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Studies on sex determination mechanisms and sex chromosome evolution have previously focused on a small number of model organisms. However, recent studies of a large number of non-model organisms such as fish, reptiles, and mosquitoes have found that known models and knowledge of the evolution of sex-determining genes and sex chromosomes in humans, birds, and fruit flies are almost completely inapplicable.
On July 18, 2024, Qi Zhou's lab at Zhejiang University was invited to write a review article titled "Evolution and regulation of animal sex chromosomes" in Nature Reviews Genetics. This article summarizes a series of recent research results from Zhou Qi's lab and other domestic and international counterparts, and delves into the unique evolutionary regulatory patterns of animal sex chromosomes, providing a detailed explanation for understanding the complex diversity behind the ubiquitous biological phenomenon of gender.
This paper summarizes the classical model of the origin and evolution of sex chromosomes, and proposes a new model that needs to be further re-understood by summarizing different "exceptional" events, including how sex reversal is common in fish and amphibians, how it mediates the rapid replacement of the sex chromosome system, and how the special phenomenon of the platypus ten-line sex chromosome complex is formed due to the violent rearrangement between chromosomes. It is the exploration of the sex chromosomes of these different species that makes the sex chromosome evolution model more abundant and perfect.
性染色体演化和性染色体颠换的经典模型示意图（Credit: Nature Reviews Genetics）
In addition, recombination inhibition is a very critical event in the origin and evolution of sex chromosomes. The repressed regions on the sex chromosomes are no longer genetically recombined, and the degree and mechanism of inhibition vary greatly among different species, further enhancing the diversity of the sex chromosome system.
模式图（Credit: Nature Reviews Genetics）
Frequent transposition of sex-determining genes between species is also one of the reasons for the diversity of sex chromosome systems, and the underlying mechanisms behind this phenomenon are further discussed in this paper. Few studies have linked the rapid evolution of sex chromosomes to the transient expression window of upstream sex-determining genes (so-called master sex determining genes) during embryonic development. Although these genes play an important role in the sex determination process, their expression tends to be less persistent than downstream genes and is fleeting in both evolution and development. This paper thus compares this mechanism to the United Kingdom constitutional monarchy. The so-called "dominant sex-determining genes" (such as the Sry gene, which determines human males), like the Queen of United Kingdom, do not actually have the ability to be decisive, but only disrupt the delicate balance between male and female gonadal development during gonadal development. On the contrary, a large number of downstream conserved genes involved in the gonadal pathway (i.e., the ovaries that develop into females or the testicles of males) are pitted against each other by forming a pattern similar to that of a "parliament", and these genes are the forces that really determine the fate of sex differentiation. Through this analogy, this paper unravels the complexity and diversity of the gender determination process. Finally, the mechanism of the global dose compensation effect of sex chromosomes is discussed in detail. After the Y chromosome loses most of its genes, species need to develop new mechanisms to balance the dose effect. Different species achieve this balance through different mechanisms within the three-dimensional nuclear space, which further demonstrates the diversity and complexity of sex chromosome evolution.
This article summarizes the current research progress in this field and provides a new perspective for understanding the evolution and regulation of sex chromosomes.
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https://www.nature.com/articles/s41576-024-00757-3
Editor-in-charge|Explore Jun
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