Angiosperms are also of particular interest for empirical studies of sex chromosome evolution, because they probably evolved separate sexes repeatedly and relatively recently.
In these species, X and Y pairing in male meiosis is confined to the tips (Westergaard, 1958; Parker, 1990; Lardon et al, 1999), and recombination is absent for most of the Y chromosome.
Many dioecious species with hermaphrodite relatives have evident rudiments of opposite sex structures in flowers of plants of each sex, suggesting recent evolution of unisexual flowers (Darwin, 1877).
The low frequency and scattered taxonomic distribution of dioecy and sex chromosomes suggest that cosexuality is the ancestral angiosperm state (Figure 1) (Charlesworth, 1985; Renner and Ricklefs, 1995.
Some data come from crosses between dioecious plants and related monoecious or hermaphrodite species (Westergaard, 1958).
In Silene dioica and latifolia, there is direct evidence from cytological studies of Y chromosome deletions.