BackgroundThe taxonomy and systematics of Salix subgenus Salix s.l. is difficult. The reliability and evolutionary implications of two important morphological characters (number of stamens, and morphology of bud scales) used in subgeneric classification within Salix remain untested, and a disjunct Old–New World distribution pattern of a main clade of subgenus Salix s.l., revealed by a previous study, lacks a reasonable explanation. To study these questions, we conducted phylogenetic analyses based on 4,688 bp of sequence data from four plastid (rbcL, trnD–T, matK, and atpB–rbcL) and two nuclear markers (ETS and ITS) covering all subgenera of Salix, and all sections of subgenus Salix s.l.ResultsSubgenus Salix came out as para- or polyphyletic in both nrDNA and plastid trees. The plastid phylogeny successfully resolved relationships among the major clades of Salix, but resolution within subgenus Salix s.l. remained low. Nevertheless, three monophyletic groups were identifiable in subgenus Salix s.l.: the ‘main clade’ of subgenus Salix s.l., with New and Old World species being reciprocally monophyletic; the section Triandroides clade; and the subgenus Pleuradenia clade. While nrDNA regions showed higher resolution within subgenus Salix s.l., they failed to resolve subgeneric relationships. Extensive, statistically significant gene-tree incongruence was detected across nrDNA–plastid as well as nrDNA ETS–ITS phylogenies, suggesting reticulate evolution or hybridization within the group. The results were supported by network analyses. Ancestral-state reconstructions indicated that multiple stamens and free bud scales represent the plesiomorphic states within Salix, and that several significant shifts in stamen number and bud scale morphology have occurred.ConclusionsSubgenus Salix s.l. is not monophyletic, and the evolutionary history of the subgenus has involved multiple reticulation events that may mainly be due to hybridization. The delimitation of subgenus Salix s.l. should be redefined by excluding section Triandrae and subgenus Pleuradenia from it. The evolutionary lability of bud-scale morphology and stamen number means that these characters are unreliable bases for classification. The disjunct Old–New World distribution of subgenus Salix s.l. appears to be linked to the profound climatic cooling during the Tertiary, which cut off gene exchange between New and Old World lineages.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0311-7) contains supplementary material, which is available to authorized users.

By following the evolution of populations that are initially genetically homogeneous, much can be learned about core biological principles. For example, it allows for detailed studies of the rate of emergence of de novo mutations and their change in frequency due to drift and selection. Unfortunately, in multicellular organisms with generation times of months or years, it is difficult to set up and carry out such experiments over many generations. An alternative is provided by “natural evolution experiments” that started from colonizations or invasions of new habitats by selfing lineages. With limited or missing gene flow from other lineages, new mutations and their effects can be easily detected. North America has been colonized in historic times by the plant Arabidopsis thaliana, and although multiple intercrossing lineages are found today, many of the individuals belong to a single lineage, HPG1. To determine in this lineage the rate of substitutions—the subset of mutations that survived natural selection and drift–, we have sequenced genomes from plants collected between 1863 and 2006. We identified 73 modern and 27 herbarium specimens that belonged to HPG1. Using the estimated substitution rate, we infer that the last common HPG1 ancestor lived in the early 17th century, when it was most likely introduced by chance from Europe. Mutations in coding regions are depleted in frequency compared to those in other portions of the genome, consistent with purifying selection. Nevertheless, a handful of mutations is found at high frequency in present-day populations. We link these to detectable phenotypic variance in traits of known ecological importance, life history and growth, which could reflect their adaptive value. Our work showcases how, by applying genomics methods to a combination of modern and historic samples from colonizing lineages, we can directly study new mutations and their potential evolutionary relevance.

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Salix is a taxonomically difficult genus and its generic limits and infrageneric division are still highly controversial. We employed sequences of the chloroplast rbcL gene, trnD‐T spacer and atpB‐rbcL spacer for phylogenetic analyses of Salix. The results strongly support that Salix, Toisusu and Chosenia form a monophyletic group, confirming the merge of Toisusu and Chosenia with Salix. Within Salix s.l., two major clades are recognizable; one contains species of subgenus Salix (excluding sections Triandrae and Urbanianae), with the New World and the Old World species forming strongly supported clades, respectively; and the other includes sections Triandrae, Urbanianae, Chosenia and all species of subgenera Chamaetia and Vetrix, with the latter two subgenera forming a robustly supported subclade but with low resolution. A new infrageneric classification of Salix is herein proposed. The previously recognized subgenus Salix is split into three subgenera: Salix, Chosenia and Triandrae, and subgenera Chamaetia and Vetrix are combined as subgenus Vetrix.

The Hengduan Mountains (HDM) biodiversity hotspot exhibits exceptional alpine plant diversity. Here, we investigate factors driving intraspecific divergence within a HDM alpine species Salix brachista (Cushion willow), a common component of subnival assemblages. We produce a high-quality genome assembly for this species and characterize its genetic diversity, population structure and pattern of evolution by resequencing individuals collected across its distribution. We detect population divergence that has been shaped by a landscape of isolated sky island-like habitats displaying strong environmental heterogeneity across elevational gradients, combined with population size fluctuations that have occurred since approximately the late Miocene. These factors are likely important drivers of intraspecific divergence within Cushion willow and possibly other alpine plants with a similar distribution. Since intraspecific divergence is often the first step toward speciation, the same factors can be important contributors to the high alpine species diversity in the HDM.

The hierarchical open-structured core–shell Cu(OH)2@CoNiCH NTs/CF achieves extraordinary OER performance with a low overpotential and low Tafel slope.

Developing low-cost and high-performance bifunctional electrocatalysts is an essential prerequisite for obtaining clean hydrogen energy via overall water splitting. Herein, we present a metal–organic framework-derived route for the synthesis of N-doped carbon encapsulated Co x Fe1–x P nanoparticles (Co x Fe1–x P/NC), which show remarkable electrocatalytic activity toward both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media. The optimized Co0.17Fe0.79P/NC shows low overpotentials at a current density of 10 mA cm–2 (139 and 299 mV) and small Tafel slopes (57 and 44 mV dec–1) for HER and OER, respectively. The water electrolyzer using Co0.17Fe0.79P/NC as both anode and cathode reaches the current density of 10 mA cm–2 at a voltage of 1.66 V, and exhibits excellent long-term stability for 35 h. These excellent catalytic performances are mainly ascribed to the facile NC scaffold and slight Co incorporation, which provide large surface contact area between catalyst and electrolyte, fast charge and mass transport pathways, and abundant and stable active sites.

It is still a challenge for design and fabrication of cost-effective and efficient bifunctional electrocatalysts for both cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) for overall water splitting. Herein, we design and synthesize amorphous NiFe nanotube arrays on nickel foam (NiFe NTAs-NF) with high electrocatalytic activity and excellent durability for both OER and HER in overall water splitting. The as-synthesized NiFe NTAs-NF only requires relatively low overpotentials of 216 mV for the OER and 181 mV for the HER to reach current densities of 50 and 10 mA cm–2, respectively. Moreover, when used as bifunctional catalysts for water splitting, the designed electrode only needs a low cell voltage of 1.62 V to obtain 10 mA cm–2 for the overall water splitting, with an extremely excellent durability. The excellent performance of the NiFe NTAs-NF might be attributed to the synergistic effect and amorphous phase of NiFe alloy as well as the well-defined nanotube array architecture with large surface area, abundant active sites, and sufficient gas and electrolyte diffusion channels.