Origin of chromosomal abnormalities in nuclear transplants—A reevaluation of nuclear differentiation and nuclear equivalence in amphibians

Abstract

Previous transplantation tests of amphibian nuclei derived from advanced somatic and germinal cell types have demonstrated that most of these nuclei have acquired developmental restrictions during embryogenesis. The nature of these restrictions has been revealed in part through chromosomal studies of abnormal nuclear transplants. In these studies, it has been found that the developmental restrictions in the nuclear transplants are accompanied in most cases by karyotypic alterations. In order to determine the origin of these chromosomal aberrations and thereby gain some understanding of the chromosomal changes accompanying normal embryogenesis, we have studied the nuclear and chromosomal morphology of nuclear transplants within the first 8 hours after nuclear transplantation. These studies carried out on nuclei of determined endodermal cells demonstrate that in most cases the origin of these chromosomal aberrations is traceable to the initial responses of the transplanted nucleus to the cytoplasm of the recipient egg. Many transplanted endodermal nuclei fail to undergo sufficient nuclear enlargement and chromatin decondensation. As a result of these events, chromatid bridges arise during the first mitotic cleavage and subsequent to chromatid breakage at late anaphase, variable deletions in chromosomes occur. These different amounts of genetic losses then lead to variable abnormal development in nuclear transplants and ultimately are the cause of developmental arrest. The results of these studies indicate that during embryogenesis the chromosomes of many endodermal nuclei have acquired restrictions in the form of condensed or compact chromatin (heterochromatin). Furthermore, these restrictions are not reversed completely by transplantation into young egg cytoplasm; and consequently, the complete expression of the transplanted nucleus is prevented. The significance of these findings is discussed in relation to our current understanding of nuclear differentiation in amphibians.