Sex chromosomes in plants.
A majority of plant species are cosexuals forming male (stamens) and female (pistils) sexual organs in each flower. About 5% of species are strictly dioecious and form unisexual flowers, either male or female, on different individuals. A high number of plant species represent intermediate stages, i.e., different forms of flowers are present on one individual (e.g. monoecy) or sexually different individuals occur in populations of plants (e.g. gynoecy). It is well documented that these sexual forms also represent intermediate steps in the evolution of sexuality. Similarly as in animals, there are two basic mechanisms of sex determination in plants: genetic and environmental (hormonal). Among the dioecious species, only a few of them have evolved heteromorphic sex chromosomes, especially white campion (Silene latifolia) and common sorrel (Rumex acetosa). In these two classical species, different sex chromosome-based mechanisms have been described: white campion has the male dominant chromosome Y (the mammalian type of sex determination), while in sorrel the sexuality is controlled by a ratio between the number of X chromosomes and the number of autosomal sets (the drosophila system). Recent molecular analyses show that the plant sex chromosomes are evolutionarily much younger compared with the sex chromosomes in animal species. This fact makes them the optimum models to study early stages of sex chromosome evolution.