SISTER CHROMATID SEGREGATION DURING MITOSIS IN POLYPLOID WHEAT
HE chromosomes of eukaryotes replicate in a semi-conservative manner (TAYLOR, WOODS and HUGHES 1957; PRESCOTT and BENDER 1963). Utilizing this observation, we have studied the segregation of sister chromatids during mitosis of plant and animal cells (LARK, CONSIGLI and MINOCHA 1966; LARK 1967). Autoradiographic measurements of segregation at the second mitosis following incorporation of radioactive label indicated that sister chromatids were not always segregated randomly. Instead, chromatids
... stead, chromatids synthesized during any particular generation tended to remain together and to segregate as a group. These experiments were instigated by measurements of DNA synthesis and segregation in bacteria (LARK 1966) . Their rationale has been discussed in detail elsewhere (LARK 1969). The experiments with eukaryotes indicated that different systems might segregate in a more or less random manner depending on the ploidy of the system (LARK, CONSIGLI and MINOCHA 1966; LARK 1967). Recently, the interpretation of these experiments has been challenged by results obtained using a different experimental approach (HEDDLE, WOLFF, WHISSELL and CLEAVER 1967; CUEVOS-SOSA 1968 ). These authors have used a different technique to study sister chromatid segregation. Whereas we have measured the amount of radioactivity (number of silver grains) associated with a chromatid set, they have measured the number of labelled chromosomes which remain associated during the second and third cycles of mitosis and division which follow radioactive labeling. A common organism, Vicia faba, was studied by HEDDLE et al. (1967) and by ourselves. They observed a random distribution of radioactive chromosomes in second and third mitosis, we observed a non-random distribution of radioactivity among sister-chromatid sets. HEDDLE et al. (1967) explained the difference by attributing o w distribution to the random segregation of a few radioactive chromosomes (in Vicia this would be six or less at the second or third mitosis after growth) which could lead to an asymmetric distribution of radioactive label. On the other hand, sister chromatid exchanges at the kinetochore would obscure a non-random distribution which might have existed in their studies and no data were presented to compare the frequency with which one or both arms of the metacentric chromosome were labeled. PRESCOTT and BENDER (1963) have estimated that the frequency of sister chromatid exchange at the kinetochore will account for 25% of all sister chromatid exchanges.