene (e.g. [33], [34], [35], [12]) have been events detected in a number of phylogeographic studies conducted on the plants and animals of the North American arid regions
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