Department of Botany, Indiana University, Bloomington, Indiana 47401.
Genetics. 1972 May;71(1):73-96. doi: 10.1093/genetics/71.1.73.
Knobbed regions of the regular maize complement frequently are eliminated at the second microspore division in spores which have two or more B chromosomes. Evidence is presented that no or little loss occurs in spores with one B and that the rate is not increased in spores with more than two B's.-The B chromosomes from an unrelated strain proved as effective in inducing loss as did the B's of the original high loss stock.-Chromatin loss induced by B's is restricted to knobbed A chromosomes and occurs only at the second microspore division. Knobbed chromosomes 3, 5, and 9 have been tested and all interact with B's to give loss. Chromosomes with large knobs are more frequently broken than are those with smaller knobs and knobless chromosomes show negligible loss.-Although knobs and B's are essential for chromatin elimination, modifying genes can markedly affect the rate of loss.--Two knobbed heterologous chromosomes undergo simultaneous loss more frequently than expected from independent events. The data indicate that joint loss occurs in competent cells and that preferential assortment of the two deficient chromosomes to specific poles is unlikely.-B chromosomes and deficient chromosomes assort independently at the second microspore anaphase.-Genetic data from crosses with marker genes in both arms of chromosome 3 show that breakage of the postulated dicentric bridge does not occur solely at the centric region since a variety of deficient chromosomes were recovered.-Nondisjunction of B chromosomes and elimination of knobbed chromatin take place during the second microspore mitosis. The argument is advanced that the two phenomena result from faulty replication of heterochromatic segments. The position of the nonreplicating segment in the two kinds of chromosomes determines whether nondisjunction or breakage takes place.-Finally, it is suggested that all of the reported effects of the B chromosome can be accounted for if the B is a parasitic entity having no genetic function other than controlling the replication of its proximal heterochromatic knob and increasing the ability of B-containing sperm cells to compete successfully for fertilization of the egg.
有两个或更多 B 染色体的二分体花粉在第二次减数分裂时，常会把正常玉米染色体上的瘤状结构区域除掉。本文提供的证据表明，当花粉只有一个 B 染色体时，几乎没有丢失发生，而当花粉有两个以上的 B 染色体时，丢失率也不会增加。来自于无关品系的 B 染色体，和原来高丢失品系的 B 染色体一样，都能有效地引起丢失。B 染色体引起的染色质丢失仅限于瘤状 A 染色体，而且只发生在第二次减数分裂。已经检测了 3、5 和 9 号瘤状染色体，它们都与 B 染色体相互作用导致丢失。大瘤状染色体比小瘤状染色体更容易断裂，而无瘤状染色体则几乎没有丢失。尽管瘤状结构和 B 染色体是染色质消除所必需的，但修饰基因能显著影响丢失的速度。两个瘤状异源染色体同时丢失的频率比独立事件预期的要高。这些数据表明，在有能力的细胞中发生联合丢失，而且两个缺陷染色体优先分配到特定的两极是不太可能的。B 染色体和缺陷染色体在第二次减数分裂后期是独立分配的。来自与 3 号染色体臂上标记基因杂交的遗传数据表明，假定的双着丝粒桥的断裂不是仅发生在着丝粒区域，因为回收了各种缺陷染色体。B 染色体的不分离和瘤状染色质的消除发生在第二次减数分裂的二分体花粉中。本文提出的论点是，这两个现象是由于异染色质片段的错误复制所致。两种染色体中非复制片段的位置决定了不分离或断裂是否发生。最后，如果 B 染色体是一种寄生实体，除了控制其近端异染色质瘤状结构的复制和增加含 B 染色体精子细胞成功竞争受精卵子的能力外，没有遗传功能，那么所有报道的 B 染色体的效应都可以得到解释。
