The 22,000-year-old cave painting of an Atlantic salmon (Salmo salar) near the Vézère River in France is a reminder of our fascination with, and dependence on, Atlantic salmon throughout human history. Atlantic salmon belongs to the salmonid lineage which comprises 11 genera, with at least 70 species that exhibit a wide range of ecological adaptations and use a variety of marine and freshwater life history strategies 1 . Salmonids hold important positions as socially iconic species and economic resources within aquaculture, wild fisheries and recreational sport fisheries. Moreover, they serve as key indicator species of the health of North Atlantic and Pacific coastal and river ecosystems.All teleosts share at least three rounds of whole-genome duplication (WGD), 1R and 2R before the divergence of lamprey from the jawed vertebrates 2 , and a third teleost-specific WGD (Ts3R) at the base of the teleosts ~320 million years ago (Mya)  . Very little is known about the mechanisms of genomic and chromosomal reorganization after WGD in vertebrates because the 1R, 2R and Ts3R occurred so long ago that few clear signatures of post-WGD reorganization events remain. In contrast, a fourth WGD (the Ss4R salmonid-specific autotetraploidization event) occurred in the common ancestor of salmonids ~80 Mya after their divergence from Esociformes ~125 Mya 6-8 (Fig. 1), and the continued presence of multivalent pairing at meiosis and evidence of tetrasomic inheritance in salmonid species suggests that diploidy is not yet fully re-established 6,9,10 . Salmonids thus appear to provide an unprecedented opportunity for studying vertebrate genome evolution after an autotetraploid WGD 11,12 over a time period that is long enough to reveal long-term evolutionary patterns, but short enough to give a high-resolution picture of the process. In addition, they provide an excellent setting for contextualizing genome evolution with a dramatic post-WGD species radiation and intricate adaptations to a whole range of life history regimes.Here we present a high-quality reference genome assembly of the Atlantic salmon, and use it to describe major patterns characterizing the post-Ss4R salmonid genome evolution over the past 80 million years (Myr). Our results challenge the recent claim that rediploidization in salmonids has been a gradual process unlinked to significant genome rearrangements 13 . They also challenge current views about the relative importance of sub-and neofunctionalization in vertebrate genomes (reviewed in ref. 14), and the importance of dosage balance as a gene duplicate retention mechanism 15 . Genome characterizationThe Atlantic salmon reference genome assembly (GenBank: GCA_000233375.4) adds up to 2.97 gigabases (Gb) with aThe whole-genome duplication 80 million years ago of the common ancestor of salmonids (salmonid-specific fourth vertebrate whole-genome duplication, Ss4R) provides unique opportunities to learn about the evolutionary fate of a duplicated vertebrate genome in 70 extant lineages. Here we present a high...
A key goal of biology is to understand phenotypic characteristics, such as health, disease and evolutionary fitness. Phenotypic variation is produced through a complex web of interactions between genotype and environment, and such a 'genotype-phenotype' map is inaccessible without the detailed phenotypic data that allow these interactions to be studied. Despite this need, our ability to characterize phenomes - the full set of phenotypes of an individual - lags behind our ability to characterize genomes. Phenomics should be recognized and pursued as an independent discipline to enable the development and adoption of high-throughput and high-dimensional phenotyping.
Atlantic cod (Gadus morhua) is a large, cold-adapted teleost that sustains long-standing commercial fisheries and incipient aquaculture1,2. Here we present the genome sequence of Atlantic cod, showing evidence for complex thermal adaptations in its haemoglobin gene cluster and an unusual immune architecture compared to other sequenced vertebrates. The genome assembly was obtained exclusively by 454 sequencing of shotgun and paired-end libraries, and automated annotation identified 22,154 genes. The major histocompatibility complex (MHC) II is a conserved feature of the adaptive immune system of jawed vertebrates3,4, but we show that Atlantic cod has lost the genes for MHCII, CD4 and Ii that are essential for the function of this pathway. Nevertheless, Atlantic cod is not exceptionally susceptible to disease under natural conditions5. We find a highly expanded number of MHCI genes and a unique composition of its Toll-like receptor (TLR) families. This suggests how the Atlantic cod immune system has evolved compensatory mechanisms within both adaptive and innate immunity in the absence of MHCII. These observations affect fundamental assumptions about the evolution of the adaptive immune system and its components in vertebrates.
Haplodiploid organisms comprise about 20% of animals. Males develop from unfertilized eggs while females are derived from fertilized eggs. The underlying mechanisms of sex determination, however, appear to be diverse and are poorly understood. We have dissected the complementary sex determiner (csd) locus in the honeybee to understand its molecular basis. In this species, csd acts as the primary sex-determining signal with several alleles segregating in populations. Males are hemizygous and females are heterozygous at this locus; nonreproducing diploid males occur when the locus is homozygous. We have characterized csd by positional cloning and repression analysis. csd alleles are highly variable and no transcription differences were found between sexes. These results establish csd as a primary signal that governs sexual development by its allelic composition. Structural similarity of csd with tra genes of Dipteran insects suggests some functional relation of what would otherwise appear to be unrelated sex-determination mechanisms.
A fundamental goal in biology is to achieve a mechanistic understanding of how and to what extent ecological variation imposes selection for distinct traits and favors the fixation of specific genetic variants. Key to such an understanding is the detailed mapping of the natural genomic and phenomic space and a bridging of the gap that separates these worlds. Here we chart a high-resolution map of natural trait variation in one of the most important genetic model organisms, the budding yeast Saccharomyces cerevisiae, and its closest wild relatives and trace the genetic basis and timing of major phenotype changing events in its recent history. We show that natural trait variation in S. cerevisiae exceeds that of its relatives, despite limited genetic variation, and follows the population history rather than the source environment. In particular, the West African population is phenotypically unique, with an extreme abundance of low-performance alleles, notably a premature translational termination signal in GAL3 that cause inability to utilize galactose. Our observations suggest that many S. cerevisiae traits may be the consequence of genetic drift rather than selection, in line with the assumption that natural yeast lineages are remnants of recent population bottlenecks. Disconcertingly, the universal type strain S288C was found to be highly atypical, highlighting the danger of extrapolating gene-trait connections obtained in mosaic, lab-domesticated lineages to the species as a whole. Overall, this study represents a step towards an in-depth understanding of the causal relationship between co-variation in ecology, selection pressure, natural traits, molecular mechanism, and alleles in a key model organism.
BackgroundThe Atlantic salmon genome is in the process of returning to a diploid state after undergoing a whole genome duplication (WGD) event between 25 and100 million years ago. Existing data on the proportion of paralogous sequence variants (PSVs), multisite variants (MSVs) and other types of complex sequence variation suggest that the rediplodization phase is far from over. The aims of this study were to construct a high density linkage map for Atlantic salmon, to characterize the extent of rediploidization and to improve our understanding of genetic differences between sexes in this species.ResultsA linkage map for Atlantic salmon comprising 29 chromosomes and 5650 single nucleotide polymorphisms (SNPs) was constructed using genotyping data from 3297 fish belonging to 143 families. Of these, 2696 SNPs were generated from ESTs or other gene associated sequences. Homeologous chromosomal regions were identified through the mapping of duplicated SNPs and through the investigation of syntenic relationships between Atlantic salmon and the reference genome sequence of the threespine stickleback (Gasterosteus aculeatus). The sex-specific linkage maps spanned a total of 2402.3 cM in females and 1746.2 cM in males, highlighting a difference in sex specific recombination rate (1.38:1) which is much lower than previously reported in Atlantic salmon. The sexes, however, displayed striking differences in the distribution of recombination sites within linkage groups, with males showing recombination strongly localized to telomeres.ConclusionThe map presented here represents a valuable resource for addressing important questions of interest to evolution (the process of re-diploidization), aquaculture and salmonid life history biology and not least as a resource to aid the assembly of the forthcoming Atlantic salmon reference genome sequence.
Vitellogenin is a female-specific glucolipoprotein yolk precursor produced by all oviparous animals. Vitellogenin expression is under hormonal control, and the protein is generally synthesized directly before yolk deposition. In the honeybee (Apis mellifera), vitellogenin is not only synthesized by the reproductive queen, but also by the functionally sterile workers. In summer, the worker population consists of a hive bee group performing a multitude of tasks including nursing inside the nest, and a forager group specialized in collecting nectar, pollen, water, and propolis. Vitellogenin is synthesized in large quantities by hive bees. When hive bees develop into foragers, their juvenile hormone titers increase, and this causes cessation of their vitellogenin production. This inverse relationship between vitellogenin synthesis and juvenile hormone is opposite to the norm in insects, and the underlying proximate processes and life-history reasons are still not understood. Here we document an alternative use of vitellogenin by showing that it is a source for the proteinaceous royal jelly that is produced by the hive bees. Hive bees use the jelly to feed larvae, queen, workers, and drones. This finding suggests that the evolution of a brood-rearing worker class and a specialized forager class in an advanced eusocial insect society has been directed by an alternative utilization of yolk protein.
Functionally sterile honey bee workers synthesize the yolk protein vitellogenin while performing nest tasks. The subsequent shift to foraging is linked to a reduced vitellogenin and an increased juvenile hormone (JH) titer. JH is a principal controller of vitellogenin expression and behavioral development. Yet, we show here that silencing of vitellogenin expression causes a significant increase in JH titer and its putative receptor. Mathematically, the increase corresponds to a dynamic dose-response. This role of vitellogenin in the tuning of the endocrine system is uncommon and may elucidate how an ancestral pathway of fertility regulation has been remodeled into a novel circuit controlling social behavior.
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