A re-examination of chiasma terminalization and chiasma frequency in male mice

The distribution and frequency of chiasmata have been analyzed in male BALB/c mice. Bivalents were classified in terms of the number of interstitial chiasmata (CH) and terminal associations (TA) present as follows: 1CH, 2CH, 1TA, 2TA, 1CH X 1TA, 1CH X 2TA, 2CH X 1TA, 2CH X 2TA and 0. We provide evidence that the TA frequently dissociates during 1st meiotic prophase. Consequently six of the observed bivalents may be derived from three basic bivalent types (namely 2CH X 2TA, 1CH X 2TA and 2TA) by dissociation of the TA according to the following schemas: (1) 2CH X 2TA leads to 2CH X 1TA leads to 2CH, (2) 1CH X 2TA leads to 1CH X 1TA leads to 1CH, and (3) 2TA leads to 1TA leads to 0. We also provide evidence that interstitial chiasmata do not move, which implies that a TA can not be formed by chiasma terminalization. The chiasma frequency estimated by assuming that terminal associations do not result from terminalized chiasmata is 17.2+/-2.4 compared to a value of 25.4+/-2.2 calculated on the assumption of chiasma terminalization.