The sex expression and sex determining mechanism in species.

Generally, species are dioecious, but monoecious strains in several populations have been found, providing excellent models for studying sex differentiation and sex determination mechanisms. Although the mechanisms of sex determination and sex evolution have been extensively studied, related research on heterozygous woody plants is limited. Here, we discuss the expressions of various sex types, which showed broad diversity and complex instability. We have also reviewed the sex determination systems in the plant kingdom and the morphological, cytological, physiological, and molecular aspects of the sex-linked markers in trees. Moreover, hypotheses to explain the origin of monoecy are discussed, which is more likely to be the interaction between sex-related genes and environment factors in female plants. Besides, further prospects for the utilization of monoecious resources and the research directions of sex determination mechanism are proposed. This study provides important information on sex expression and provides more insights into sex differentiation and determination.