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 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.     

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.     

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.     

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.     

Origin of sex

The competitive advantage of sex consists in being able to use redundancy to recover lost genetic information while minimizing the cost of redundancy. We show that the major selective forces acting early in evolution lead to RNA protocells in which each protocell contains one genome, since this maximizes the growth rate. However, damages to the RNA which block replication and failure of segregation make it advantageous to fuse periodically with another protocell to restore reproductive ability. This early, simple form of genetic recovery is similar to that occurring in extant segmented single stranded RNA viruses. As duplex DNA became the predominant form of the genetic material, the mechanism of genetic recovery evolved into the more complex process of recombinational repair, found today in a range of species. We thus conclude that sexual reproduction arose early in the evolution of life and has had a continuous evolutionary history. We cite reasons to reject arguments for gaps in the evolutionary sequence of sexual reproduction based on the presumed absence of sex in the cyanobacteria. Concerning the maintenance of the sexual cycle among current organisms, we take care to distinguish between the recombinational and outbreeding aspects of the sexual cycle. We argue that recombination, whether it be in outbreeding organisms, self-fertilizing organisms or automictic parthenogens, is maintained by the advantages of recombinational repair. We also discuss the role of DNA repair in maintaining the outbreeding aspects of the sexual cycle.     

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.     

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.     

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.     

Human sex ratios and sex distribution in sibships of size 2

We previously analyzed data from the U.S. National Health Interview Survey (NHIS, 1998 to 2002) on families with two biological children (10 years of age and younger) and found that the distribution of families with two boys, two girls, and one boy + one girl did not statistically conform to a binomial distribution regardless of the boy/girl sex ratio used. Using the best estimate of the sex ratio from the data, we found that there were significantly more families with opposite-sex siblings than families with same-sex siblings. No biological mechanism could explain these results at the time. In the present study we conducted an analysis of the first two children in sibships of size 3 from the same data source and found that there are significantly more same-sex sibships than unlike-sex sibships. Combining the two sets of data for the first two children produced observed numbers in close agreement with the expected numbers. A hypothesis of parental choice (family planning) appears to be strongly supported as an explanation for the discrepancies in the two sets of data individually. For example, parents who have a boy and a girl (either order) as their first two children are more likely to stop having children ("stopping rule") than are parents whose first two children are of the same sex.     

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.