M. D. UPADHYA and A. T. NATARAJAN
Division of Botany, Indian Agricultural Research Institute, New Delhi - 12, India
The occurence of supernumerary constriction in the nucleolar organizing region of the satellited chromosomes (the supernumerary constriction being separated from the secondary constriction by a minute chromosome segment) has been recorded by BHADURI and GHOSH (Cytologia, 20, 148 - 49, 1956 ) in Triticum macha DEK. et MEN and by UPADHYA and SWAMINATHAN (WIS. No. 13. p 9 : 1961) in T. Zhukovskyi MEN. et ER. Both the pairs of Sat-chromosomes exhibit this characteristic supernumerary constriction in the former, while it has been recorded consistently for only one pair, in the latter: BHADURI and GHOSH have (1. cit.) also stressed the utility of these identifiable pairs of chromosomes in cytological studies as well as in crosses as markers.
During the extensive survey of Karyotypes of different species and varieties of Triticum, the supernumerary constriction at the Sat-region has been recorded in several instances sporadically, as well as in some of the established induced mutants (vide Table 1).
Frequent occurrence of such supernumerary constrictions specifically associated with the secondary constriction region, indicates the probable structure of secondary constriction of the chromosomes of Triticinae to be of bipartiate nature, but is revealed only occassionally depending on the preparation of the slide and the orientation of the chromosome , a situation very similar to the quadripartiate nature of primary constriction as revealed by pachytene studies of Rye chromosome by LIMA DE FARIA (Chromosoma, 1952: 5: 1-68).
