Birds are the most species-rich class of tetrapod vertebrates and have wide relevance across many research fields. We explored bird macroevolution using full genomes from 48 avian species representing all major extant clades. The avian genome is principally characterized by its constrained size, which predominantly arose because of lineage-specific erosion of repetitive elements, large segmental deletions, and gene loss. Avian genomes furthermore show a remarkably high degree of evolutionary stasis at the levels of nucleotide sequence, gene synteny, and chromosomal structure. Despite this pattern of conservation, we detected many non-neutral evolutionary changes in protein-coding genes and noncoding regions. These analyses reveal that pan-avian genomic diversity covaries with adaptations to different lifestyles and convergent evolution of traits.

Using next-generation sequencing technology alone, we have successfully generated and assembled a draft sequence of the giant panda genome. The assembled contigs (2.25 gigabases (Gb)) cover approximately 94% of the whole genome, and the remaining gaps (0.05 Gb) seem to contain carnivore-specific repeats and tandem repeats. Comparisons with the dog and human showed that the panda genome has a lower divergence rate. The assessment of panda genes potentially underlying some of its unique traits indicated that its bamboo diet might be more dependent on its gut microbiome than its own genetic composition. We also identified more than 2.7 million heterozygous single nucleotide polymorphisms in the diploid genome. Our data and analyses provide a foundation for promoting mammalian genetic research, and demonstrate the feasibility for using next-generation sequencing technologies for accurate, cost-effective and rapid de novo assembly of large eukaryotic genomes.

The existence of males and females, which are often strikingly different in morphology, reproductive strategies and behavior, is one of the most widespread phenomena in biology. However, the genetic mechanisms that generate this ubiquitous pattern are surprisingly diverse and do not follow a phylogenetic pattern. Sex-determination mechanisms can differ between even closely related species and arise frequently and independently. Fish provide a paradigmatic example, as their sex-determination mechanisms range from environmental to different modes of genetic determination. The evolutionary meaning of this remarkable plasticity is unknown. For genetic sex determination, where the trigger for female or male development comes from the genetic constitution of the individual, the evolution of sex-determination mechanisms is connected to a very peculiar genomic process, namely the formation of sex chromosomes [1][2][3][4] .To improve understanding of the function and evolution of sex chromosomes, their genetic organization must be deciphered.However, owing to their degenerate nature and high repetitive DNA content, sex chromosomes pose almost insurmountable problems in deciphering their gene content and organization. So far, only the human 5 , chimpanzee 6 and rhesus macaque Y chromosomes 7 and the male-specific region on the Y chromosome of one fish, the medaka 8 , have been sequenced. These analyses have nevertheless provided important insights into the evolution of Y chromosomes, their genomic organization and their degeneration processes, as well as predictions as to their likely evolutionary fate 9-12 .Much less genomic information exists on W chromosomes because, as with Y chromosomes, they are predominantly highly repetitive in nature. The prevailing theory of the evolution of sex chromosomes predicts that degeneration of the heterogametic sex chromosome is a stepwise process that occurs over an extended period of time. We therefore reasoned that an evolutionarily young W chromosome Whole-genome sequence of a flatfish provides insights into ZW sex chromosome evolution and adaptation to a benthic lifestyle Genetic sex determination by W and Z chromosomes has developed independently in different groups of organisms. To better understand the evolution of sex chromosomes and the plasticity of sex-determination mechanisms, we sequenced the whole genomes of a male (ZZ) and a female (ZW) half-smooth tongue sole (Cynoglossus semilaevis). In addition to insights into adaptation to a benthic lifestyle, we find that the sex chromosomes of these fish are derived from the same ancestral vertebrate protochromosome as the avian W and Z chromosomes. Notably, the same gene on the Z chromosome, dmrt1, which is the male-determining gene in birds, showed convergent evolution of features that are compatible with a similar function in tongue sole. Comparison of the relatively young tongue sole sex chromosomes with those of mammals and birds identified events that occurred during the early phase of sex-chromosome evolution. Pertinent to...

Sex-specific chromosomes, like the W of most female birds and the Y of male mammals, usually have lost most genes owing to a lack of recombination. We analyze newly available genomes of 17 bird species representing the avian phylogenetic range, and find that more than half of them do not have as fully degenerated W chromosomes as that of chicken. We show that avian sex chromosomes harbor tremendous diversity among species in their composition of pseudoautosomal regions and degree of Z/W differentiation. Punctuated events of shared or lineage-specific recombination suppression have produced a gradient of “evolutionary strata” along the Z chromosome, which initiates from the putative avian sex-determining gene DMRT1 and ends at the pseudoautosomal region. W-linked genes are subject to ongoing functional decay after recombination was suppressed, and the tempo of degeneration slows down in older strata. Overall, we unveil a complex history of avian sex chromosome evolution.

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