The Blochmann bodies: hereditary intracellular symbionts of insects.

Summary The term ‘Blochmann body’ was originated by Wheeler in 1889 for bacteria-like particles in the cytoplasm of cockroach eggs. These particles can be traced during embryonic development to definitive somatic cells, the mycetocytes. These and similar particles of other insects have so far not been cultivated in bacteriological media nor injected into host animals to produce either pathological or benign infections. Their structure and composition indicate them to be evolutionary descendants of free-living micro-organisms; operationally, they appear to belong to the class of cell particles designated by Lederberg (1952) as plasmids. Genetic studies have shown the Blochmann bodies to be transmitted through the maternal line. Their presence in the egg cytoplasm at some stage in oocyte development is easily demonstrated, but studies by a number of workers have so far yielded a variety of conflicting claims or suggestions as to how the particles get into the germ cells or oocytes. The Blochmann particles of cockroaches, besides existing in the mycetocytes and eggs, occur embedded in a dense tangle of microvilli which are extensions of the plasma membrane of young oocytes. Essentially particle-free strains of cockroaches can be produced by feeding aureo-mycin or high levels of urea, or by withholding manganese. The effect is produced only by treating females, and is delayed one generation. In generations following the first (symbiont-free) generation, the Blochmann symbionts gradually reappear, suggesting that elimination was not absolute. Blochmann bodies in both the mycetocytes and the ovarioles of the cockroach carry out oxidative metabolism, as indicated by their ability to reduce tetrazolium. Glycolysis has not been demonstrated. The generalization that symbionts of the Blochmann type represent an adaptation to compensate for dietary deficiencies is inapplicable, since deficiencies have not been demonstrated for the diets of cockroaches, weevils, or homopterans—the major insect groups in which the symbionts occur. The symbionts of cockroaches and homopterans appear to be involved in the utilization of nitrogenous waste products in synthetic metabolism. In most instances the Blochmann bodies lack the central nucleoid body characteristic of the growing phase of free-living bacteria, thus resembling the Kappa and Mu particles of Paramecium and the endosymbiont of the protozoan Crithidia oncopelti. Both histochemical tests and electron-microscope studies indicate a DNA component that is widely dispersed within the particle. Blochmann bodies are without internal cristae. The cockroach symbionts contain muramic acid, a diagnostic feature of the cell wall of bacteria. Response to various nuclear and cytoplasmic reagents is intermediate between those of typical mitochondria and free-living bacteria. The envelopes of the Blochmann particles are generally thinner than those of free-living bacteria. The function of plasmids of the Blochmann type may be that they, like the bacteroids of the Rhixobium-legume symbiosis, extend the range of metabolic potential of the host cell by a process of mutual host-symbiont adjustment. Possible roles could be subsumed under the headings of bacteria-like, mitochondria-like, or nucleus-like functions.