Aspergillus genomes: secret sex and the secrets of sex

The genomic sequences of three species of Aspergillus, including the model organism A. nidulans (which is homothallic: having no differentiated mating types, a strain being able to cross with itself), suggest that A. fumigatus and A. oryzae, considered to be asexual, might in fact be heterothallic (having two differentiated mating types, a strain being able to cross only with strains of opposite mating type). The genomic data have implications for the understanding of the evolution and the mechanism of sexual reproduction in this genus. We propose a model of epigenetic heterothallism to account for the reproductive patterns observed in Aspergillus nidulans.     

Influence of sex of antecedent siblings on the human sex ratio

Analysis of family configurations in a population of 649,366 American secondary school students confirmed that sex of later-born children is influenced by the sex of antecedent siblings. Antecedent brothers decrease the probability of subsequent male births. This observation, a confirmation of an earlier report in a substantially smaller sample, is consistent with an immunologic influence on human sex determination.     

The effects of sex preselection on the sex ratio of families

Advances in the potential for couples to predetermine the sex of their children will have significant consequences for many aspects of society. Among the likely demographic impacts are changes in both population size and the sex ratio. The aim of this article is to assess the effects of sex preselection on the sex ratio of families. The expected family sex ratio is derived and characterized for couples with particular preferences for the sex composition of their families. When couples desire k children of one sex and none of the other, the proportion of children in the completed family that are of the desired sex falls with increasing k. Constraints on total family size further reduce this proportion. When couples have a desire for a balanced composition of one boy and one girl, and when they have a preference for, say, a boy to be born first, they can expect a proportion of boys in the family that at first rises, and then falls, as sex preselection methods improve.     

Origin and maintenance of sex: the evolutionary joys of self sex

Sex is generally thought of as meiosis, conjugation, and syngamy, with the primary function of sex believed to be genetic mixing. However, conjugation does not occur with complete automixis, whereas syngamy does not occur with restitutional automixis. Self sex in the forms of automixis and autogamy does not include genetic mixing. Yet sex, including self sex, is necessary for most eukaryotic lineages. What is the purpose of sex without genetic mixing? Obligate self sex is not an evolutionary dead end, but holds the key to understanding the evolutionary origin, function, maintenance, and ubiquity of sex. We extend the rejuvenescence hypothesis that sex provides a necessary developmental reset for multicellular eukaryotes and even many unicellular eukaryotes. Sex reduces additive genetic variance of epigenetic signals, especially cytosine methylation, and of ploidy levels. Furthermore, we argue that syngamy is a modified form of meiosis that maintains ploidy and resets epigenetic signals. Epigenetic resetting is consistent with sex being induced by starvation or desiccation. Diminution of additive genetic variance is consistent with the origin and maintenance of an adaptive trait, sex, that has been present for approximately two billion years. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 707–728.     

Disentangling the benefits of sex

Understanding the evolutionary advantage of sexual reproduction remains one of the most fundamental questions in evolutionary biology. Most of the current hypotheses rely on the fact that sex increases genetic variation, thereby enhancing the efficiency of natural selection; an important body of theoretical work has defined the conditions under which sex can be favoured through this effect. Over the last decade, experimental evolution in model organisms has provided evidence that sex indeed allows faster rates of adaptation. A new study on facultatively sexual rotifers shows that increased rates of sex can be favoured during adaptation to new environmental conditions and explores the cause of this effect. The results provide support for the idea that the benefits of increasing genetic variation may compensate for the short-term costs of sexual reproduction.     

人类的性别与性别畸形

从细胞遗传学和分子遗传学的角度阐述了人类性别的形成机理和性别畸形的致病机理。人类性别的形成是以SRY基因为主导的、多基因参与和调控的、有序表达的生理过程。性别畸形的形成是由于性染色体数目或结构异常、与性别形成有关的基因缺失、突变或与其表达调控相关的其他基因突变所致。     

Evolution of the avian sex chromosomes and their role in sex determination

Is it the female-specific W chromosome of birds that causes the avian embryo to develop a female phenotype, analogous to the dominance mode of genic sex differentiation seen in mammals? Or is it the number of Z chromosomes that triggers male development, similar to the balance mode of differentiation seen in Drosophila and Caenorhabditis elegans? Although definite answers to these questions cannot be given yet, some recent data have provided support for the latter hypothesis. Moreover, despite the potentially common features of sex determination in mammals and birds, comparative mapping shows that the avian sex chromosomes have a different autosomal origin than the mammalian X and Y chromosomes.     

Evolution of sex I. Primitive sex

Sex is a process of fusion of separate hereditary determinants. The advantages which could accrue from such fusions are protection against deleterious mutations and the possibility of combining favorable alleles into a single individual. If the fitness of the aggregate resulting from fusion is greater than its parts there will be strong selective pressure to perpetuate the aggregate in all progeny. Continued fusion presents problems though and new environmental conditions may occur which favor segregation. Segregation is also favored because of the existence of favorable recessive mutations. It is argued that the balance between these alternative goals of phenotype stability versus variety achieved an effective compromise with the development of the meiosis-fusion-mitosis cycle.     

H-Y antigen in the study of sex determination and control of sex ratio

It has been proposed, on the basis of widespread phylogenetic conservation, that H-Y antigen is the inducer of primary sex, causing the undifferentiated XY gonad to become a testis in male heterogametic species such as the human and bovine. That proposition has withstood extensive testing in vivo and in vitro. Freemartin gonads are H-Y⁺, for example, and masculinization of the freemartin gonad has been attributed to soluble H-Y, borne and transmitted in the serum of the bull twin, and bound in ovarian receptors of the female. We have applied monoclonal H-Y antibodies to the identification of gender in embryos of the bovine. Our preliminary results imply presence of H-Y in bovine embryos of the morula and blastocyst stages recovered at about 6–12 days of gestation. Assignment of H-Y phenotype -- positive in males and negative in females -- allows selective implantation of male and female during embryo transfer. Thus in an early study, we correctly identified gender in 6 of 7 calves born healthy at term, after transfer of 8 blastocysts.     

Decoding sex: Elucidating sex determination and how high-quality genome assemblies are untangling the evolutionary dynamics of sex chromosomes

Sexual reproduction is a diverse and widespread process. In gonochoristic species, the differentiation of sexes occurs through diverse mechanisms, influenced by environmental and genetic factors. In most vertebrates, a master-switch gene is responsible for triggering a sex determination network. However, only a few genes have acquired master-switch functions, and this process is associated with the evolution of sex-chromosomes, which have a significant influence in evolution. Additionally, their highly repetitive regions impose challenges for high-quality sequencing, even using high-throughput, state-of-the-art techniques. Here, we review the mechanisms involved in sex determination and their role in the evolution of species, particularly vertebrates, focusing on sex chromosomes and the challenges involved in sequencing these genomic elements. We also address the improvements provided by the growth of sequencing projects, by generating a massive number of near-gapless, telomere-to-telomere, chromosome-level, phased assemblies, increasing the number and quality of sex-chromosome sequences available for further studies.     

Group sex ratio and sex reversal

In hermaphroditic fishes, the initiation of sex reversal by male removal explains the replacement of lost males but does not explain how the number of males in a group may increase. Since numerous species apparently cannot produce primary males, a second means of initiating sex reversal must exist. In the present study we formulate a model which suggests the existence of an additional mechanism governing sex change: as soon as the ratio of adult females to males within a group exceeds a certain threshold value, a female changes sex even though no male has been removed. This process is inferred from comparison of data collected in the Red Sea and the western Indian Ocean with the model's predictions concerning size at sex reversal and the sex ratio of groups. The results suggest how several ecological factors may influence the occurrence rate of sex reversal and the development and growth of social groups.     

Plant sex determination and sex chromosomes

Sex determination systems in plants have evolved many times from hermaphroditic ancestors (including monoecious plants with separate male and female flowers on the same individual), and sex chromosome systems have arisen several times in flowering plant evolution. Consistent with theoretical models for the evolutionary transition from hermaphroditism to monoecy, multiple sex determining genes are involved, including male-sterility and female-sterility factors. The requirement that recombination should be rare between these different loci is probably the chief reason for the genetic degeneration of Y chromosomes. Theories for Y chromosome degeneration are reviewed in the light of recent results from genes on plant sex chromosomes.     

New tools for sex identification and the study of sex allocation in birds

The recent development of simple, DNA-based methods for the determination of an individual's sex will make possible large-scale studies of sex allocation and the consequence of gender in birds. Birds provide ideal systems for studying these questions in vertebrates, as so much is known about their biology and determinants of fitness. Until recently, however, little quantitative work has been possible because of the difficulty in determining gender in most cases. Recent studies suggest that biased sex allocation be more widespread in birds than has been realized.     

Estimators of the human effective sex ratio detect sex biases on different timescales

Determining historical sex ratios throughout human evolution can provide insight into patterns of genomic variation, the structure and composition of ancient populations, and the cultural factors that influence the sex ratio (e.g., sex-specific migration rates). Although numerous studies have suggested that unequal sex ratios have existed in human evolutionary history, a coherent picture of sex-biased processes has yet to emerge. For example, two recent studies compared human X chromosome to autosomal variation to make inferences about historical sex ratios but reached seemingly contradictory conclusions, with one study finding evidence for a male bias and the other study identifying a female bias. Here, we show that a large part of this discrepancy can be explained by methodological differences. Specifically, through reanalysis of empirical data, derivation of explicit analytical formulae, and extensive simulations we demonstrate that two estimators of the effective sex ratio based on population structure and nucleotide diversity preferentially detect biases that have occurred on different timescales. Our results clarify apparently contradictory evidence on the role of sex-biased processes in human evolutionary history and show that extant patterns of human genomic variation are consistent with both a recent male bias and an earlier, persistent female bias.