Parental genomes

When we studied the compositional distribution of six genes (or small multigene families) and one family of transposable elements, Tntl, in DNA fractions from tobacco Nicotiana tabacum) separated according to base composition, we could show that gene distribution is bimodal and that such bimodality is due to the different base composition of the two parental genomes of tobacco N. sylvestris and N. tomentosiformis) and to the different parental origin of the genes tested (Matassi et al., 1991). 

These results indicate a conservation of the compositional patterns of the two parental genomes, as well as a conservation of the gene localization. (Fig. 8.17) Moreover, they suggest the absence of any extensive recombination between two genomes which have been in the same nucleus for a time which has been estimated to be less than 6 million years, whereas the two parental genomes have an estimated life of about 75 million years (Oka-muro and Goldberg, 1985). 

Gleba Y.Y., Parokonny A., Kotov V., Negrutiu I., Momot V. (1987). Spatial separation of parental genomes in hybrids of somatic plant cells. Proc. Natl. Acad. Sci. USA 84 3709-3713. 

Somatic hybrids therefore provide unique opportunities for investigating the effect of parental genome dosage, ploidy and nuclear and cytoplasmic interaction on physiological traits. 

A second- or subsequent-generation QSAR model originates from parent models by mutation and recombination. Mutation occurs through random addition, elimination, or exchange of one or more variables. Upon recombination, two offspring models are created from two parent models by exchanging partial sequences of the parent genomes. 

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. 

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. 

As shown above, consideration of both the replication of the two parental genomes and of their expression (i.e., transcription) in interspecific hybrids leads to the notion of a non-species-specific coordination, and the remainder of this paper will be devoted to a discussion of its possible nature and mechanism(s). 

We wish to surest that the primary factor of this unified control is the constitution of a single nucleus, and that the enclosure of the two parental genomes within a single nuclear membrane results in the establishment of a new basic rate of DNA synthesis and chromosome replication which defines the new cell cycle to which all subsequent activities of the parental genomes are thereafter adjusted. 

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