Abstract: Sex differentiation in angiosperms has a tremendous contribution to genetic diversity, offspring and environmental adaptation. Unisexual flower species, which are widely available in nature, play significant roles in economic development and social life, and their formation is an important way to avoid self-fertilization, promote cross-breeding, maintain genetic diversity and avoid sexual functional interference. The distribution of unisexual flower species in different evolutionary branches indicates that sex differentiation may be important in the evolution of genetic diversity and have a complicated regulatory network. Therefore, the study of the mechanism of sexual differentiation has vital theoretical significance and practical application value. Sex expression research involves morphology, biochemistry, cytology and molecular genetics. After reviewing the literature published in recent years, this paper presents a systematic summary of studies on sex differentiation from four aspects. 1) The formation of unisexual flowers originated from the ancestors of the bisexual flower, mainly through the two systematic evolutionary pathways of sexual organ loss or degradation. From the perspective of floral position and structures, unisexual flowers that have a variety of external morphological structures are divided into dioecy and monoecy. In terms of the flower development, the phenotypic characteristics of unisexual flowers are formed during four developmental stages, i.e., stage 0 (before the appearance of the sexual organ primordium), stage 1 (early development of the sexual organ primordium), stage 2 (pre-meiosis) and stage 3 (post-meiosis). The sex regulatory mechanisms of the species, which are involved in stages 0, 1 and 3, were discovered and proven. 2) Six major phytohormones, such as gibberellin, auxin and ethylene, play important roles in the regulation of sex differentiation. Gibberellin mainly promotes the development of stamens, while auxins, cytokinins and ethylene mainly promote the growth of carpels. The regulation mechanisms of related hormones has been elucidated, such as the ethylene synthesis pathway to regulate sex differentiation. 3) The molecular basis of organ loss or abortion has been investigated in different types of unisexual flowers. We integrated the sex-related or sex-determining genes in the identified species, and we explain the mechanisms of epigenetic regulation of sex formation, such as DNA methylation and small RNA, as well as the XY system, the X:A system and the ZW chromosome system, that are all used to regulate the pattern of sex differentiation. 4) Effects of environmental factors such as temperature, light intensity and mineral nutrition on sex differentiation. This paper presents a review of existing studies on sex differentiation in order to summarize different regulatory mechanisms of unisexual flower development, it deepens our understanding of sex expression, phylogeny evolution and regulatory mechanisms of sex differentiation.

Key words: angiosperms, sex differentiation, sex chromosomes, plant hormones, environmental factors