Over the past few years, we have made considerable progress in understanding the genetics of flowering time in maize. As a part of our studies, we have determined the knob composition of a number of inbred lines. These include some of the currently used inbreds, or those which have been used extensively in the present day maize germplasm in the U.S.A. The cytological analyses regarding the number, size and position of knobs were performed with acetocarmine squashes of microsporocytes mostly at pachytene stage during meiosis. With regard to size, the knobs were divided into six arbitrary classes. This classification was subjective since no attempt was made to measure the size of the knob.
The data presented in Table 1 provide information on 35 inbred lines regarding the number, size and position of knobs together with some background information. The knob number ranged from 1 to 6 with most lines having 3 to 4 knobs. Of 23 possible locations, the knobs were found at 13 in the lines analyzed. The knob on the long arm of chromosome 7 (7L knob) was the most frequent, being present in all the lines (or races) except C103, F2, Apachito and Early Early Synthetic. Two other knobs were also found with high frequency; one on the long arm of chromosome 4 (4L) and the other on the short arm of chromosome 9 (9S). The knobs on chromosome 1 were the least frequent while no knob was observed on chromosome 10. Two Mexican flints (Apachito and Azu1) and the Argentine flint (Colorado Halidaisi majorado) have supernumerary B chromosomes in their genomes. Three flints, Wilbur's, Parker's and Tama (not shown in the table), were also analyzed, and as expected none had a cytologically observable knob.
The Lancaster Sure Crop and its derivatives such as C103 and Mo17, the cold tolerant French inbred F2, and Early Early Synthetic (probably the earliest maturing line of maize) had only one knob. The inbreds related to Oh43 family have the highest average knob number while those related to Iowa Stiff Stalk Synthetic have an average knob number intermediate between those of the Lancaster derivatives and the Oh43 family.
Table 1 (continued). Number, size and position of heterochromatic knobs in maize (Zea mays W inbreds and varieties.
Sajjad R. Chughtai and Dale M. Steffensen
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