A male transmissible deficiency induced by B chromosomes in maize -- Saraiva, LS, Carvalho, CR Rhoades and Dempsey accumulated extensive data about the unusual phenomenon of interaction between B chromosomes and heterochromatic knobs on A chromosomes causing breaks and loss of markers on these chromosomes. This high-loss phenomenon occurs at the second microspore division and produces deficient chromosome. A survey was made for plants with deficient chromosomes transmissible through the gametes resulting from high loss. Because the Yg2 locus is close to the terminal knob, selection of exceptional plants which have lost this locus identifies deficiencies of various lengths of 9S as well as for more complex rearrangements.

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.
 
 
Number of seedlings
Phenotype
Df C/N9 yg2 c 
as male
Df C/N9 yg2 c 
as female
White
91
111
Green-white striped
20
25
Green-yellow green striped
24
29
Yellow green
102
15
* Combined data from two ears for each kind of cross.

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 male gametes.
 
 

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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