A Structural Change in the Chromosomes of Maize Leading to Chain Formation

Abstract

(1) The M-sterile race of maize studied shows eight bivalents and a chain of four chromosomes at diakinesis. About 24 per cent. of the pollen grains abort and the ears are incompletely filled. (2) M-steriles selfed give partially sterile and normal offspring in the ratio of 2:1. The cross M-sterile [female] x normal [male] gives a like result. The reciprocal combination, normal [female] x M-sterile [male], however, produces equal numbers of partially sterile and normal offspring. (3) When M-steriles are used as pistillate parents about half the partially sterile offspring give amounts of aborted pollen considerably in excess of 24 per cent. These plants are termed "high steriles." (4) It is assumed that in M-sterile plants a terminal segment of one chromosome has become detached from its normal position and affixed by its broken end to a nonhomologous whole chromosome. (5) The three-armed chromosome structure expected on this hypothesis in the heterotypic prophase has been identified and is illustrated. (6) At the heterotypic metaphase, commonly, either the end members of the chromosome chain pass to one pole and the other two to the opposite pole or alternate chromosomes go to the same pole. (7) Of the four resulting types of spores one is deficient for the translocated segment and aborts, one carries a normal chromosome complement and one the two chromosomes modified by the translocation. The fourth type carries the "receiver" chromosome with the displaced piece attached and a normal "donor" chromosome; it is disomic for the translocated segment. (8) The difference in reciprocal crosses is due to the non-transmission of gametes disomic for the translocated segment through the male gametophytes. (9) Cytological evidence is presented showing that the "high steriles" are hyperploids. The trisomic condition of the translocated segment leads to frequent nondisjunction. (10) The M-sterile chain possesses one pair of chromosomes in common with the semi-sterile-2 ring.