SCIENTIA SINICA Vitae, Volume 49, Issue 4: 403-420(2019) https://doi.org/10.1360/N052018-00208

Evolution of sex determination mechanisms and sex chromosomes

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  • ReceivedOct 7, 2018
  • AcceptedDec 28, 2018
  • PublishedApr 15, 2019


The evolution of sex determination mechanisms and sex chromosomes has always been a central research topic in evolutionary biology. Because the sex determination process occurs during the early stage of development, and the regulation of sex-linked genes often involves non-coding RNAs and epigenetic modifications, this topic is an interdisciplinary hotspot with developmental biology and molecular biology. This review elaborates on the importance and origin of sex, numerous ways of sex determination, and the mechanisms of sex chromosome evolution. We will summarize the reported sex-determining genes and introduce the population genetic models under which the sex chromosomes evolve without homologous recombination. To date, only a few sex-determining genes have been discovered, but they have already exhibited a great diversity beyond biologists’ expectation. Future research directions will focus on identifying more upstream sex-determining genes for plants and animals and their downstream sex-determining pathways. New genome research and gene knockout techniques will develop other appropriate model organisms in this area, and to ultimately address the basic biological questions that why different organisms need to evolve such a great variety of sex-determining ways and how they transit to each other.


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  • Figure 1

    Sex chromosome systems and known sex determining genes of different species. The symbol in the figure indicates different sex determination system of each species, and the rightmost column is the sex determination gene of the species. TSD refers to temperature-dependent sex determination; haplodiploids refers to sex determination mechanisms of ant and honey bees etc., which depends on the ploidy of the individual

  • Figure 2

    Vertebrate sex determination pathway[18,47~51]. A: The upstream and downstream genes in the male sex-determining pathway. Genes in dark are expressed in males, while in grey are suppressed in males. B: The upstream and downstream genes in the female sex-determining pathway. Genes in gray are expressed in females, while in dark are inhibited in females. The genes in the dotted line are genes shared by the sex-determining pathways of red-eared turtle, mouse, chicken, and medaka. Arrows indicate promotion of expression and bluntheads indicate inhibition of expression. Black lines represent male-related sex-determining pathways, grey lines represent female-related sex-determining pathways

  • Figure 3

    General pattern of sex chromosome evolution. The black horizontal solid line on the sex chromosome represents the sex-determining gene, and the black horizontal dotted line represents the sexually antagonistic gene, the gray area represents the region where the recombination is suppressed. PAR refers to the pseudoautosomal region, and sex chromosomes still have recombination in PAR. A: Sex chromosomes are usually presumed to have evolved from a pair of ordinary autosomes. B: Mutations cause certain genes on the original autosome to acquire the sex-determining function. C-D: The sexually antagonistic alleles are expected to accumulate surrounding the sex-determining gene and gradually spread along the Y/W chromosome, driving more events of recombination suppression and resulting in the loss of most gene functions on the Y/W chromosomes. E: The Y/W chromosomes are completely degenerated, forming highly differentiated sex chromosomes. F: Many species have evolved dosage compensation mechanisms to balance the gene expression between X/Z chromosomes and autosomes in different sexes

  • Figure 4

    Models of Y/W-chromosome degeneration. There are currently four models for the degeneration of Y/W chromosomes. Gray bars represent the retained sequences and white bars represent the sequences that are removed from the population later. A: Background selection: Alleles with strong deleterious mutations are rapidly purged in the natural population, resulting in a decrease in the polymorphism level of the Y/W chromosomes that cannot be recovered. Because the size of the effective population becomes smaller, deleterious mutations start to accumulate and cause the degeneration of Y/W chromosomes. B: Muller’s ratchet: Y/W chromosomes without mutations or carrying the least number of deleterious mutations are randomly lost in the population. This process is irreversible in the absence of recombination, resulting in the fixation and accumulation of deleterious mutations on the Y/W chromosomes. C: Genetic hitchhiking: By positive selection of beneficial mutations in the non-recombination regions of the Y/W chromosomes, other linked deleterious mutations are also fixed on the chromosomes. D: Maladaptation of Y/W chromosomes: natural selection will eliminate severely deleterious mutations on the non-recombination regions of Y/W chromosomes, but will also remove other linked beneficial mutations, resulting in a slower rate of adaptive evolution of Y/W chromosomes compared to X chromosomes or autosomes with recombination (Modified by Bachtrog D Curr Opin Genet Dev 2006 [95])

  • Figure 5

    Dosage compensation mechanisms for different species. Different strategies and degrees of dosage compensation mechanisms are employed in different organisms. Drosophila, Caenorhabditis elegans and mammals have global dosage compensation: the transcription of the X chromosome in male Drosophila is doubled, the XX hermaphroditic C. elegans effectively reduces the expression of each X chromosome to half, and one of the two X chromosomes is randomly inactivated in female mammals. Bombyx mori has partial dosage compensation for certain genes only at specific developmental time points and tissues

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