The comparative cytology of two inbreds of maize, namely Mo17 and B73, has been studied in order to characterize these inbreds by chromosome morphology. Samples of these inbreds were collected at the Suburban Experiment Station at Waltham. Using the customary methods of staining, they were analysed at various stages of meiosis, with special emphasis on pachytene and diplotene stages in order to provide information such as presence or absence of knobs, the number, size and location of knobs, condition of knobs, whether homozygous or heterozygous for the particular knob in question. There is some controversy regarding the knob size, and this is considered to be purely subjective. But even as such it can be seen that in any particular location for a particular chromosome, the presence or absence and the size is always consistent. In these observations the knob size has been classified as small, medium and large.
Inbred Mo17: The sample from this inbred showed normal meiosis. The cytoplasm showed a tendency to stain faster than the chromosomes. At pachytene, synapsis was complete and ten bivalents are formed. A sporadic case of a pericentric inversion was observed (Figure 1). There was no sign of bridges or fragments. The chromosome involved in this inversion has not been identified yet. Of the ten pairs of chromosomes, only one large knob was observed, and it is perfectly homozygous. The chromosome showed all the characteristics of chromosome 7 in its morphology in that it had an arm ratio of 2.6 and a conspicuous heterochromatic segment in the long arm adjacent to the centromere. The knob is located on the long arm of chromosome 7.
Inbred B73: Meiosis is normal except that it differed from Mo17 in that it showed two large knobs. Based on their total lengths and arm ratios the chromosomes have been identified as chromosome 7 and 8 of the complement. In both cases the knobs were located in the long arm. In this inbred an interesting feature is the occurrence of a small fragment in almost all the cells analysed. It did not show any association with any one particular chromosome but was seen sticking to one or the other and in some cases free from the rest of the complement (Figure 2). It is not a "B" chromosome.
Significance of these studies is to emphasize the importance of using chromosome analysis especially pachytene morphology with special reference to knob number, location, size and such other features to identify the individual inbreds used in breeding programs. Inbreds have already been fingerprinted based on morphological characters and other traits, and one more criterion based on cytology would improve the sophistication of the existing classification. Variation of knob constitution in different inbreds could be used in identification and purity of the inbreds involved.
Fig. 1 - Camera lucida drawing of a pericentric invasion
Fig. 2 - Camera lucida drawing of the fragment
Chandra V. Pasupuleti
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