Interspecific somatic hybrids

Regulation of the Cell Cycle in Mammalian Cells Inferences and Speculations Based on Observations of Interspecific Somatic Hybrids... 

Intercellular control of the functions of mammalian cells has been long since evidenced by observations of experimental embryologists and, in recent years, one form of it, contact inhibition, discovered by Abercrombie and Heayesman (1954), has been a most popular subject of study. Contact inhibition is certainly one type of control which is generally not found in bacteria and this difference must be directly correlated with differences between the structures of bacterial and mammalian cell surfaces. Other differences between the structures of bacterial and mammalian cells are the presence in the latter of a nuclear membrane and of an elaborate mitotic apparatus. We shall have little to say about the latter and shall consider mainly the nuclear membrane, to which, we think, are delegated certain functions assumed, in bacteria, by the cell membrane and concerned with die coordinated replication of the chromosomes (Jacob et al., 1963). This idea represents one of a number of inferences from recent observations made in our laboratories on interspecific somatic hybrids between mammalian cells, and it is the primary purpose of this paper to present the evidence on which it is based (Note 2). 

Before doing so, we wish to call attention to the fact that the karyotypic and phenotypic characteristics of the interspecific somatic hybrids will be presented in the order in which they were observed accordingly, we will give consideration first to rat X mouse and hamster X mouse hybrids. The implications of the properties of these hybrids will be discussed, and an attempt will be made to evaluate their significance in contributing to a general speculative picture of the coordination of replication processes in mammalian cells. We shall see later (Section IV, E) that, in the light of recent observations on some other interspecific hybrids, some of our conclusions may have to be amended. 

The viability of these hybrid cells permits an inference of interest from both the embryological and evolutionary points of view. The continuous multiplication and apparent permanence of hybrids between somatic cells of species phylogenetically as remote as the hamster, the rat, and the mouse, shows that, between somatic cells, there is no incompatibility similar to that observed in sexual interspecific crosses. It will be recalled that this incompatibility, in the extreme cases, results in the elimination of the foreign chromatin from the fertilized egg. It is clear therefore that the inviabiUty of sexual hybrids between remote forms, when they do contain both parental genomes, is due to their inadequacy for directing development and differentiation rather than to the inability of hybrid genotypes to control the balanced metabolism involved in cell replication. 

The validity of this inference depend of comse on the demonstration of the continuous presence in interspecific somatic hybrids of the two foreign parental genomes, on the one hand, and on that of their functional activity, on the other. Information on these two points is given in the following two sections. 

Returning to our speculations on the coordination of biochemical events in mononucleate interspecific somatic hybrids, we would like to go one step further and suggest that the synchronization of the two parental cell cycles to produce a single new cycle is in part the consequence of the attachment of (some or all ) chromosomes of both species to the single nuclear membrane and hence of their simultaneous response to the reaction of the nuclear membrane to the signal initiating DNA synthesis. 

Owing to the apparently frequent differences in physical properties of enzymes of remote mammalian species, similar experiments should be possible with interspecific somatic hybrids and should thus provide evidence on the vahdity of these speculations. In principle (and we emphasize in principle ), the approximate alignment due to linear transcription should result in more or less clearly spaced bursts of homologous enzymes in the cell cycle of synchronized populations. Coordination by an induction-repression mechanism should, on the contrary, result in the simultaneous synthesis of homologous enzymes. 

While our speculations have been prompted by observations of interspecific somatic hybrids, they apply also to, and could have been based on, earlier observations (some of which will be quoted) of the numerous intraspecific (mouse X mouse) hybrids produced in our laboratories. However, the foreignness of the two genomes of interspecific hybrids presents the problems of coordination of their activities in a particularly acute form and simultaneously offers possibilities for experimental tests of the various hypothetical solutions. 

Weiss, M. C., and Ephhussi, B. (1966b). Studies of interspecific somatic hybrids. 

The use of protoplasts in studies of stress physiology and biochemistry expands the advantages of cell culture systems discussed in the preceding sections. Additional applications are related to the fusion of protoplasts. Intraspecifie and interspecific protoplast fusion greatly enhance genetic variability of the fused protoplasts (Kumar Cocking, 1987). The resulting somatic hybrids provide cells which can be used for selection of specific traits (e.g. environmental stress tolerance) provided by one or both donor cells and for basic studies on cytoplasmic and nuclear inheritance of desired characteristics. 

Carputo, D. C. T., Speggiorin, M., Zoina, A., Frusciante, L. (1997). Resistance to blackleg and tuber soft rot in sexual and somatic interspecific hybrids with different genetic background. American Potato Journal 74, 161-172. 

An efficient protoplast plant system has been developed, based on which interspecific hybrids not available by traditional improvement have been produced by a method of somatic hybridization. 

Gancel, A.L., D. OUe, P. OlUtraut, F. Luro, and J.M. BriUouet, 2002. Leaf and peel volatile compounds of an interspecific citrus somatic hybrid (Citrus aurantifolia Swing, x Citrus parodist Macfayden). Flavour Fragr. J., 17 416-424. 

Thus, judging from these three nonselective enzymes, examined in a number of rat X mouse and hamster X mouse somatic hybrids, it can be stated that both parental genomes are indeed functional in these cells. Moreover, it appears that in the quoted cases, the products of homologous genes, in spite of the evolutionary divergence of the latter, are similar enough to form fully functional hybrid molecules such hybrid molecules are probably very common in interspecific hybrid cells since many proteins are composed of subunits. 

Weiss, M. C., and Ephrussi, B. (1966a). Studies of interspecific (rat X mouse) somatic hybrids. I. Isolation, growth and evolution of the karyotype. Genetics 54, 1095-1109. 

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