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حيوانات غريبة : صور من المتحف البريطاني
يحوي هذا الكتاب الرائع مجموعة فريدة من الصور المقتناة في المتحف البريطاني، تعود إلى الفترة ما بين القرن الخامس عشر وبداية القرن التاسع عشر، وتحيط القارئ بنمط من الثقافة البصرية نابض بالحياة. ويعرض الكتاب أعمالا فنية لدورر وغويا وستوبس وبيويك، فضلا عن صور بديعة لفنانين أقل شهرة، وهي تضم جمعا من المخلوقات الجميلة والغريبة، كالحوت الجانح على الشاطئ، والخنزير المسخ، والنيص الشهير. وتعبر صور الكتاب عن الفضول تجاه العالم الطبيعي، الذي تزامن مع اختراع الحفر الطباعي في أوروبا، فلقد كانت الصور المطبوعة قبل ظهور التصوير الفوتوغرافي هي الوسيلة الرئيسة لتبادل المعلومات (صحيحها وخاطئها) حول الأنواع الحيوانية الجديدة والمعروفة على حد سواء. وصيغت النظرة إلى الحيوانات بشكل أساسي عبر علاقتها بعالم البشر، فقد جرى تمثيل الكثير منها كي تفسر وفق ما ترمز إليه أو ما تقدمه من مواعظ أخلاقية للبشرية، في حين تظهر صور الصيد، وتربية المواشي، وحدائق الحيوان، اعتماد البشر الدائم عليها.
BOOK
How to make a sex chromosome
Sex chromosomes can evolve once recombination is halted between a homologous pair of chromosomes. Owing to detailed studies using key model systems, we have a nuanced understanding and a rich review literature of what happens to sex chromosomes once recombination is arrested. However, three broad questions remain unanswered. First, why do sex chromosomes stop recombining in the first place? Second, how is recombination halted? Finally, why does the spread of recombination suppression, and therefore the rate of sex chromosome divergence, vary so substantially across clades? In this review, we consider each of these three questions in turn to address fundamental questions in the field, summarize our current understanding, and highlight important areas for future work.
Sex chromosome evolution begins when recombination between a homologous pair of chromosomes is halted. Here, Wright
et al.
review our current understanding of the causes and mechanisms of recombination suppression between incipient sex chromosomes and suggest future directions for the field.
Journal Article
Sexual selection drives evolution and rapid turnover of male gene expression
Significance Genes with different expression between males and females (sex-biased genes) show rapid rates of sequence and expression divergence in a range of taxa. These characteristics have led many to assume that sex-biased genes are the product of sexual selection and sexual conflict, but this assumption remains to be rigorously tested. Using a phylogenetically controlled analysis of birds that exhibit diverse levels of sexual selection, we show a rapid turnover in sex-biased gene expression primarily through evolution of male expression levels and that the degree of sexual selection predicts the proportion of male-biased genes but does not account for rates of coding sequence evolution. We also discuss the impact of allometry on gene expression studies, an issue rarely discussed in the literature.
The profound and pervasive differences in gene expression observed between males and females, and the unique evolutionary properties of these genes in many species, have led to the widespread assumption that they are the product of sexual selection and sexual conflict. However, we still lack a clear understanding of the connection between sexual selection and transcriptional dimorphism, often termed sex-biased gene expression. Moreover, the relative contribution of sexual selection vs. drift in shaping broad patterns of expression, divergence, and polymorphism remains unknown. To assess the role of sexual selection in shaping these patterns, we assembled transcriptomes from an avian clade representing the full range of sexual dimorphism and sexual selection. We use these species to test the links between sexual selection and sex-biased gene expression evolution in a comparative framework. Through ancestral reconstruction of sex bias, we demonstrate a rapid turnover of sex bias across this clade driven by sexual selection and show it to be primarily the result of expression changes in males. We use phylogenetically controlled comparative methods to demonstrate that phenotypic measures of sexual selection predict the proportion of male-biased but not female-biased gene expression. Although male-biased genes show elevated rates of coding sequence evolution, consistent with previous reports in a range of taxa, there is no association between sexual selection and rates of coding sequence evolution, suggesting that expression changes may be more important than coding sequence in sexual selection. Taken together, our results highlight the power of sexual selection to act on gene expression differences and shape genome evolution.
Journal Article
Convergent recombination suppression suggests role of sexual selection in guppy sex chromosome formation
Sex chromosomes evolve once recombination is halted between a homologous pair of chromosomes. The dominant model of sex chromosome evolution posits that recombination is suppressed between emerging X and Y chromosomes in order to resolve sexual conflict. Here we test this model using whole genome and transcriptome resequencing data in the guppy, a model for sexual selection with many Y-linked colour traits. We show that although the nascent Y chromosome encompasses nearly half of the linkage group, there has been no perceptible degradation of Y chromosome gene content or activity. Using replicate wild populations with differing levels of sexually antagonistic selection for colour, we also show that sexual selection leads to greater expansion of the non-recombining region and increased Y chromosome divergence. These results provide empirical support for longstanding models of sex chromosome catalysis, and suggest an important role for sexual selection and sexual conflict in genome evolution.
It has been suggested that sex chromosomes arise as a result of sexual conflict, resulting in selection against recombination between chromosomes. Here, the authors resequence laboratory and wild guppy populations with differing levels of sexual antagonism, providing support for this long-held view.
Journal Article
Extreme heterogeneity in sex chromosome differentiation and dosage compensation in livebearers
Once recombination is halted between the X and Y chromosomes, sex chromosomes begin to differentiate and transition to heteromorphism. While there is a remarkable variation across clades in the degree of sex chromosome divergence, far less is known about the variation in sex chromosome differentiation within clades. Here, we combined whole-genome and transcriptome sequencing data to characterize the structure and conservation of sex chromosome systems across Poeciliidae, the livebearing clade that includes guppies. We found that the Poecilia reticulata XY system is much older than previously thought, being shared not only with its sister species, Poecilia wingei, but also with Poecilia picta, which diverged roughly 20 million years ago. Despite the shared ancestry, we uncovered an extreme heterogeneity across these species in the proportion of the sex chromosome with suppressed recombination, and the degree of Y chromosome decay. The sex chromosomes in P. reticulata and P. wingei are largely homomorphic, with recombination in the former persisting over a substantial fraction. However, the sex chromosomes in P. picta are completely nonrecombining and strikingly heteromorphic. Remarkably, the profound degradation of the ancestral Y chromosome in P. picta is counterbalanced by the evolution of functional chromosome-wide dosage compensation in this species, which has not been previously observed in teleost fish. Our results offer important insight into the initial stages of sex chromosome evolution and dosage compensation.
Journal Article
Ecology and allometry predict the evolution of avian developmental durations
The duration of the developmental period represents a fundamental axis of life-history variation, yet broad insights regarding the drivers of this diversity are currently lacking. Here, we test mechanistic and ecological explanations for the evolution of developmental duration using embryological data and information on incubation and fledging for 3096 avian species. Developmental phases associated primarily with growth are the longest and most variable, consistent with a role for allometric constraint in determining the duration of development. In addition, developmental durations retain a strong imprint of deep evolutionary history and body size differences among species explain less variation than previously thought. Finally, we reveal ecological correlates of developmental durations, including variables associated with the relative safety of the developmental environment and pressures of breeding phenology. Overall, our results provide broad-scale insight into the relative importance of mechanistic, ecological and evolutionary constraints in shaping the diversification of this key life-history trait.
Developmental duration is a key life-history trait. Cooney et al. compile data on 3096 bird species to quantify the degree to which phylogenetic history, body size and ecological variables like predation risk or breeding phenology influence variation in developmental duration.
Journal Article
Masculinization of Gene Expression Is Associated with Exaggeration of Male Sexual Dimorphism
Gene expression differences between the sexes account for the majority of sexually dimorphic phenotypes, and the study of sex-biased gene expression is important for understanding the genetic basis of complex sexual dimorphisms. However, it has been difficult to test the nature of this relationship due to the fact that sexual dimorphism has traditionally been conceptualized as a dichotomy between males and females, rather than an axis with individuals distributed at intermediate points. The wild turkey (Meleagris gallopavo) exhibits just this sort of continuum, with dominant and subordinate males forming a gradient in male secondary sexual characteristics. This makes it possible for the first time to test the correlation between sex-biased gene expression and sexually dimorphic phenotypes, a relationship crucial to molecular studies of sexual selection and sexual conflict. Here, we show that subordinate male transcriptomes show striking multiple concordances with their relative phenotypic sexual dimorphism. Subordinate males were clearly male rather than intersex, and when compared to dominant males, their transcriptomes were simultaneously demasculinized for male-biased genes and feminized for female-biased genes across the majority of the transcriptome. These results provide the first evidence linking sexually dimorphic transcription and sexually dimorphic phenotypes. More importantly, they indicate that evolutionary changes in sexual dimorphism can be achieved by varying the magnitude of sex-bias in expression across a large proportion of the coding content of a genome.
Journal Article
Single-cell consequences of X-linked meiotic drive in stalk-eyed flies
Sex-linked meiotic drivers limit the inheritance of the alternate sex chromosome in the heterogametic sex, subsequently skewing the offspring sex ratio. They consequently have large impacts on genome evolution, adaptation, and the emergence and maintenance of sexually selected traits. Despite this, our understanding of their molecular basis and consequences for gametogenesis and sex chromosome regulation more broadly has focused on a handful of model organisms, primarily Drosophila and mouse, which are not representative of the broad diversity of reproductive modes and drive systems in nature. Here, we employ single-cell RNA sequencing (scRNA-seq) to investigate a sex-linked meiotic driver in the Malaysian stalk-eyed fly, Teleopsis dalmanni . First, we produce a comprehensive single-cell atlas of the male T. dalmanni gonad and identify major testis cell types. We then provide a comprehensive profile of the cellular and transcriptional landscape of the testis, providing evidence for a lack of complete meiotic sex chromosome inactivation and complex trajectory of dosage compensation. Second, by contrasting single-cell expression data between drive and standard testes, we provide insight into the consequences of a meiotic driver for the transcriptomic landscape of the testis and sex chromosome regulation. Importantly, we show that the presence of a meiotic driver does not perturb fundamental patterns of X-linked regulation. Our results provide insight into how the meiotic driver might bias its transmission to the next generation and highlight genes with perturbed expression as a potential consequence of the disruption of spermatogenesis.
Journal Article
Positive Selection Underlies Faster-Z Evolution of Gene Expression in Birds
The elevated rate of evolution for genes on sex chromosomes compared with autosomes (Fast-X or Fast-Z evolution) can result either from positive selection in the heterogametic sex or from nonadaptive consequences of reduced relative effective population size. Recent work in birds suggests that Fast-Z of coding sequence is primarily due to relaxed purifying selection resulting from reduced relative effective population size. However, gene sequence and gene expression are often subject to distinct evolutionary pressures; therefore, we tested for Fast-Z in gene expression using next-generation RNA-sequencing data from multiple avian species. Similar to studies of Fast-Z in coding sequence, we recover clear signatures of Fast-Z in gene expression; however, in contrast to coding sequence, our data indicate that Fast-Z in expression is due to positive selection acting primarily in females. In the soma, where gene expression is highly correlated between the sexes, we detected Fast-Z in both sexes, although at a higher rate in females, suggesting that many positively selected expression changes in females are also expressed in males. In the gonad, where intersexual correlations in expression are much lower, we detected Fast-Z for female gene expression, but crucially, not males. This suggests that a large amount of expression variation is sex-specific in its effects within the gonad. Taken together, our results indicate that Fast-Z evolution of gene expression is the product of positive selection acting on recessive beneficial alleles in the heterogametic sex. More broadly, our analysis suggests that the adaptive potential of Z chromosome gene expression may be much greater than that of gene sequence, results which have important implications for the role of sex chromosomes in speciation and sexual selection.
Journal Article
Detecting signatures of selection on gene expression
A substantial amount of phenotypic diversity results from changes in gene expression levels and patterns. Understanding how the transcriptome evolves is therefore a key priority in identifying mechanisms of adaptive change. However, in contrast to powerful models of sequence evolution, we lack a consensus model of gene expression evolution. Furthermore, recent work has shown that many of the comparative approaches used to study gene expression are subject to biases that can lead to false signatures of selection. Here we first outline the main approaches for describing expression evolution and their inherent biases. Next, we bridge the gap between the fields of phylogenetic comparative methods and transcriptomics to reinforce the main pitfalls of inferring selection on expression patterns and use simulation studies to show that shifts in tissue composition can heavily bias inferences of selection. We close by highlighting the multi-dimensional nature of transcriptional variation and identifying major unanswered questions in disentangling how selection acts on the transcriptome.
This paper examines the main approaches for studying gene expression evolution, tests their inherent biases and discusses open questions about the evolution of the transcriptome.
Journal Article