| Formation of anaphase bridges with or without fragments,
in rye (Secale cereale L.) J.A. SAIZ de OMENACA E.T.S.I. Agronomos, Universidad de Cordoba, spain Dicentric bridges at meiosis have been found in inbred plant material. Several causes have been mentioned as the origin of anaphase bridges. Anaphase bridges originated by crossing-over in inversion heterozygotes and not accompanied by fragments were explained in several ways, such as difficult observation by the presence of numerous univalents (KOLLER 1938) and small size (BLANCO 1948). Anaphase bridges have been found in rye populations in many instances. Inbreeding in rye increases the frequency of bridges with or without fragments; REES and THOMPSON (1955) found asymmetrical bivalents and interpreted them as acentric fragments originated by chromosome breakage and retained by chiasmata. Later there would be a U-type sister union between chromatids and the bridges can be formed at the first or the second anaphase division. In 1966. LEWIS and JOHN proposed that U-type changes could result from erroneous crossing-over. JONES (1967, 1968 and 1969) and JONES and BRUMPTON (1971) obtained results which reinforce the hypothesis of both types being due to breakages and U-type reunions resulting from alterations in the normal crossing-over process. LEWIS and JOHN (1966) and STUTZ (1976) explained the bridge formation as the result of genetically controlled chromatid and subchromatid breakage. Eight inbred lines of rye and their F1 hybrids were studied. Table 1 gives in columns 2nd to 4th respectively: the frequency of abnormal cells, the frequency of cells with one bridge and one fragment, and the frequency of cells with one bridge and no detectable fragment. These bridges, showing no knobs, were formed between sister chromosomes of a bivalent or between the sister chromatids of an univalent undergoing equational division. Even in this last case fragments can occasionally being present. The fragments are usually rather small, punctiform and variable in size even between PMC from the same anther. Fragments unaccompanied by bridges were also observed and their frequencies are recorded in the 5th column. The presence of bridges without fragments is constant in our material. The fragments can pass unnoticed on occasions, but not in the proportion necessary to support the assumption that all the bridges formed are accompanied by fragments. In our material the presence of bridges is surely not a consequence of inversion heterozygosis because of the material's inbreeding history. On the other hand, we observed spontaneous chromosome breakage in our material, a high frequency of bridges in lines and F1s with relatively low chiasma frequencies, and a pointed discrepancy between the number of bridges and the number of fragments observed, apart from other irregularities such as interweaven bivalents. All these abnormalities indicate (LEWIS and JOHN 1966) that bridges and fragments can be a result of chromosome breakage. The usually small size of the fragment is in harmony with the localization of chiasmata, which even in inbred rye is distal, with some exceptions. All this supports the assumption that the place in which the crossing-over occurs in the chromosome is the same in which the U-type changes giving rise to bridges accompanied fragments take also place. JOHN and LEWIS (1965) and LEWIS and JOHN (1966) proposed that breakage and reunion at half-chromatid level is a cause-mechanism of anaphase bridges. The change could take place between half chromatids of sister chromatids or between half chromatids of non-sister chromatids. If the union is U-type, dicentric bridges and acentric fragments would form, the latter retained in the middle part forming a knob, which does not coincide with what we observed in our material. Should a fragment break free, it would not be easy to distinguish the bridges originating at half-chromatid level of the resulting U-type union between chromatids. A half-chromatid breakage followed by and X-type union could possibly cause the appearance of anaphase bridges without fragments, but with a knob in its middle. These bridges have not been observed in our material, with few exceptions, and can be due to other causes. STAR (1970), STUTZ (1976) and others also explained irregularities on the basis of subchromatid exchanges during synapsis. But there are still more reasons not to consider that anaphasic bridges are a result of changes at half-chromatid level; by instance, there have been considerable efforts to demonstrate the existence of the half-chromatid unit, but the proofs against its existence are strongly enough to think that each chromatid is formed by a single molecule of DNA. |
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