Fusion (Biology)

FUSION (conplantation), a term proposed by Isaev to denote such transplantations (see) where the matter concerns the joining of significant sections of two organisms to each other or the pairing of two whole organisms. Fusion of the latter type has received the name parabiosis (see). For the first time, fusion in animals was carried out by Trembley in 1774 in work with hydras. Trembley succeeded in fusing pieces of one and the same individual and pieces of different individuals of the same species, i.e., auto- and homoplastic conplantations.

Figure 1. Sectorial chimeras in hydras: a—Pelmatohydra oligactis; b—Hydra vulgaris; c—fusion operation; d—chimera from the side; e—chimera from above. (From Isaev.)

Subsequently, a number of authors succeeded in heteroplastic conplantations on hydras. In Wetzel's experiments, attempts at fusion of the brown hydra (Hydra fusca) with the green hydra (Hydra viridis) remained unsuccessful. The joined pieces stuck together for some time, but then separated. Conversely, the fusion of Hydra fusca and H. grisea succeeds very well. The joined parts, as a rule, retained their inherent pigmentation. Isaev produced two types of fusion between the common hydra (H. vulgaris) and the gray stalked hydra (Pelmatohydra oligactis). In one series of experiments, each hydra was cut lengthwise and unfolded into a plate; two such plates from different hydras were placed together and held in contact for some time, after which a connection occurred and a single organism was formed, colored gray on one side and having long tentacles on that side, and colored reddish on the other side with shorter tentacles (Fig. 1). Subsequently, some regulatory changes occurred, and the number of tentacles decreased. In another series of experiments, Isaev threaded an individual of one species of hydra inside an individual of another species. For some time, the coloration of the fused parts was preserved, but then the reddish parts were, as it were, displaced by the gray ones. This replacement, however, does not mean the complete disappearance of the properties of the common hydra (Hydra vulgaris) in the fused individual. When such fused hydras bud, they produce some offspring completely identical to the normal gray hydra and some that carry the traits of the fused parent. If we denote normal gray hydras with the letter P, and intermediate ones, called by Isaev "oligactoids", with the letters PH, then the descendants of the fused hydra can be schematically denoted as follows: Generations

III IV

Average hydra <G

V <1 PH <^P PH

The results presented indicate that after fusion, with the visible disappearance of the properties of one of the partners, cellular elements of both partners remain in the fused organism, which are revealed during budding. The presence of these elements was shown in a number of Goetsch's experiments with the fusion of hydras, wherein the cellular elements of one of the partners could migrate through the body of the other partner, reaching areas sufficiently distant from the site of fusion. Besides experiments on hydras, successful results were yielded by the fusion of worms. For such experiments, different species of turbellarian flatworms and oligochaete roundworms were used. T. Morgan, in experiments with the planarian Bipalium kewense, obtained fusions of large pieces of worms by compressing them between plates, by which contact of the wound surfaces was achieved. L. Morgan used the planarians Phagocata gracilis and Planaria maculata for the experiment. Success of fusion was achieved by placing the pieces between moist strips of very thin paper, the ends of which were fixed with pieces of glass and pins. L. Morgan discovered that with careful joining of the parts, regeneration does not occur, and the fused organism remains unchanged. With unsuccessful joining of the pieces, regeneration begins on the surface of the cut, and the fused individuals separate. A condition for the preservation of fused individuals in connection with each other is the connection of the ends of their nervous system. If this condition is not met, then regeneration begins along the nerve cords with subsequent separation of the partners. Experiments on fusion in annelid worms were set up on different species of earthworm. Jost sutured pieces of the earthworms Lumbriculus rubellus and Allobophora terrestris auto-, homo-, and heteroplastically. He obtained diverse combinations like "short" worms, in which the middle part was cut out and the front and back ends were fused; "long" worms, composed of the front and back end of one worm and the middle of another; two front or two back ends were fused to each other; T-shaped conplantations were carried out, etc. Heteroplastic fusions succeed with great difficulty; however, in Jost's work, a worm obtained by the fusion of pieces of Lumbriculus and Allobophora lived for more than 8 months, with the parts retaining their characteristic species features. The influence of one partner was not detected even during the regeneration of the other. Thus, during the regeneration of a part of Lumbriculus, the regenerating part retained the typical pale coloration; the darkly colored part of the fused individual, taken from Allobophora, did not exert an influence on the regenerate of the part taken from Lumbriculus (Fig. 2). Crampton fused butterfly pupae, cutting them in different directions,

Figure 2. Fusion in earthworms; formation of a regenerate on the grafted section from three segments. (According to Jost from Korschelt.)

Figure 3. Fusion of butterfly pupae, cut in different directions, and their various parts. (According to Crampton from Korschelt.)

folding the cut surfaces together and sealing the connection site with paraffin (Fig. 3). From such fused pupae, it was sometimes possible to rear butterflies; mutual influences were manifested in the fact that the metamorphosis of the fused pieces occurred simultaneously, but there were usually no changes in coloration; each part, during heteroplastic conplantations, as a rule, retained its features. However, in Crampton's experiments, there are also such data: the posterior part of the abdomen of a Callosamia promethea pupa was fused with a Samia cecropia pupa and in the butterfly, it took on the coloration of Samia; the same was observed during the fusion of a piece of a pupa of the latter species with a Telea polyphemus pupa. The circulation of hemolymph of the larger partner may, in Morgan's opinion, be the cause of such a change in coloration. It should be noted that in Crampton's experiments, fusion of only the integuments took place; the internal organs remained separated. Experiments on fusion were also set up on vertebrate animals, with particularly demonstrative results obtained on amphibian tadpoles. For the first time, these studies were conducted by Born, extracting embryos from the egg membrane. He cut them into two parts and then, bringing the wound surfaces into contact, held the pieces of tadpoles with silver wires. Homoplastic conplantations in Born's experiments were easy to succeed, however, the fusion of Rana esculenta with R. fusca or R. arvalis is also quite possible. Connections of parts of R. esculenta and Bombina bombina are very short-lived, as is the connection of embryos of tailed and tailless amphibians (Triton taeniatus and Rana esculenta). In cases of successful connections, the internal organs—the nervous, digestive, and circulatory systems—grew together, and the tadpole, normally feeding, underwent metamorphosis into a frog. Effective heteroplastic conplantations were performed on amphibians by Harrison. Fusing the front part of a Rana sylvatica embryo with the back part of a R. palustris embryo, he observed the preservation of the characteristic coloration by each part (Fig. 4). In the majority of fusion experiments, the authors came to the conclusion about the absence of influence of one partner on the other in the sense of the development of morphological traits. At the same time, the developmental processes of the fused individuals proceeded synchronously (butterflies, tadpoles), which indicates the undoubted presence of mutual influences. The results of some of Crampton's experiments cited above speak to the same thing. It is necessary to note that many of the works related to this are presented insufficiently detailed, without the necessary consideration of possible interactions, and require re-examination. The method of conplantations promises to yield much of value in solving a number of fundamental problems of biology concerning the properties of the organism as a whole, the connection of its parts, their relative autonomy, their local specific features, etc.

Figure 4. Tadpoles obtained by fusion of the front half of Rana sylvatica with the back half of Rana palustris. (According to Harrison from Korschelt.)

rearing tadpoles by fusion

feeding normally

metamorphosis into a frog.