The presence of sequences homologous to the internal fragment of the Mu transposable element in B chromosomes of Black Mexican lines has been already reported (MNL 61:77, 1987). In situ hybridization experiments with the entire Mu-1 element confirm the localization of these sequences only in the two heterochromatic blocks adjacent to the proximal euchromatic region of the long arm of the supernumerary chromosome.
A parallel investigation by Southern blot analysis of genomic DNA from the Black Mexican lines with or without B chromosomes (never visited or having lost them) of different year harvests has been carried out. The DNAs, digested with several restriction enzymes (HinfI, EcoRI, HindIII, XbaI) and probed with the internal fragment, denote the occurrence of three (Mu-L, Mu-1, Mu-S1, according to Walbot, MNL 59:96, 1985) elements of the Mu family and equal patterns of hybridizing bands, also independent of the year harvested, in the three Black Mexican lines. However, comparison of the EcoRI digestions probed with the entire element show among several hybridizing fragments the absence of a 4.2 Kb band in the Black Mexican line never visited by the B chromosome. The overall results could support the idea that B chromosomes may derive from a modified heterochromatinized A chromosome.
Southern blot investigation has been extended to other distantly or closely related cereals (respectively wheat, rye, barley, triticale or teosintes, sorghum, coix) that show the presence of partially homologous sequences to both the internal or entire Mu-1 element. This rather surprising result could have interesting application in gene tagging provided that such sequences have maintained transposable activity.
C. Patrosso, L. Bernard, N.E. Pogna1 and A. Viotti
1Instituto Sperimentale Cerealicoltura, S. Angelo Lodigiano, Italy
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