PATTERN AND FREQUENCY OF NOCODAZOLE INDUCED MEIOTIC NONDISJUNCTION INOOCYTES OF MICE CARRYING THE TOBACCO MOUSE METACENTRIC RB(16.17)7BNR

Oocytes from (C3H/HeH x 102/H)F-1 and Rb(16.17)7Bnr homozygous females were exposed to a range of doses of nocodazole in vitro. The spindle poison caused a dose dependent increase in metaphase I(MI) arrest and hyperploidy. A concentration of 0.03 mu g/ml was found to induce a max imum hyperploid frequency of 3.1% and 11.6% respectively without a hig h level of MI arrest. Between 0.03 and 0.05 mu g/ml MI arrest increase d substantially and reached a frequency of approximately 90%. In a fur ther experiment oocytes from Rb7 homozygous, heterozygous and 3H1 fema les were exposed to 0.03 mu g/ml nocodazole 4, 6 or 8 h after the onse t of maturation. The phase at which the spindle was inhibited resulted in a specific pattern of nondisjunction which in turn was dependent o n whether the female carried an Rb metacentric. 3H1 oocytes gave a nor mally distributed pattern of increase in aneuploid frequency (over the spontaneous value) centering around a 6 h application. This was thoug ht to be due to the interaction of chromosomes with the microtubules o f the spindle during attachment and/or alignment. In contrast both Rb homozygotes and heterozygotes gave the same biphasic response, with a high frequency of aneuploidy in the oocytes when nocodazole was applie d 4 and 8 h after the onset of maturation. In Rb homozygotes we demons trated that the Rb bivalent underwent nondisjunction more frequently t han the average acrocentric, when nocodazole was administered early. I t can be assumed that the Rb trivalent in Rb heterozygotes showed a si milar response. This early Rb specific effect, in combination with a d elayed-version of the acrocentric effect found in the 3H1 mice was tho ught to generate the biphasic pattern. We discuss the implications of (a) the different meiotic behaviours of metacentrics and acrocentrics and (b) the meiotic delay in Rb mice.