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。     

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.     

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.     

Evolutionarily stable sex ratios and sex allocations

The paternal fitness of a sexual individual is equated with the fitness of those eggs of its potential mates which it is able to fertilize. This property enables the total sexual fitness of individuals to be expressed in terms of female gamete contributions in separate equations for a cosex (an individual in a population composed of a single sexual class which combines male and female functions) and for parents in a dioecious population. The general equations are used in phenotypic models of selection which examine conditions maximizing the fitness advantage of one phenotype over another with a different sex ratio or allocation. As an example, it is shown that finite population size confers full stability on the sexual allocations in a cosexual population and on the sex ratio in a dioecious population.The use of fitness advantages provides the outcome of selection for all frequencies of contrasted phenotypes. It is therefore possible to redefine an ESS to allow for persistent variability in a population. A phenotype is an ESS in a population if, from any initial frequency, it is protected from loss by its fitness advantage. The conditions for a rare mutant to spread invariably coincide with those for its fixation only if an individual of any phenotype affects the fitness of other individuals of all phenotypes in identical ways.     

The primary sex ratio under environmental sex determination

The ESS primary sex ratio (male/female) under environmental sex determination (ESD) is shown to be equal to the ratio of the average fertility of a female to the average fertility of a male. Thus, depending upon how male and female fertility change over the environmental variable causing ESD, the primary sex ratio may be either male or female biased, or neither. The primary sex ratio thus contains information as to how male and female fertilities change with the environment.     

Alternative strategies of fetal sex diagnoses and sex preselection

Abstract

Motives for sex control include avoidance of sex‐linked disease and realization of preferred sex compositions of children. Currently, the only wholly effective means of sex control is diagnosis of fetal sex by mid‐trimester karyotyping of amniotic fluid cells followed by corrective abortion when diagnosis is adverse. Unfortunately the delays involved in karyotyping mean that abortion cannot be minimum‐risk suction curretage. Radioimmunoassay procedures allow somewhat earlier diagnosis and therefore less risky abortion, but entail more diagnostic error. In the first part of the paper, several assay procedures are evaluated in terms of relative expense as compared to karyotyping, gestational age when reliability is highest, and level of that reliability. Later portions of the paper focus on use of radioimmunoassay to diagnose fetal sex for purposes of regulating the sex composition of offspring. Three strategies are compared with respect to their efficiency and expected levels of diagnosis and abortion.     

Novel sex cells and evidence for sex pheromones in diatoms

Background

Diatoms belong to the stramenopiles, one of the largest groups of eukaryotes, which are primarily characterized by a presence of an anterior flagellum with tubular mastigonemes and usually a second, smooth flagellum. Based on cell wall morphology, diatoms have historically been divided into centrics and pennates, of which only the former have flagella and only on the sperm. Molecular phylogenies show the pennates to have evolved from among the centrics. However, the timing of flagellum loss – whether before the evolution of the pennate lineage or after – is unknown, because sexual reproduction has been so little studied in the ‘araphid’ basal pennate lineages, to which Pseudostaurosira belongs.

Methods/Principal Finding

Sexual reproduction of an araphid pennate, Pseudostaurosira trainorii, was studied with light microscopy (including time lapse observations and immunofluorescence staining observed under confocal scanning laser microscopy) and SEM. We show that the species produces motile male gametes. Motility is mostly associated with the extrusion and retrieval of microtubule-based ‘threads’, which are structures hitherto unknown in stramenopiles, their number varying from one to three per cell. We also report experimental evidence for sex pheromones that reciprocally stimulate sexualization of compatible clones and orientate motility of the male gametes after an initial ‘random walk’.

Conclusions/Significance

The threads superficially resemble flagella, in that both are produced by male gametes and contain microtubules. However, one striking difference is that threads cannot beat or undulate and have no motility of their own, and they do not bear mastigonemes. Threads are sticky and catch and draw objects, including eggs. The motility conferred by the threads is probably crucial for sexual reproduction of P. trainorii, because this diatom is non-motile in its vegetative stage but obligately outbreeding. Our pheromone experiments are the first studies in which gametogenesis has been induced in diatoms by cell-free exudates, opening new possibilities for molecular ‘dissection’ of sexualization.     

Environmental sex reversal,Trojan sex genes,and sex ratio adjustment: conditions and population consequences

The great diversity of sex determination mechanisms in animals and plants ranges from genetic sex determination (GSD, e.g. mammals, birds, and most dioecious plants) to environmental sex determination (ESD, e.g. many reptiles) and includes a mixture of both, for example when an individual’s genetically determined sex is environmentally reversed during ontogeny (ESR, environmental sex reversal, e.g. many fish and amphibia). ESD and ESR can lead to widely varying and unstable population sex ratios. Populations exposed to conditions such as endocrine‐active substances or temperature shifts may decline over time due to skewed sex ratios, a scenario that may become increasingly relevant with greater anthropogenic interference on watercourses. Continuous exposure of populations to factors causing ESR could lead to the extinction of genetic sex factors and may render a population dependent on the environmental factors that induce the sex change. However, ESR also presents opportunities for population management, especially if the Y or W chromosome is not, or not severely, degenerated. This seems to be the case in many amphibians and fish. Population growth or decline in such species can potentially be controlled through the introduction of so‐called Trojan sex genes carriers, individuals that possess sex chromosomes or genes opposite from what their phenotype predicts. Here, we review the conditions for ESR, its prevalence in natural populations, the resulting physiological and reproductive consequences, and how these may become instrumental for population management.     

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.     

人类性别决定和性别分化研究进展

SRY基因在人类性别分化中起着关键作用,目前研究认为SRY仅是涉及性别决定过程的基因之一,其他基因和SRY相关基因SOX9,抗副中肾激素基因AMH,编码缁类因子的基因SF1,X－连锁的DAX基因,wilm‘s肿瘤抑制基因WT1等基因都参与了人类性腺分化和发育,本文拟就人类性别决定基因的研究进展及其与人类性别分化的关系作一综述。     

Visualizing trypanosome sex

Genetic exchange in Trypanosoma brucei is now well characterized. It is a key tool that has enabled an understanding of important parasite genetic traits and underpinned the Trypanosoma brucei genome project. However, a key aspect that has eluded us is the point in the trypanosome life cycle where genetic exchange occurs. Research using green and red fluorescent trypanosomes for visualizing genetic crosses has now identified this stage.