We have carried out an experiment of in situ hybridization with a Mu sequence (generously provided by V. Walbot). The entire plasmid pAB5 (Taylor et al., Maydica 31:31-45, 1986) containing the internal 650 bp fragment of the Mu1 element has been cloned in the single EcoRI site of the insertional lambda vector 1149. The entire recombinant phage was labelled by nick-translation with tritiated thymidine to a specific activity of about 20 million dpm per microgram of DNA. In situ hybridization in the presence of dextran sulphate was performed on microsporocytes from several Black Mexican lines containing 4 to 12 B chromosomes. Silver grains on B chromosomes have been observed in the two heterochromatic blocks adjacent to the proximal euchromatic region. Interestingly, the number of silver grains present in B chromosomes of the same microsporocytes varied from zero to ten. This suggests that sequences related to the Mu element preferentially accumulate in the same chromosomal region. Few silver grains were also observed in the A chromosome region, but there was no evidence of induction of mutable alleles in the self progeny of these A C R Su Black Mexican lines. However, when they were crossed as male parents with stocks recessive for endosperm marker genes, kernels with purple spots or coloured-purple kernels with SU sectors were observed.
A. Viotti, L. Bernard and N.E. Pogna
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