Asynchronous metaphase

Fig. 4. Asynchronous metaphase of hybrid between cells of the Chinese hamster melanoma 3460 and of the mouse heteroploid line 2472.

Suppose that, at the moment of fusion, cell A is in C2 (i.e., has terminated DNA synthesis) while its mate, B, is still in S or in Ci suppose further tbat in the fusion product (a binucleated cell ), nucleus A, without waiting for B to catch up with it, emits a signal to which B responds without having completed the intervening reactions comprised in S and G2 then one would observe asynchronous metaphases like the ones recorded in the hamster-species hybrids (Indeed, the picture would not differ from that observed by Stubble-... 

As can be seen in Table 2, hybrids between cells of a Syrian hamster melanoma line and aneuploid mouse cells of Sanford s high cancer line are listed as asynchronous although, at 29 C, numerous hybrid mitoses are observed 2 days after the initiation of mixed cultures, in up to 50% of these metaphases, the chromosomes of the two species appear to be in very different stages of contraction or coiling [this is so whether colchicine is used or not and whedier, prior to karyological analysis, the cultures are incubated for 5 hours at 29°... 

Starting from the mildest of the three described aberrations—the apparently transitory mitotic asynchrony in the hamster species hybrids described by Yerganian and Nell (1966)—one must remark, to begin with, that since the percent of viable immediate products of cell fusion ( newborn hybrids ) is unknown, one cannot be entirely sure that the second set of (synchronous ) metaphases observed by these authors represents indeed the progeny of the first (asynchronous ) set, i.e., that we are really dealing with transitory asynchrony. On the other hand, since the hybrid metaphases we observe in the vast majority of our hybridization experiments are certainly not the first mitoses of newborn hybrids, we cannot be sure either that, in all crosses, most of the first hybrid mitoses are not asynchronous. It is possibly significant that in the only case where most of the observed mitoses certainly are the first mitoses of newborn hybrids (crosses of the 3460 X 2472 series, at 29°), up to 50 of the metaphases are asynchronous indeed. 

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