The centromere structure in Robertsonian wheat-rye translocation chromosomes indicates that centric breakage-fusion can occur at different positions within the primary constriction

Univalent chromosomes at meiotic metaphase I have a tendency to misdivide a t the centromeres. Fusion of the misdivision products may produce Robertson ian translocations. The fine structure of the centromeres in Robertsonian w heat-rye translocation chromosomes was analyzed by fluorescence in situ hyb ridization (FISH) using two centromere-specific DNA clones: pRCS1, derived from rice, and pAWRC1, derived from rye. Clone pRCS1 hybridizes to the cent romeres of all grasses including wheat and rye, whereas clone pAWRC1 is rye specific and hybridizes only to the centromeres of rye. Four of the six wh eat-rye translocations derived from a single centric misdivision event (Ist generation translocations) had hybrid centromeres, with approximately half of the centromere derived from rye and half from wheat. In the two other I st generation translocations, the entire centromere was derived from rye. A mong eight reconstructed wheat and rye chromosomes that originated from two consecutive centric misdivision-fusion events (2nd generation translocatio ns), T1BS . 1BL (derived from T1BS . 1RL and T1RS . 1BL) and one of three T 2BS . 2BL (derived from T2RS . 2BL and T2BS . 2RL) had hybrid centromeres. T1RS . 1RL (derived from T1BS . 1RL and T1RS . 1BL), two of three T2BS . 2B L, and all three T2RS . 2RL (derived from T2RS . 2BL and T2BS . 2RL) had ry e centromeres. All three 3rd generation translocations had hybrid centromer es with approximately half of the centromere derived from rye. There were n o indications that the composite structure of the centromere in these chrom osomes affected their behavior in mitosis or meiosis. These observations su pport the notion of a compound structure of the centromere in higher organi sms, and indicate that during the centric breakage-fusion event, centromere breakage may occur in different positions along the segment of the chromos ome that interacts with the spindle fibers. Normal behavior of the 1st, 2nd , and 3rd generation centric translocations in mitosis and meiosis indicate s that, at least in wheat and rye, centromeres are not chromosome specific.