Unisexual Maize Plants and Their Bearing on Sex Differentiation in Other Plants and in Animals.

Differences in the size or shape of the chromosomes of the two sexes have been noted in many dioecious plants as well as in animals. A compilation by SINOTO (1929) gives 7. species of bryophytes and 51 species of angiosperms so characterized. In all but one case the female gametes are alike. The male gametes are of two kinds. They either differ in size or shape of one pair of chromosomes, or one class of gametes has one or more chromosomes that the other class does not have. In 33 dioecious species of bryophytes and angiosperms examined, no allosomes have been found. Animals also have species in which no inequality in chromosome construction is evident. WINGE (1922) does not find any visible difference in the chromosomes of Lebistes, although there is definite evidence that one chromosome pair is associated with sex differentiation. Many other animal species have been examined and no heteromorphic chromosomes found. Nevertheless, WINGE (1923) states: “I have no doubt that sex chromosomes are found altogether throughout the whole of the vegetable kingdom in dioecious species, though it will not perhaps always be possible to demonstrate the same. . . . Unisexual individuals of either sex can in their autosomes be equipped with male and probably female tendencies, but the sex chromosomes are the normal regulating mechanism which as a rule determines the sex.” According to this view one particular pair of chromosomes carries the main sex-determining agency whether or not the members are visibly differentiated. It is obvious that differences in the amount of chromatin can not be the primary genetic agency in sex determination. Unisexual plants, produced in controlled pedigree cultures, from a monoecious species showing equal chromosome pairs, give an illustration of a working mechanism that differentiates the two sexes. Dioecious maize has been developed and propagated through four generations. Two classes of sex-determining gametes are produced by the male plants; the female plants are monogametic.