Transitory asynchrony

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. 

While this scheme seems to account for the transitory asynchrony of the hamster-species hybrids, its application to the particular case of the hybrids of the 3460 X 2472 series encounters serious diflBculties, as follows (a) In these hybrids the lagging chromosomes are always those of the hamster parent, (b) Fifty percent of the hybrid metaphases appear synchronous, yet clones of viable hybrids are not obtained. Therefore one must assume either that synchrony of some of the first mitoses is purely fortuitous (and due to fusion of cells in the same late phase of preparation for mitosis) and is not maintained in the succeeding divisions or that the inviability of these hybrids is due to causes unrelated to the observed mitotic asynchrony. The possible nature of these causes will be discussed below. Before we do so, we would like to point out that the application of the above hypothesis to the human X mouse hybrids encounters similar difiSculties. Since, on this hypothesis, a certain fraction of hybrids must result from the fusion of cells in the same phase of the life cycle, one should find a fraction of hybrids containing the full complements of mouse and human chromosomes. In fact, however, such a condition of these hybrids appears to be ephemeral. 

By the use of UV-inactivated Sendai virus, Yerganian and Nell (1966) have obtained hybrids between somatic cells of the Armenian and Chinese hamsters (transformed by human adenoviruses, types 18 and 7, respectively). Two days after the exposure of die mixed cell suspension to the virus, 18 hybrid metaphases were recorded. These metaphases contained the expected chromosome complements of die two species but these were markedly out of phase with respect to spiralization or contraction of the chromosomes. This asynchrony was apparently transitory, for a second karyological analysis, performed 7 days after exposure of the cells to the virus, showed no such asynchrony in the hybrid metaphases. 

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