The high-loss strain used had several B chromosomes and chromosome 9 carried a large knob terminating the short arm. Marker genes on this arm included the dominant Yg2 and C alleles with Yg2 close to the knob. Pollen from the high-loss strain was applied to the silks of yg2 c tester plants and the C seeds produced were planted to search for deficient chromosomes due to non-correspondence between embryo and endosperm after the breaks occur at the second microspore division. The yellow green seedlings represented deficiencies of chromosome 9.
The cytological analysis of yellow green exception number 364-2 showed that it had one chromosome 9 with a small terminal knob and a knobless homologue. Because the high-loss strain used as the paternal parent was homozygous for a large knob in chromosome 9 and the yg2 tester stock was homozygous for a small knob, the chromosome 9 constitution of plant 364-2 obviously arose by loss of the knob from the high-loss strain although cytologically there was no apparent or easily detectable deletion of the euchromatic tip. Pollen from this plant was completely fertile and uniform in size. Following pollination by a c male parent, an ear with 148 C and 145 c kernels was produced, showing a normal female transmission of the supposedly deficient chromosome, with the dominant C allele. Normal transmission was also obtained when plant 364-2 was used as the male in a test cross (55 C: 49 c).
To determine if the knobless chromosome 9 was in fact deficient, individuals of the presumably Df9 (deficient Yg2) C/ N9 yg2 c constitution were crossed as male and female parents with a stock homozygous for McClintock’s wd chromosome and containing the Wd ring with the dominant Yg2 and Wd alleles which cover the deficiency in the wd chromosome. Because the ring chromosome is somatically unstable and frequently lost, wd plants possessing the ring are green-white striped. The progenies of the above crosses were planted in the sandbench and seedlings were scored (Table 1).
Table 1. Seedling classification of
progenies from reciprocal crosses of Df9 C/N9 yg2 c plants
with individuals homozygous for wd and containing the Wd ring (*).
The Df9 was derived from plant 364-2.
|Green-yellow green striped||
Four phenotypic classes were observed. Chromosome 9 constitutions of these classes are as follow: white (Df9/wd), yellow green (N9/wd), green-yellow green striped (N9/wd plus ring) and green-white striped (Df9/wd plus ring).
The occurrence of white and green-white
striped seedlings means that plant 364-2 had a chromosome 9 deficient for
the Yg2 and Wd loci. The 1:1 ratios of white plus green-white
striped versus yellow green plus green-yellow striped seedlings demonstrate
the normal transmission of the deficient chromosome by both female and
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