MITOTIC INSTABILITY OF B-CHROMOSOMES DURING EMBRYO DEVELOPMENT IN LOCUSTA-MIGRATORIA

Mitotic instability of B chromosomes during embryo development has bee n studied in the locust Locusta migratoria. Direct cytological observa tion of B chromosome nondisjunction in embryos has shown that it occur s in 2.7 per cent of anaphase and telophase cells, and that this frequ ency is not significantly different among embryos of 5-9 days of devel opment. We have defined three indices which have been shown to be very useful to quantifying mitotic instability: R, the ratio of embryos sh owing B chromosome instability, M, the median of the distribution of B chromosome numbers in a sample of embryo cells, which has been shown to be a good estimator of the original number of B chromosomes present in the zygote, and MI, the sum of all deviations (in absolute value) of B numbers with respect to M in the same embryo. Mitotic instability of B chromosomes is already apparent in 3-day-old embryos and reaches its maximum value on the fifth day of development. The intensity of m itotic instability, as measured by MI, varies significantly during the developmental period analysed but no definite trend was observed.