The Incompatibility Mechanism of the Biennis Group I Races of Oenothera.

EARLY all of the North American euoenotheras east of the Rocky Mountains Nremain heterozygous generation after generation even though they have the self-pollinating habit. A specialized cytogenetic mechanism involving multiple reciprocal translocations and a balanced lethal system accounts for this paradox. These Oenotheras possess 14 chromosomes of which each set of seven, because of reciprocal translocations, is characterized by a completely different chromosomal end arrangement. As a result of pairing specificity at meiosis the 14 chromosomes become arranged in a circle with adjacent chromosomes in the circle moving toward opposite poles at anaphase I. Thus the entire maternal genome goes to one pole and the entire paternal genome to the other; ultimately only two kinds of gametes are produced which are genetically identical with the two which united to form the plant. These genomes are passed from generation to generation intact. Such genomes are known as “Renner complexes”. Because these forms are self-pollinating, the off spring should include homozygotes for each of the complexes. These do not occur, however, because of a balanced lethal system. The lethals may be zygotic or gametophytic. In the case of zygotic lethals each complex carries a different recessive lethal; in the complex heterozygote the effect of each lethal is masked. On the other hand, the complexhomozygotes do not survive because the lethals are homozygous and able to express their effect. When gametophytic lethals are present, one complex of a complex-heterozygote carries a lethal which prevents the functioning of the male gametophyte, while the lethal of its associated complex does not allow the normal development of the female gametophyte. Therefore, only one complex is transmitted through the pollen and the other through the egg. Yet the functioning of these gametophytic lethal factors is not always so sharply defined. In some races of Oenothera the complex, which is usually transmitted only through the egg, is transmitted through the pollen in a certain percentage of cases. In other cases the complex, which is usually transmitted only through the pollen, may be transmitted through the egg. It has long been the practice to designate the complex ordinarily transmitted through the egg as the alpha complex and the complex ordinarily transmitted through the pollen as the beta complex. CLELAND (1931 ) has classified the North American Oenotheras into phylogenetic groups on the basis of morphology, segmental arrangement of the chromo-