THEORETICAL ANALYSES OF CHIASMATA USING A NOVEL CHIASMA GRAPH METHOD APPLIED TO CHINESE-HAMSTERS, MICE, AND DOG

Some basic concepts of chiasma (including chiasma distribution, chiasm a frequency, interstitial and terminal chiasmata, and chiasma interfer ence) are reexamined theoretically in the light of gene shuffling, and a new method for chiasma analysis termed the chiasma graph is propose d. Chiasma graphs are developed for three mammals with greatly differe nt chromosome numbers: Chinese hamster (with n=11), mice (n=20), and a dog (n=39). The results demonstrate that interstitial chiasmata can c ontribute both to gene shuffling and to the binding of bivalents, but that so-called terminal chiasmata are in fact mostly achiasmatic termi nal associations, the main function of which is to bind bivalents. For this reason, terminal chiasmata should be excluded when chiasma frequ ency is estimated. It is also demonstrated that interstitial chiasmata distribute on bivalents randomly and uniformly, except at the centrom ere and telomere. Interference distance fluctuates almost randomly abo ve a minimum value equivalent to about 1.8% of total bivalent length a t diakinesis. These results indicate that chiasma formation in mammals is principally a random event. The demonstrated minimum interference distance seems consistent with the polymerization model for chiasma fo rmation. Some cytological aspects of crossing-over are discussed with reference to the minimum interaction theory for eukaryotic chromosome evolution.