Sex Chromosome Differentiation
Dioecism is presumed to have arisen independently in different plant taxa and the origin of cytologically distinguishable sex chromosomes might have been a gradual process. The effects of sex chromosomes and autosomes in Melandrium have been quantified. Regarding evolution of differentiated sex chromosome, a distinct trend has been noted. This trend indicates that evolution of sex chromosomes from completely undifferentiated X and Y to types with different amounts of heterochromatin and finally to morphological differentiation has been the trend in progress. The role of the autosomes, however, is evident from the variation in sex expression in plants with the same sex chromosome constitution but with different autosome combinations. With increasing gene dosage or polyploidy, the reversal to autosomal stage or suppression of sex-determining property of sex chromosomes is rather significant. Sex chromosomal mechanism of sex determination in dioecious species of higher plants thus reveals conflicting results in a number of taxa.