Cytoplasmic Influence on Lethality in Nucleocytoplasmic and Natural Hybrids between Bufo Calamita Laur and Bufo Bufo L. (Amphibia, Anura).

Nucleocytoplasmic interactions have been studied through nuclear and cytoplasmic grafts. Bufo bufo nuclei transplanted in Bufo calamita eggs provoked 95% lethality during gastrulation. Influence of the cytoplasm in such a nucleocytoplasmic incompatibility has been studied through cytoplasmic fractionation and grafting of the fractions obtained. Fractionation of egg cytoplasm gave three fundamental fractions: lipid, hyaline cytoplasm and vitellus. After injection of B. calamita fractions into B. bufo fertilized eggs, the hyaline cytoplasmic fraction provoked a gastrulation block in the same ratio (95%) as in the interspecific nuclear graft experiment. Influence of the hyaline cytoplasmic fractions of both species has been tested on nucleocytoplasmic and natural hybrid developments. The B. calamita hyaline cytoplasmic fraction provoked in each case a developmental block at the beginning of gastrulation. The B. bufo hyaline cytoplasmic fraction acts later at the yolk plug stage and in some cases partially corrects the developmental bolck or exogastrulation of young hybrid gastrula. It can be generalized that in both kinds of hybrids, the hyaline cytoplasmic fraction of B. calamita exerted a lethal effect on B. bufo nucleus.