Historic demography changes of plant species adapted to New World arid environments could be consistent with either the Glacial Refugium Hypothesis (GRH), which posits that populations contracted to refuges during the cold-dry glacial and expanded in warm-humid interglacial periods, or with the Interglacial Refugium Hypothesis (IRH), which suggests that populations contracted during interglacials and expanded in glacial times. These contrasting hypotheses are developed in the present study for the giant columnar cactus Cephalocereus columna-trajani in the intertropical Mexican drylands where the effects of Late Quaternary climatic changes on phylogeography of cacti remain largely unknown. In order to determine if the historic demography and phylogeographic structure of the species are consistent with either hypothesis, sequences of the chloroplast regions psbA-trnH and trnT-trnL from 110 individuals from 10 populations comprising the full distribution range of this species were analysed. Standard estimators of genetic diversity and structure were calculated. The historic demography was analysed using a Bayesian approach and the palaeodistribution was derived from ecological niche modelling to determine if, in the arid environments of south-central Mexico, glacial-interglacial cycles drove the genetic divergence and diversification of this species. Results reveal low but statistically significant population differentiation (FST = 0.124, P < 0.001), although very clear geographic clusters are not formed. Genetic diversity, haplotype network and Approximate Bayesian Computation (ABC) demographic analyses suggest a population expansion estimated to have taken place in the Last Interglacial (123.04 kya, 95% CI 115.3–130.03). The species palaeodistribution is consistent with the ABC analyses and indicates that the potential area of palaedistribution and climatic suitability were larger during the Last Interglacial and Holocene than in the Last Glacial Maximum. Overall, these results suggest that C. columna-trajani experienced an expansion following the warm conditions of interglacials, in accordance with the GRH.
EUSO-Balloon is a pathfinder for JEM-EUSO, the mission concept of a spaceborne observatory which is designed to observe Ultra-High Energy Cosmic Ray (UHECR)-induced Extensive Air Showers (EAS) by detecting their UltraViolet (UV) light tracks “from above.” On August 25, 2014, EUSO-Balloon was launched from Timmins Stratospheric Balloon Base (Ontario, Canada) by the balloon division of the French Space Agency CNES. After reaching a floating altitude of 38 km, EUSO-Balloon imaged the UV light in the wavelength range ∼290–500 nm for more than 5 hours using the key technologies of JEM-EUSO. The flight allowed a good understanding of the performance of the detector to be developed, giving insights into possible improvements to be applied to future missions. A detailed measurement of the photoelectron counts in different atmospheric and ground conditions was achieved. By means of the simulation of the instrument response and by assuming atmospheric models, the absolute intensity of diffuse light was estimated. The instrument detected hundreds of laser tracks with similar characteristics to EASs shot by a helicopter flying underneath. These are the first recorded laser tracks measured from a fluorescence detector looking down on the atmosphere. The reconstruction of the direction of the laser tracks was performed. In this work, a review of the main results obtained by EUSO-Balloon is presented as well as implications for future space-based observations of UHECRs.
ABSTRACT. The endemic Mexican cactus, Mammillaria pectinifera, shows low dispersal capabilities and isolated populations within the highly dissected landscape of Tehuacán Valley. These characteristics can restrict gene flow and act upon the genetic divergence and speciation in arid plants. We conducted a phylogeographic study to determine if the origin, current distribution, and genetic structure of M. pectinifera were driven by Quaternary geomorphic processes. Sequences of the plastids psbA-trnH and trnT-trnL obtained from 66 individuals from seven populations were used to estimate genetic diversity. Population differentiation was assessed by an analysis of molecular variance. We applied a stepwise phylogenetic calibration test to determine whether species origin and genetic divergence among haplotypes were temporally concordant with recognizable episodes of geomorphic evolution. The combination of plastid markers yielded six haplotypes, with high levels of haplotype diversity (h = 0.622) and low nucleotide diversity (π = 0.00085). The populations were found to be genetically structured (F ST = 0.682; P < 0.00001), indicating that geographic isolation and limited dispersal were the primary causes of genetic population differentiation. The estimated origin and divergence time among haplotypes were 0.017-2.39 and 0.019-1.237 mya, respectively, which correlates with Pleistocene tectonics and erosion events, supporting a hypothesis of geomorphically-driven geographical isolation. Based on a Bayesian skyline plot, these populations showed long term demographic stability, indicating that persistence in confined habitats has been the main response of this species to landscape changes. We conclude that the origin and haplotype divergence of M. pectinifera were a response to local Quaternary geomorphic evolution.
The morphological evolution of the basins in the Sierra Madre del Sur (SMS), southern M exico is poorly understood. This work explains for the first time the geomorphological development of the tectonic, fluviallyinterconnected SMS basins named San Juan Raya (SJRb) and Zapotitl an (ZAPb). The evolution of the SJRb and ZAPb are analysed within the context of the transformations of the well-studied Tehuac an basin (TEHb). A new interpretation of a series of tectonic features of the TEHb valley area is also presented. Published geological data and extensive field work provided the basis for our geomorphological and evolutionary interpretation of basin evolution of this part of Mesoamerica during the late Cenozoic. Stratigraphic and sedimentary records suggest that after the late Cretaceous-early Cenozoic orogeny the TEHb and ZAPb were closed basins, and that the TEHb graben system was activated during the Paleogene as a response to the dominant regional NW-SE trending faults. We propose that the ZAPb and SJRb formed sequentially during the Neogene as a result of new E-W, N-S and NE-SW faults. The continuation of the TEHb extension during the Oligocene widened its lowland area and allowed the formation of an extensive lake. No alluvial or fluvial records of this interval are found in the ZAPb and SJRb. No sedimentation rather than formation and subsequent erosion of such sediments is supported by the basin morphology and by the absence of re-worked alluvial deposits at the outlet area where both connect to the TEHb. By middle to late Miocene the TEHb lost its endorheic configuration, ending the lake-type deposition while new faults initiated the opening of the ZAPb. Intensive tectonics, alluvial deposition and the confinement of the Tehuac an lake to the north sector of this basin characterised the Pliocene. During the late Pliocene to the early Pleistocene the formation of the SJRb was initiated. Quaternary faulting related to basin extension along the north watershed of the SJRb and ZAPb is supported by independent data on the biogeography of the cactus Mammillaria pectinifera. We introduce the idea that the departure from the regional NW-SE fault alignment that formed the major Miocene basins to a more local E-W trend that formed Neogene-Quaternary basins was probably a response to the latest post-orogenic relaxation of the crust in the Mixteca terrane.
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