Chromosomal and embryological analyses of nuclear changes occurring in embryos derived from transfers of nuclei between Rana pipiens and Rana sylvatica.

Abstract Diploid nuclei from normal blastulae of Rana pipiens were transplanted into enucleated eggs of Rana sylvatica. Following 10–12 divisions in the foreign cytoplasm, their nuclear descendants were transferred back into enucleated pipiens eggs. All but one of the embryos derived from these eggs developed abnormally and were arrested at various stages ranging from late blastulae to young tadpoles. These results, like those obtained by Moore, 1958a , Moore, 1958b , Moore, 1960 , demonstrate that pipiens nuclei are changed as a consequence of replicating in sylvatica cytoplasm and are no longer capable of promoting normal development when returned to their own type of cytoplasm. The main objective of this investigation was to determine whether the restrictions in the developmental capacity of these nuclei involve cytologically detectable alterations in the chromosome complement. Analysis of metaphase plates from the abnormally developing back-transfer embryos showed that all of these embryos did in fact possess abnormal numbers and types of chromosomes as follows: 1. 1. The chromosomal abnormalities were most pronounced in back-transfer embryos arresting in early stages of development (gastrulae, neurulae, and early tailbud embryos). When compared with the normal pipiens karyotype, the chromosome complements of these embryos were found to possess the following abnormalities: (a) gains and losses of from one to four chromosomes, (b) ring chromosomes, minute chromosomes, and in one case, acentric fragments, and (c) a decrease in the number of large chromosomes coupled with an increase in the number of small ones. The karyotypes were so abnormal that it was not possible to specify which chromosomes were involved in the rearrangements. 2. 2. The chromosomal alterations in back-transfer embryos arresting in later stages of development (late tailbud embryos and abnormal tadpoles) were less extensive and consequently could be analyzed in greater detail. These embryos were aneuploid by one or two chromosomes and, unlike the more deficient embryos described above, did not possess rings or minutes. A detailed karyotype analysis of one clone revealed that the chromosome complements of these embryos lacked one or more specific chromosomes known to be present in the normal diploid set and, at the same time, possessed new structural types normally not found in Rana pipiens. Some of these new types of chromosomes appeared to be the result of a translocation involving a distinctive chromosome (no. 10) of the normal pipiens complement. Karyotypic abnormalities of the types just described can account for the changes in the developmental capacity of pipiens nuclei as a consequence of replicating in sylvatica cytoplasm and for the results obtained by Moore (1960) , which show that these changes are irreversible. According to the evidence presented in this report, alterations in the chromosome complements arise during the time that pipiens nuclei are dividing in sylvatica cytoplasm but exactly when and how they are produced remains to be worked out.