Sex Determination in Sphaerocarpos

Sex determination was first worked out in a haploid-dominant organism in the bottle liverwort Sphaerocarpos donnellii, a bryophyte whose gametophyte generation, which is haploid, shows heteromorphic (differently shaped) chromosomes between the sexes. Both the male and the female gametophyte are independent haploid organisms carrying eight chromosomes (n=8) in total. Seven of those eight chromosomes are ordinary autosomes, identical in appearance between the male and female gametophytes. The eighth chromosome, however, differs: in the female gametophyte it is a comparatively large chromosome called X, while in the male gametophyte the corresponding eighth chromosome is a comparatively small chromosome called Y. The diploid sporophyte generation, which develops from the union of male and female gametes, is always heterogametic (carrying one X and one Y), and when it undergoes meiosis to produce haploid meiospores, it produces two classes in roughly equal num …