In previous reports (MNL 60:103, 1986; MNL 61:78-79, 1987) transposition of knobs from terminal to internal chromosomal positions in diploperennis-Chapalote hybrids and knob amplification in diploperennis-annual teosinte hybrids were described. Another unusual phenomenon in the inheritance of knobs has been observed in the F1 and F2 progeny of crosses between diploperennis and Palomero Toluqueno and is reported here.
The Palomero Toluqueno parent plant was grown from seed provided by M.M. Goodman (Mex 6, INIA, R-6, 70-71) and had 3 internal knobs. The diploperennis parent was grown from seed provided by H. Iltis (Upper las Joyas, January 1979, Iltis et al. #1250) and it had 7 terminal knobs. When F1 sporocytes from the cross between these 2 parents were examined, no knobs were cytologically visible. Evidently correlated with the disappearance of knobs was the appearance of "nuclear bodies" that look like micronucleoli. Walters' (Chromosoma 17:78, 1965) study of nuclear bodies in maize revealed they contain RNA, protein, and phospholipids but no DNA. They appear in the earliest stages of meiosis and are believed to arise from the chromosomes.
In F2 sporocytes derived from selfing the knobless F1 progeny, knobs had reappeared. One internal knob and 2 terminal knobs were observed in the F2. In the F1, 1 to 6 nuclear bodies were present in every cell, but in the F2, nuclear bodies were observed in only 55% of the cells and no more than 2 were observed in any cell. These data imply that there is an inverse correlation between the cytological appearance of knobs and nuclear bodies.
It is significant that all knobs appear lost cytologically in the F1 but reappear in the F2. If knobs are transmitted by Mendelian inheritance, they could be lost in the F1, but they would not reappear in the F2. These findings suggest that knob DNA sequences may exist in alternating heterochromatic and euchromatic states. If they have this capacity to change states, it is conceivable that in the euchromatic state they might be transcribed, and thus, under certain conditions such as the genomic shock of interspecific hybridization, play a regulatory role in genomic expression.
Mary W. Eubanks
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