Meiotic loss of the B chromosomes of maize is influenced by the B univalent co-orientation and the TR-1 knob constitution of the A chromosomes.
The suppression of meiotic loss when the maize B chromosomes are unpaired is genetically determined. Two genotypes were selected in 1B x 0B crosses: the H line where the B transmission rate is Mendelian (50%) and the L line where the B is present in only about 40% of the progeny. Using the ZmBs probe located at the centromere and at the distal portion of the B chromosome in FISH, we found that the centromeric and telomeric ends of the B univalent co-orient at metaphase I. This feature seems to promote proper centromere orientation causing the lack of meiotic loss of the unpaired B. The co-orientation was observed in both lines, however in the L line the B univalents were not always properly oriented, showing amphitelic orientation in about 25% of the metaphase I cells. We also studied plants of the H and L lines with FISH to test the possible relation between the knob constitution and B loss. It has been found that the plants of both lines are similarly variable for the 180-bp knob repeat, but they differ in the TR-1 350-bp repeat, the L line having more TR-1 knobs. The use of a 45S rDNA probe which labels chromosome 6, allowed us to determine that this chromosome shows the main variability between the two lines: the L line has TR-1 in both arms, showing a large TR-1 knob on the long arm. The H line has only one, generally located on the short arm besides the NOR.
González-Sánchez
M
M
Departamento de Genética, Facultad de Biología, Universidad Complutense, Madrid, Spain.
González-García
M
M
Vega
J M
JM
Rosato
M
M
Cuacos
M
M
Puertas
M J
MJ
D000705:Anaphase; D032461:Chromosomes, Plant / Q000235:genetics*; D017404:In Situ Hybridization, Fluorescence; D008540:Meiosis; D008677:Metaphase; D048629:Micronuclei, Chromosome-Defective; D012091:Repetitive Sequences, Nucleic Acid / Q000235:genetics*; D003313:Zea mays / Q000166:cytology* / Q000235:genetics*