Abstract
On the basis of differences in crossing-over frequency and in radiation sensitivity, two stocks of Phryne cincta can be distinguished. The higher sensitivity of the Alpine stock (as), compared with the Berlin stock (bs), seemed to be due to its higher site number in tightly spiralized pronucleus chromosomes (Israelewski, 1979). A mathematical model was tested to arrive at an estimate of the maximal site number in Phryne. Deducing from this model, repair in a 2-break site would result with the probability of 1/6 in restitution, 1/6 in balanced chromosome aberration and 4/6 in unbalanced chromosome aberration. The maximal site number available is 13 in bs and 20 in as. It is suggested that a high proportion of 2-break sites are repaired after replication of the paternal pronucleus chromosomes. The proportion of prae- and postreplication repair is estimated to be 50% each, possibly in agreement with the distribution of nucleosomal and internucleosomal DNA in eukaryotic chromatin.
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