Possible role of heterochromatin in chromosome breakage induced by tissue culture in oats (Avena sativa L.)

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Abstract
Perturbations in the replication of heterochromatic regions during tissue culture may be the cause of chromosome alterations observed in regenerated hexaploid oat plants (Abena sativa L.). An idiogram based on pachytene chromosome analysis was developed depicting the heterochromatin distribution on 17 of the 21 chromosomes. Heterochromatic regions were present pericentromerically on each chromosome, at telomeres in five of the chromosomes, and in the NORs of the three satellited chromosomes. Centromeric heterochromatin covered most of the short arm in several chromosomes. Tritiated-thymidine autoradiography was used to determine the nuclear cycle duration (G1 S + G2 + mitosis = 0.8 + 5.2 + 0.3 + 3.8h = 10.1 h) in root-tip cells and revealed regions of late replication around the primary and secondary constrictions and at chromosome ends. Replication of heterochromatic regions late in the DNA synthetic period makes it plausible that these regions are more sensitive to nuclear cycle alterations caused by the culture process. Pachytene analysis of a heteromorphic chromosome pair that originated during culture showed the break in the deficient chromosome to be in centromeric heterochromatin. The presence of proximal and telomeric heterochromatin means that bridges resulting from replication problems in these regions could result in various alterations including deficiencies, duplication, and translocations and can account for heterochromatin amplification, all of which have been observed in plants that have regenerated from tissue culture.