Domestic yaks (Bos grunniens) provide meat and other necessities for Tibetans living at high altitude on the Qinghai-Tibetan Plateau and in adjacent regions. Comparison between yak and the closely related low-altitude cattle (Bos taurus) is informative in studying animal adaptation to high altitude. Here, we present the draft genome sequence of a female domestic yak generated using Illumina-based technology at 65-fold coverage. Genomic comparisons between yak and cattle identify an expansion in yak of gene families related to sensory perception and energy metabolism, as well as an enrichment of protein domains involved in sensing the extracellular environment and hypoxic stress. Positively selected and rapidly evolving genes in the yak lineage are also found to be significantly enriched in functional categories and pathways related to hypoxia and nutrition metabolism. These findings may have important implications for understanding adaptation to high altitude in other animal species and for hypoxia-related diseases in humans.

Despite the high economic and ecological importance of forests, our knowledge of the genomic evolution of trees under salt stress remains very limited. Here we report the genome sequence of the desert poplar, Populus euphratica, which exhibits high tolerance to salt stress. Its genome is very similar and collinear to that of the closely related mesophytic congener, P. trichocarpa. However, we find that several gene families likely to be involved in tolerance to salt stress contain significantly more gene copies within the P. euphratica lineage. Furthermore, genes showing evidence of positive selection are significantly enriched in functional categories related to salt stress. Some of these genes, and others within the same categories, are significantly upregulated under salt stress relative to their expression in another salt-sensitive poplar. Our results provide an important background for understanding tree adaptation to salt stress and facilitating the genetic improvement of cultivated poplars for saline soils.

Summary• The cytokinin oxidase ⁄ dehydrogenase (CKX) gene plays a principal role in controlling cytokinin levels and has been shown to be a major quantitative trait locus (QTL) affecting grain number in rice. However, the function and evaluation of the haplotypes of the wheat CKX gene have yet to be illustrated.• In this study, TaCKX6-D1, a wheat ortholog of rice OsCKX2, was cloned and its haplotype variants were determined to be significantly associated with the 1000-grain weight on the basis of linkage mapping, association analysis and gene expression analysis.• Five TaCKX6-D1 haplotypes, designated a-e, were identified. An indel marker was developed to identify haplotype a, which was associated with higher grain weight. Haplotype a showed decreased expression relative to haplotype b in seeds at 8 d after pollination. Sequence variations among modern cultivars, landraces and wild species suggest a significant domestication signature at the TaCKX6-D1 locus in Chinese wheat germplasm.• TaCKX6-D1 may serve as a useful gene for the breeding of high-yielding wheat. A strategy for allele mining and utilization of TaCKX6-D1 was proposed. Our study also sheds light on the mechanisms of grain development and domestication of wheat, as well as the functional divergence of orthologs in comparative genomics.

The hippocampus and septum play central roles in one of the most important spheres of brain function: learning and memory. Although their topographic connections have been known for two decades and topography may be critical for cognitive functions, the basis for hippocamposeptal topographic projection is unknown. We now report for the first time that Elf-1, a membrane-bound eph family ligand, is a candidate molecular tag for the genesis of the hippocamposeptal topographic projection. Elf-i is expressed in an increasing gradient from dorsal to ventral septum. Furthermore, Elf-i selectively allows growth of neurites from topographically appropriate lateral hippocampal neurons, while inhibiting neurite outgrowth by medial hippocampal neurons.Complementary to the expression of Elf-1, an eph family receptor, Bsk, is expressed in the hippocampus in a lateral to medial gradient, consistent with a function as a receptor for Elf-i. Further, Elf-1 specifically bound Bsk, eliciting tyrosine kinase activity. We conclude that the Elf-1/Bsk ligandreceptor pair exhibits traits of a chemoaffinity system for the organization of hippocamposeptal topographic projections.Topographic projection is a general feature of brain architecture, and appears to be critical for appropriate coding and processing of information (1). Nevertheless, little is known about the mechanisms that govern topographic organization. Among the many regions exhibiting topographic relations, the hippocampus and septum have been the focus of intense interest, since these structures play central roles in learning and memory (2-5). The hippocampus projects to the lateral septum and receives afferents from the medial septum (6-8). Moreover, hippocampal projections to the lateral septum are arranged in a precise order. Axons from the medial hippocampus project to the dorsal lateral septum, whereas axons from the lateral hippocampus project to the ventral lateral septum (6-8). Molecular mechanisms underlying these topographic projections are unknown.The development of topographic projections is thought to require both long-range signals to guide axons to the general target area and local cues to specify individual targets precisely for each axon terminal (9-11). Long-range signals are likely to be diffusible, such as netrins, which attract selected, distant growth cones, while repelling others (12)(13)(14)(15). In contrast, local guidance cues must match axon terminals and specific cellular targets, a requirement accommodated by matching fixed tags on afferents and corresponding targets. Complementarity of molecular tags on afferents and targets was first postulated by Sperry in his chemoaffinity hypothesis more than 50 years ago (16,17). Only recently have specific candidate molecules been identified (18)(19)(20)(21)(22). The eph family ligand and receptor, Elf-1 and Mek4, are expressed as complementary gradients in the tectum and retina, respectively (18). The repulsive axon guidance signal, a molecule closely related to Elf-1, repels the growth of retina...