Cell genetics and hereditary symbiosis.

w and scope of cytoplasmic inheritance, often centering on the diagnosis of virus 2’s. plasmagene (5, 42, 43, 45, 48, 55, 197). This review is an attempt to reconcile these views and, where possible, to look for further implications of the synthesis. This account is (quite properly) limited by space allowances. Therefore, objective reviews which already survey the experiments will be cited, when available, in lieu of original references. The treatment of material which has not been condensed previously is therefore exaggerated. An eclectic construction from the freely borrowed views of many authors has been attempted, and few of the interpretations are original. The problems of cytoplasmic heredity have often merged with those of embryonic development and somatic differentiation (56, IIS, 125, 182, 183, 202, 203, 212, 220). The genetics of somatic cells has, however, been poorly studied. Their constitution has been deduced from that of the zygote or the whole organism, except for a few, but important, studies on somatic mutation (66, 119, 123) and somatic segregation (83, 88, 189). Cytological studies have taken some exceptions to the genetic uniformity of somatic cells of the differentiated organism, but have not thereby contributed affirmatively to a comprehensive theory of development (83, 121, 220). Although the pertinence of extranuclear heredity for ontogeny has been generally accepted, in the absence of extensive evidence for it, plausible theories of development have been constructed which rely on the ultimate primacy of the nucleus (73, 131, 193, 202, 220). For technical reasons, cell genetics is best studied in organisms whose germ and soma are not irreversibly differentiated, especially in the microorganisms where vegetative proliferation is preeminent. Whether methodological obstacles with extranuclear heredity factors exaggerate the seeming dominant role of the nucleus in the genetics of higher animals and plants has been debated elsewhere (37, 55, 90, 131, 181, 220). These discussions have left a plethora of terms adrift: pangenes, bioblasts, plasmagenes, plastogenes, chondriogenes, cytogenes and proviruses, which have lost their original utility owing to the accretion of vague or contradictory connotations. At the risk of adding to this list, I propose plasmid as a generic term for any extrachromosomal hereditary determinant. The plasmid itself may be genetically simple or complex. On occasion, the nuclear reference of the general term gene will be emphasized as cltromogetze (2 rgj. This review is dedicated to the reconciliation of the attitudes that plasmids are symbiotic organisms, and that they comprise part of the genetic determination of the organic whole. The conflict may arise in part from fixed conceptions of the scope of the organism. Heuristically, the taxonomic classification of plasmids as viruses, symbionts, or plasmagenes should not obscure careful descriptions of their function, hereditary or pathological, or both. This viewpoint need not prejudge