Reciprocal Translocation In Maize - Part II

Reciprocal Translocations Cause Semi-sterility - Continued

Maize has ten chromosomes (n=10). A gamete with ten chromosomes will be sterile when one of the chromosomes is comprised of an interchange because this chromosome will be deficient for some part of the haploid complement. A gamete that has both interchange chromosomes will be fertile since the entire haploid genome will be present. A testcross of a stock heterozygous for a reciprocal translocation will result in 50% pollen abortion. Alternate segregation of a stock heterozygous for an interchange results in fertile gametes. Adjacent segregation of the heterozygous stock results in sterile gametes. One-half the gametes are sterile because adjacent segregation occurs 50% of the time. Adjacent segregation means that when a ring formation at prophase I of meiosis develops in stock heterozygous for the interchange, adjacent centromeres go to the same pole. Alternate segregation occurs when homologous, and adjacent centromeres go to opposite poles. McClintock was able to determine that homologous centromeres can go to the same pole by observing that two chromosomes with terminal knobs ended up in the same gamete. One knobbed chromosome was a normal chromosome 9 and the second knobbed chromosome was an interchange chromosome.

Brink and Cooper (1931) showed that the cross configuration of a genetic stock heterozygous for an interchange opened up to form a ring at diakinesis of prophase I of meiosis. They state that "the ring may become oriented on the spindle in two different ways. The two smaller chromosomes may lie on one side of the equator, and the two larger ones on the other. According to the second alternative, a small and a large chromosome lies on either side of the equator. If, in the separation of the chromosomes of the ring at anaphase, alternate chromosomes a, b and a'b', following either mode of orientation, go to the same poles functional spores will result since each daughter nucleus receives a full complement of the hereditary materials involved. Half the spores will receive the two modified chromosomes, a'b', and the other half, the two normal ones, a, b. If, on the other hand, following either type of arrangement on the spindle, adjacent chromosomes accompany each other to the poles, each of the resulting nuclei will not only receive some chromosome part in duplicate but will also lack a segment. Being deficient for certain genes, all the resulting spores, presumably will abort."

Brink and Cooper (1931) state that "In half the cases alternate chromosome go to same pole leading to the formation of functional spores. In the remainder of the cases adjacent chromosomes assort together, the plane of division passing through the ring in each of the two possible ways with equal frequency." This explains the 50% pollen abortion associated with genetic stock that is heterozygous for an interchange. Brink and Cooper also showed that genetic stocks which are homozygous for an interchange are fully sterile when self-fertilized. This is because each gamete, whether male or female, will contain the full haploid genomic complement. For example, in the case of a reciprocal translocation between chromosome 8 and 9, each gamete will contain one chromosome with the centromere of chromosome 8 attached to a translocated segment of chromosome 9 and the centromere of chromosome 9 with the attached translocated segment of chromosome 8. There are no missing loci in this gamete, the two translocated chromosomes complement each other.