| The appearance of bridges with fragments, of lone fragments, or even bridges
with knob can be explained by means of the processes outlined previously,
but not the appearance of anaphase bridges without knobs unaccompanied by
fragments, keeping in mind the frequency of such bridges in our material.
During the meiotic prophase a telomeric union should occur between chromatids
in a certain percentage of chromosome pairs. The different possibilities
are shown in Fig. 1 as well as its consequences
during the first and second anaphase, in accordance with the cross-over
combinations. The frequency of bridges without fragments is much lower during the second division in our material. Likewise, we only observed 21 bridges with fragments among 6623 PMC at AII. JONES and BRUMPTON (1971) came to the conclusion that even keeping in mind the conversion due to crossing-over, estimated according to the frequencies of chiasmata in MI, in 94% of the cases the U-type changes occur between non-sister chromatids, as in the X-type changes, which also implicate generally the non-fraternal chromatids. As can be seen in the corresponding figures, this is valid if it is assumed that the U-type changes are a result of an erroneous crossing-over, because this would correspond in reality to less number of "conversions" being due to crossing-over, the number of correct crossing-overs being smaller. But it is also logical that bridges appear in the first anaphase due to sister telomeric union, since a crossing-over is necessary for them in the affected chromosome arm, and we have seen that this situation is the most common one at diakinesis. The double fraternal union originates a double bridge or linked bivalents in AI, or bridges in AII, but this happens in very few cases. Acentric fragments without bridges is an infrequent succes and can be a result of stretchness and breakage. Literature Cited BLANCO, J.L. 1948. Anormalidades meioticas en relacion con la consanguidad en Zea mays L.C.S.I.C., Mision biologica de Galicia 1-81. Madrid. JOHN, B. and K.R. LEWIS 1965. The meiotic system. In: Protoplasmatologia, Vol. VI/F/1. Springer. Wien. JONES, G.H. 1967. The control of chiasma distribution in rye. Chromosoma (Berl.) 22: 60-90. JONES, G.H. 1968. Meiotic errors in rye related to chiasma formation. Mutation Res. 5: 385-395. JONES, G.H. 1969. Further correlations between chiasmata and U-type exchanges in rye meiosis. Chromosoma (Berl.) 26: 105-118. JONES, G.H. and R.J. BRUMPTON 1971. Sister and non-sister chromatid U-type exchange in rye meiosis. Chromosoma (Berl.) 33: 115-128. KOLLER, P.C. 1938. Asynapsis in Pisum sativum. J. Genet. 36: 275-306. LEWIS, K.R. and JOHN, B. 1966. The meiotic consequences of spontaneous chromosome breakage. Chromosoma (Berl.), 18: 287-304. REES, H. and B. THOMPSON 1955. Localization of chromosome breakage at meiosis. Heredity 9: 399-407. STAR, A.E. 1970. Spontaneous and induced chromosome breakage in Claytonia virginica. Am. J. Bot. 57: 1145-1149. STUTZ, H.C. 1976. Genetically controlled chromosome breakage as an isolation barrier in the origin and maintenance of Secale ancestrale. Can. J. Genet. Cytol. 18: 105-109. (Received May 10, 1977) |
| <-- Back |