Sex determination: the amphibian models

We review and discuss current knowledge about sex determination in amphibians. The astonishing wide variety of mechanisms of genotypic sex determination is presented and discussed in an evolutionary context. We recall the natural occurrence of transitory juvenile hermaphroditism in some species. Our present knowledge of the mechanisms of sex determination in amphibians is compared to that in mammals. The influence of epigenetic factors, and especially temperature is highlighted. In amphibians, the influence of temperature on sexual differentiation, that can prevail over genotypic sex determination, remains poorly considered in publications. We suggest that studies on genetic and epigenetic factors of gonadal sex differentiation in amphibians could provide substantial information on the evolutionary process of sex determination mechanisms in current living vertebrates.     

Cell death and sexual differentiation of behavior: worms, flies, and mammals

Sex differences in the nervous system are found throughout the animal kingdom. Here, we discuss three prominent genetic models: nematodes, fruit flies, and mice. In all three, differential cell death is central to sexual differentiation and shared molecular mechanisms have been identified. Our knowledge of the precise function of neural sex differences lags behind. One fruitful approach to the 'function' question is to contrast sexual differentiation in standard laboratory animals with differentiation in species exhibiting unique social and reproductive organizations. Advanced genetic strategies are also addressing this question in worms and flies, and may soon be applicable to vertebrates.     

Male and female recombination landscapes of diploid Arabidopsis arenosa

The number and placement of meiotic crossover events during meiosis have important implications for the fidelity of chromosome segregation as well as patterns of inheritance. Despite the functional importance of recombination, recombination landscapes vary widely among and within species, and this can have a strong impact on evolutionary processes. A good knowledge of recombination landscapes is important for model systems in evolutionary and ecological genetics, since it can improve interpretation of genomic patterns of differentiation and genome evolution, and provides an important starting point for understanding the causes and consequences of recombination rate variation. Arabidopsis arenosa is a powerful evolutionary genetic model for studying the molecular basis of adaptation and recombination rate evolution. Here, we generate genetic maps for 2 diploid A. arenosa individuals from distinct genetic lineages where we have prior knowledge that meiotic genes show evidence of selection. We complement the genetic maps with cytological approaches to map and quantify recombination rates, and test the idea that these populations might have distinct patterns of recombination. We explore how recombination differs at the level of populations, individuals, sexes and genomic regions. We show that the positioning of crossovers along a chromosome correlates with their number, presumably a consequence of crossover interference, and discuss how this effect can cause differences in recombination landscape among sexes or species. We identify several instances of female segregation distortion. We found that averaged genome-wide recombination rate is lower and sex differences subtler in A. arenosa than in Arabidopsis thaliana.     

Endocrine and genetic control of sex differentiation in the Malacostracan Crustacea

Summary

Sex differentiation in Malacostraca is controlled by hormone secreted from the androgenic glands. Experimentally induced sex inversions in isopods and amphipods proved that the genetic female and male possess primordia of the androgenic glands, gonads, and gonoducts, along with sexual characteristics of both sexes. During the sensitive period, the presence or absence of androgenic gland hormone (AGH) affects the differentiation of these primordia.

Genetic control of the development of androgenic gland primordium seems to be brought about assuming of the following: 1. Both genetic female and male possess gene(s) (AGH-G) responsible for the AGH-synthesis situated on the homologous loci of the sex chromosomes and/or on the autosomes. 2. The gene(s) are activated spontaneously with the lack of inhibition of the major sex factor carried by the W or X chromosome. The W and X factors are hypostatic to major sex factor carried by the Y chromosome. The Z factor does not seem to influence sex differentiation. Sufficient allochthonous AGH seems to render the W and X factors ineffective.     

Genetic control of quiescence in hematopoietic stem cells

Cellular quiescence is a reversible cell cycle arrest that is poised to re-enter the cell cycle in response to a combination of cell-intrinsic factors and environmental cues. In hematopoietic stem cells, a coordinated balance between quiescence and differentiating proliferation ensures longevity and prevents both genetic damage and stem cell exhaustion. However, little is known about how all these processes are integrated at the molecular level. We will briefly review the environmental and intrinsic control of stem cell quiescence and discuss a new model that involves a protein-to-protein interaction between G0S2 and the phospho-nucleoprotein nucleolin in the cytosol.     

Sex determination in flatfishes: Mechanisms and environmental influences

Flounder of the genus Paralichthys exhibit a unique mode of sex determination where both low and high temperatures induce male-skewed sex ratios, while intermediate temperatures produce a 1:1 sex ratio. Male differentiation is thus easily induced in genetic females creating a combination of genetic (GSD) and environmental sex determination (ESD). Since male flounder become reproductively fit at substantially smaller body sizes than females, temperature or other environmental variables that elicit lower growth rates may also influence sex differentiation toward male development. This review covers our current knowledge of sex determination and differentiation in flatfishes including possible adaptive significance of ESD and involvement of factors such as aromatase (cyp19).     

Sex-chromosome evolution: recent progress and the influence of male and female heterogamety

It is now clear that sex chromosomes differ from autosomes in many aspects of genome biology, such as organization, gene content and gene expression. Moreover, sex linkage has numerous evolutionary genetic implications. Here, I provide a coherent overview of sex-chromosome evolution and function based on recent data. Heteromorphic sex chromosomes are almost as widespread across the animal and plant kingdoms as sexual reproduction itself and an accumulating body of genetic data reveals interesting similarities, as well as dissimilarities, between organisms with XY or ZW sex-determination systems. Therefore, I discuss how patterns and processes associated with sex linkage in male- and female-heterogametic systems offer a useful contrast in the study of sex-chromosome evolution.     

遗传学教学中性别决定关键基因的阐述

有性生殖的出现是生物进化中的重大事件。性别作为生物的一种重要而又复杂的表型, 由基因和环境因素共同控制, 其中遗传因素即基因起到非常关键的作用。 然而, 并不是每个相关基因对于生物的性别都具有相同的作用, 性别决定关键基因对生物性别的决定和性别的分化具有重要作用, 因而研究和理解性别决定的关键基因具有重要意义。随着现代遗传学的发展, 目前关于生物性别决定方式以及性别决定关键基因的研究已取得了很大的进展。文章就生物的基因性别决定机制以及基因性别决定机制的研究策略进行了综述, 以期在遗传学教学中能更好地理解和阐述。     

In search of determinants: gene expression during gonadal sex differentiation

The diversity of inputs that guide sexual fate during development is both intriguing and daunting. In the field of fish biology, the study of sex determination is of great importance. For example, in aquaculture, sexually dimorphic growth rates and overall size leads to one sex being more marketable than the other. Moreover, for breeding purposes it is important to maintain balanced sex ratios. Furthermore, sex determination is sensitive to environmental factors, such as temperature and contaminants, which can lead to skewed sex ratios, intersexes and sterility in wild or farmed fish. The gonad is typically the first organ to exhibit morphological signs of sexual dimorphism and therefore is likely to be the primary organ system whose fate is controlled by the sex determination cues in many fish species. Additionally, the sexual fate of the gonad has been shown to fully or partially control organismal sex differentiation. Thus, understanding the genetic regulation of gonadal sex differentiation is critical in studies of fish sex determination. This review summarizes recent knowledge of genes expressed during gonadal sex differentiation in gonochoristic teleost fish. Three species are discussed, which serve as excellent model systems for probing teleost sex differentiation: the Oreochromis niloticus, Oryzias latipes and Danio rerio. The similarities and differences between gonadal gene expression in these three species and in comparison to mammals suggest conserved roles during vertebrate gonadal sex differentiation. In the future, it will be essential to develop tools to assay the function of genes expressed during gonadal sex differentiation in fish.     

Zebrafish monosex population reveals female dominance in sex determination and earliest events of gonad differentiation

The zebrafish is a popular model for genetic analysis and its sex differentiation has been the focus of attention for breeding purposes. Despite numerous efforts, very little is known about the mechanism of zebrafish sex determination. The lack of discernible sex chromosomes and the difficulty of distinguishing the sex of juvenile fish are two major obstacles that hamper the progress in such studies. To alleviate these problems, we have developed a scheme involving methyltestosterone treatment followed by natural mating to generate fish with predictable sex trait. Female F1 fish that gave rise to all-female offspring were generated. This predictable sex trait enables characterization of gonadal development in juvenile fish by histological examination and gene expression analysis. We found the first sign of zebrafish sex differentiation to be ovarian gonocyte proliferation and differentiation at 10 to 12 days post-fertilization (dpf). Somatic genes were expressed indifferently at 10 to 17 dpf, and then became sexually dimorphic at three weeks. This result indicates clear distinction of male and female gonads derived independently from primordial gonads. We classified the earliest stages of zebrafish sex determination into the initial preparation followed by female germ cell growth, oocyte differentiation, and somatic differentiation. Our genetic selection scheme matches the prediction that female-dominant genetic factors are required to determine zebrafish sex.     

How do genes regulate simple behaviours? Understanding how different neurons in the vertebrate spinal cord are genetically specified

Understanding how the vertebrate central nervous system develops and functions is a major goal of a large body of biological research. This research is driven both by intellectual curiosity about this amazing organ that coordinates our conscious and unconscious bodily processes, perceptions and actions and by the practical desire to develop effective treatments for people with spinal cord injuries or neurological diseases. In recent years, we have learnt an impressive amount about how the nerve cells that communicate with muscles, motoneurons, are made in a developing embryo and this knowledge has enabled researchers to grow motoneurons from stem cells. Building on the success of these studies, researchers have now started to unravel how most of the other nerve cells in the spinal cord are made and function. This review will describe what we currently know about spinal cord nerve cell development, concentrating on the largest category of nerve cells, which are called interneurons. I will then discuss how we can build and expand upon this knowledge base to elucidate the complete genetic programme that determines how different spinal cord nerve cells are made and connected up into neural circuits with particular functions.     

Drosophila sex combs as a model of evolutionary innovations

The diversity of animal and plant forms is shaped by nested evolutionary innovations. Understanding the genetic and molecular changes responsible for these innovations is therefore one of the key goals of evolutionary biology. From the genetic point of view, the origin of novel traits implies the origin of new regulatory pathways to control their development. To understand how these new pathways are assembled in the course of evolution, we need model systems that combine relatively recent innovations with a powerful set of genetic and molecular tools. One such model is provided by the Drosophila sex comb—a male‐specific morphological structure that evolved in a relatively small lineage related to the model species D. melanogaster. Our extensive knowledge of sex comb development in D. melanogaster provides the basis for investigating the genetic changes responsible for sex comb origin and diversification. At the same time, sex combs can change on microevolutionary timescales and differ spectacularly among closely related species, providing opportunities for direct genetic analysis and for integrating developmental and population‐genetic approaches. Sex comb evolution is associated with the origin of novel interactions between Hox and sex determination genes. Activity of the sex determination pathway was brought under the control of the Hox code to become segment‐specific, while Hox gene expression became sexually dimorphic. At the same time, both Hox and sex determination genes were integrated into the intrasegmental spatial patterning network, and acquired new joint downstream targets. Phylogenetic analysis shows that similar sex comb morphologies evolved independently in different lineages. Convergent evolution at the phenotypic level reflects convergent changes in the expression of Hox and sex determination genes, involving both independent gains and losses of regulatory interactions. However, the downstream cell‐differentiation programs have diverged between species, and in some lineages, similar adult morphologies are produced by different morphogenetic mechanisms. These features make the sex comb an excellent model for examining not only the genetic changes responsible for its evolution, but also the cellular processes that translate DNA sequence changes into morphological diversity. The origin and diversification of sex combs provides insights into the roles of modularity, cooption, and regulatory changes in evolutionary innovations, and can serve as a model for understanding the origin of the more drastic novelties that define higher order taxa.     

The Decapentaplegic morphogen gradient: from pattern formation to growth regulation

Morphogens have been linked to numerous developmental processes, including organ patterning and the control of organ size. Here we review how different experimental approaches have led to an unprecedented level of molecular knowledge about the patterning role of the Drosophila melanogaster morphogen Decapentaplegic (DPP, the homologue of vertebrate bone morphogenetic protein, or BMP), the first validated secreted morphogen. In addition, we discuss how little is known about the role of the DPP morphogen in the control of organ growth and organ size. Continued efforts to elucidate the role of DPP in D. melanogaster is likely to shed light on this fundamental question in the near future.