Duplication of the nucleolus organizer in the genome of maize

Cytogenetic analysis was undertaken in an alien addition stock of maize derived from an intergeneric cross between recessive marker gene stock for chromosome 2 in the maize parent and Tripsacum dactyloides L. In this strain a duplication of the nucleolus organizing region (NOR) was observed. The two chromosomes, each carrying a segment of the NOR, form two distinct nucleoli each with a bivalent attached to it. Microsporogenesis is normal and at pachytene the two chromosomes involved in this cytologically visible abnormality do not show any pairing relationships. This accounts for the absence of higher associations like quadrivalents at diakinesis and metaphase stages of meiosis. The break must have occurred in the NOR itself, resulting in a breakage-transposition-duplication of the nucleolus organizer. At the tetrad stage of meiosis we can see two nucleoli in each of the tetrad cells. Occasionally one of the tetrad cells shows a single large nucleolus as a result of fusion. Since there is no deficiency they are all functional. This homozygous translocation is transmitted normally and the progeny breed true.

Chandra V. Pasupuleti
 
 


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