Abstract:

Background & Aim: Cryptic dioecy refers to a population that is phenotypically non-dioecious, but individuals actually function as males or females due to the morphologically hermaphroditic flower that typically exhibits female or male sterility. As a widespread transitional from hermaphroditism to dioecy, understandings of adaptive strategy of cryptic dioecy could help of illuminating the evolution of diverse sexual systems within angiosperms.
Progress: The morphological sex expressions of cryptic dioecy are diverse and can be classified into four types: androdioecy (type I), hermaphrodite (type II), gynodioecy (type III) and others (type IV), constituting 48.9%, 47.5%, 2.7%, and 0.9% of reported species, respectively. In the APG IV system, cryptic dioecy occurs across 22 orders, 36 families and 65 genera in around 221 species which accounts for approximately 34.4% of orders, 8.7% of families, 0.5% of genera and merely about 0.1% of angiosperms species. Most cryptic dioecious species are woody, biotic pollinated and distributed in tropics or subtropics. They have nectar or pollen as a floral reward. Phylogenetically, the sexual system appears in magnolia, monocotyledon and eudicots, particularly prevalent among more advanced core eudicots. Several hypotheses have been proposed to explain the evolutionary significance of sterile sex organs in cryptic dioecy, including ancestry hypothesis, genetic constraints hypothesis, promotion of wind-borne pollen dispersal hypothesis, pollinator attraction hypothesis, mimicry and deception hypothesis, but empirical studies remain little known. In this study, we provide a comprehensive summary of the forms of sex expression and types of sterile sex organs of cryptic dioecious in plant lineages. Families with cryptic dioecious species evolved are mapped into an updated phylogeny of angiosperms and systematic evolution of taxa with this sexual system in. Furthermore, five hypotheses regarding evolutionary significance of sterile sex organs in cryptic dioecious are also presented.
Prospects: The future trajectory of related research is presented, with the aim of providing insights into the empirical studies of selection underlying the evolution of cryptic dioecy in angiosperms.

Key words: cryptic dioecy, sex expression, sexual system, phylogeny, evolutionary significance, angiosperms