Preferential segregation in maize

M. M. Rhoades,  Genetics,  27:395-407. 1942.

An abnormal type of chromosome 10, found by Longley in maize from the s.-w. part of the U. S., is preferentially segregated during megasporogenesis. More than 70% of the ovules receive the abnormal chromosome instead of the 50% expected with random segregation. At pachytene the length of the extra piece of chromatin in the abnormal chromosome is slightly greater than the short arm of chromosome 10. The proximal and distal portions of the extra piece are euchromatic, but a large and conspicuous knob lies between the 2 euchromatic portions. The origin of this extra piece is unknown. Pollen with the abnormal chromosome 10 is only partially successful in competing with pollen possessing a normal chromosome 10. Extra chromatin present in the abnormal chromosome may impair pollen-tube growth. The excess of ovules with the abnormal type of chromosome is due neither to abortion of ovules with a normal chromosome nor to megaspore competition. The R locus, known to lie in the long arm of chromosome 10, proved to be closely linked to the extra piece of chromatin which is believed to be inserted near the tip of the long arm. The observed % of recombination (1-2%) is probably less than the amt. occurring distal to R in stocks carrying 2 normal chromosomes 10. Crossing-over in the gR region is not affected when the abnormal chromosome is heterozygous, and crossing-over in this interval did not affect preferential segregation. Evidence was obtained indicating the influence of the environment on the degree of preferential segregation.