Evolution of Sex Chromosomes and Dosage Compensation in Plants

Sex chromosomes are the chromosomes involved in sex determination and are characterized by reduced recombination, specialized gene content and dosage compensation. These are also called as allosomes1. Plants based on sexuality can be broadly classified into two classes namely, monomorphic species and polymorphic species. There are three sex chromosome systems in plants, (a) XY: male heterogamety (b) ZW: female heterogamety and (c) UV: haplo-diploid system. There are two important models to explain evolution of sex chromosomes in plants viz., two mutational model and sexual antagonism model2. With SNR (sex specific non recombining region) degeneration, the heterogametic sex in ZW and XY systems has lower levels of expression than the homogametic sex, which can be deleterious. A mechanism called dosage compensation has evolved in some species that allows for similar male and female expression levels. There are three different dosage compensation mechanisms, namely random inactivation of female X-chromosome, hyper-activation of male X -chromosome and hypo-activation of female X- chromosomes. An advanced genetic and molecular study on the plant sex chromosomes will help to develop markers for sex determination of dioecious plants before flowering, so this in turn increase the proportion of desired sex types like females in case of grapevines, kiwi fruit, date palms, strawberries and Cannabis sativa. Males in asparagus and hermaphrodites in case of papaya and also early sex determination of young seedlings could enhance the breeding programme and also lowers the cost of production