Effects of heterochromatic knobs in maize sister inbred lines

Resumo

Maize heterochromatic knobs are chromosomal structures composed of highly repetitive DNA repeats. They are variable among races. A well-known effect of knobs is meiotic drive, causing their preferential segregation to egg cells, that occurs when knobs are converted into neocentromeres in the presence of the abnormal chromosome 10. It was demonstrated that distinct kinesin motors drive two types of maize neocentromeres composed of the K180 and TR1 repeats. We selected tropical sister inbred lines, homozygous for different knob compositions, to investigate knob effects. A group of these lines showed embryogenic response in tissue culture. Low frequency of mitotic instability was observed in the cultures and breaks occurred in chromosome possessing large knobs. Chromosome breakage was followed by breakage-fusion-bridge (BFB) cycles giving origin to aberrant chromosomes that suffered addition of telomeres. Fertile plants regenerated from short-term cultures had normal chromosomes. Correlations between genome size, knob content and flowering time has been reported. These relationships were not found in the inbred lines differing in the presence of four knobs and early flowering time was detected in lines and hybrids possessing the knob K9S.