On the Questionable Existence of Sex Chromosomes in the Angiosperms

Nucleolar suppression, its inheritance and association with taxonomy and sex in the genus Ribes

“…The genus Salix may represent a later stage in the evolution of dioecy following hybridisation. Jensen (1940) disagrees with the normal classification of Salix as a strictly dioecious genus, since numerous instances of perfect flowers and all gradations between this state and monoecy and dioecy occur, and considers the genus is not yet fully sexually stabilised. It is probably significant that in a genus in which all species are more or less dioecious, all interspecific hybrids investigated have shown differential amphiplasty, and the species themselves show evidence of a hybrid origin also involving amphiplasty (Wilkinson, 1944).…”

“…In young pollen grains of Cannabis sativa, the average nucleolar volume of XX (monoecious) plants was 130 per cent, compared with the 100 per cent, of XY (male) plants, and the frequency distribution of nucleolar volumes for male plants showed a tendency towards a double peak, whereas XX types showed only a single peak (Herich, 1961). Jensen (1940) pointed out that nearly all reported cases of sex chromosomes in plants relate to species which are members of genera which also contain monoecious or hermaphrodite species, suggesting a recent origin for the unisexual condition; also many species with "apparent" sex chromosomes show evidence of recent hybrid origin in their meiotic behaviour. In discussing the origin of dioecy in plants, Darlington (1937) and Lewis (1942) suggested that the genes primarily determining sex must he associated sooner or later with a structural change such as an inversion.…”