Chimeras

CHIMERAS, animal or plant organisms consisting of genetically heterogeneous tissues. Often, it is not entire organisms that are chimerically constructed, but only their individual organs. In most cases, chimeras are obtained artificially through transplantation or grafting, but they are sometimes encountered under natural conditions (apparently as a result of vegetative mutations). During the regeneration of a damaged chimeric organ, the formation of a new one consisting entirely of one type of tissue may occur. This phenomenon has been termed the "vegetative segregation" of chimeras. During sexual reproduction, their daughter individuals are also genotypically homogeneous. Several types of chimeras are distinguished: 1) in the case where tissues of one species are covered by a layer of tissues of another, they are called periclinal chimeras; 2) formations in which tissues of one species enter the tissues of another in the form of a wedge or sector are called sectorial chimeras; 3) mosaic chimeras are also distinguished, in which the tissues of both species are intricately mixed with each other; 4) finally, a new type of chimera, so-called cytomictic, was recently established by Isayev in hydras. In this case, only the inner layer of polypotent germ cells represents a mixture of cells of two species. An interesting case of chimeras is represented by myxochimeras. These are organisms formed from the fusion of different races of mold fungi. Since the body of the latter represents a plasmodium, the mixing of protoplasms occurs upon their fusion. The new mycelium and sporangiophores developing from such chimeric formations have a more or less intermediate character between both combined species. Finally, in some cases, due to the non-disjunction of chromosomes during the division of individual somatic cells of an organism, cells with tetraploid sets of chromosomes can appear, which leads to the formation of chromosomal chimeras. Similar chimeric formations can sometimes be obtained through grafting. Thus, Winkler, when grafting a tomato (24 chromosomes) and a black nightshade (72 chromosomes), obtained a chimera with 48 chromosomes. On the basis of these studies, some authors are attempting to revive the concept of "graft hybrids," i.e., hybrids obtained by grafting but capable of transmitting their chimeric properties during sexual reproduction. However, this question can by no means be considered resolved.