Symmetry and asymmetry in the development of inner organs in parabiotic twins of amphibians (Urodela).

Newt embryos of different developmental stages were combined to parabiotic twins in different positions. The exterior appearance and the symmetry relations, particularly of the internal organs (intestinal tract, heart, nuclei habenulae, and vitelline vein) were studied. Experimentally caused organ inversions allowed conclusions with respect to organ asymmetry and unilateral dominance. There was no direct correlation between appearance and symmetry of the exterior and the internal organs. All internal organs showed a continuous transition between normal and ideally inverse situs. The concordance of the organ situs differs greatly. The "left-hand side" or "right-hand side" dominance is not uniform. It depends on the type of fusion, i.e. the relative position of the parabiotic twins, and is often specific for a given organ. In some cases a non-genetic "symmetrisation factor" appears to be strongly active, depending on the fusion type and resulting in a dominant transindividual organ mirror image symmetry in the parabiotic twins. The older twin generally dominates the processes of determination and induction. The "symmetrisation factor" also acts on members of different families, i.e. genetically completely heterogeneous parabiotic twins. The development of organ asymmetry appears to be a process with several phases.

[Mirror symmetrical and "left dominant" organization in the gastrula stage molted parabiotic larva of mountain salamander]. / Spiegelsymmetrische und "linksdominante" Organogenese bei im Gastrulastadium verschmolzenen Parabioselarven des Bergmolches (Triturus alpestris).

Gastrulae of Triturus alpestris were laterally fused (body axes in equal directions). The parabiotic larvae mostly showed an undivided body with tail, but exhibited two heads. Defects of organogenesis were frequently observed in the gut, and, to some fewer extent, in the heart. The gut mostly develops undivided, the heart in singular or (with transitions) as two organs. The number of organ-inversions and partial inversions was high (table 1). In addition a transindividual organ-symmetry (mirror-image symmetry of the "pair situs") was abundant (table 3, fig. 6) and the gut and the habenular nuclei showed a strong dominance for that phenotype. The organ inversions were more abundant in the right organ or part of the organ. These right-left-differences were statistically significant referring to the inversion tendency of the heart and the habenular nuclei (table 2). This "left-dominance" observed in the organ Anlagen investigated is discussed in regard to results published previously.

[The symmetrical relation of intestines, the habenular nuclei and the intestinal venous system in dorsal-doral and ventral-ventral grown parabiontic larva of mountain salamanders (Triturus alpestris)]. / Die Symmetrieverhältnisse der Eingeweide, der Habenulakerne und des Darmvenensystems bei dorsal-dorsal und ventral-ventral verwachsenen Parabiose-larven des Bergmolches (Triturus alpestris).

Embryos of the newt Triturus alpestris were fused in their dorsal regions (DD-parabionts) of their ventral regions (VV-parabionts), while the longitudinal axes were in equal direction. In these parabionts situs and symmetry of the gut, the heart, the habenular nuclei and the vitelline vein were studied. The fusion of the embryos of the DD-parabionts was performed in three developmental stages (after Harrison): phase of neurulation (14-18, N-parabionts), late phase of neurulation (19-22, E-parabionts) and tail bud stage (23-27, S-parabionts). VV-parabionts were only fused in phase of neurulation (14-18). The external development of the parabionts in most cases was normally. DD-parabionts showed a more or less pronounced deformation of their longitudinal axes. In DD-parabionts all organs developed separately in nearly all cases. In VV-parabionts a partial fusion of the heart was sometimes observed, while fusion of large part of gut occurred regularly. In DD-parabionts complete and incomplete inversions of all organs and the vitelline vein are extremely abundant in the N-parabionts, much lower in E-parabionts and nearly absent in S-parabionts. The differences are statistically of high significance. The "pair situs" symmetries of N-parabionts (DD-parabionts) show always a strong dominance of a transindividual organ- and vitellin vein-symmetry. In VV-parabionts (N-parabionts) the tendency for inversion of the asymmetric organs is statistically significant lower as in DD-parabionts. In regard to this observation there is a preference for an asymmetric "pair situs" in the heart, habenular nuclei and vitelline vein system. In "polarisied" DD-parabionts ("left" as well as "right" parabionts) the asymmetric organs as well as the vitelline vein system inversions were more often observed in the "left" parabiont compared with the "right" parabiont in N- and E-parabionts. These right-left differences partial are statistically significant (P < 0.05). The organ situs-correlation as well as the situs-correlation between vitelline vein and gut or heart, respectively, are mostly positive (concordant). The frequency of dominance of "left" organ inversions argues against a morphogenetic "left dominance" of the amphibian embryo. The determination of organ-asymmetries is irreversible only after the end of neurulation in tail-bud-stage. The results suggest the efficiency of a "symmetry-factor", which determines the organ symmetry during neurulaphase. The differences between DD- and VV-parabionts are statistically significant and point to a dorso-ventral polarity of the embryo, with a maximal morphogenetic potency in the dorsal region.