The Silene latifolia Y chromosome, showing genes and anonymous markers that have been identified.
The deletions causing hermaphroditism (bsx mutations), and those causing complete sterility (ie early-stage anther abortion) of Y-bearing plants (asx mutations), as well as the X-43 subtelomeric sequence, are described in Farbos et al (1999) and Lardon et al (1999), and the Bgl markers are described in Donnison et al (1996).
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.
Thus separate sexes may have evolved more than 100 times in the flowering plants, given that 160 families have dioecious members.Some plant Y chromosomes are therefore at least partially genetically degenerate.Several kinds of evidence suggest the involvement of two loci in sex determination.The requirement that recombination should be rare between these different loci is probably the chief reason for the genetic degeneration of Y chromosomes.Theories for Y chromosome degeneration are reviewed in the light of recent results from genes on plant sex chromosomes.