Cytoplasm affects the number of vertebrae in carp-goldfish clones

The March 2005 issue of Biology of Reproduction contains a report of some intriguing findings in cloned offspring created when nuclei from one genus of fish were transplanted to enucleated eggs of another genus of fish.

The seven offspring, cloned from nuclei of common carp and egg cytoplasm of goldfish, were virtually identical to the nuclear donor species, Cyprinus carpio, in appearance and in most physical traits. The number of vertebrae in the clones, however, was in the range of the recipient species, Carassius auratus.

Yong-Hua Sun, Shang-Ping Chen, Ya-Ping Wang, Wei Hu, and Zuo-Yan Zhu, who conducted this work at the Institute of Hydrobiology, Chinese Academy of Sciences, in Wuhan, China, conclude that the egg cytoplasm, and not the genetic code of the transplanted nucleus, influenced this aspect of the skeleton as the cloned fish developed.

They speculate that a so-called “segmentation clock” early in embryonic development is controlled by the egg cytoplasm. Thus the egg cytoplasm of the recipient goldfish directs segmentation of the body and hence the number of vertebrae. Common carp have 33 to 36 vertebrae in their backbones, while goldfish have 26 to 28. Six of the seven cloned fish had between 26 and 28 vertebrae; one had 31.

Although the initial rate of success in producing carp-goldfish clones is low–seven offspring in 501 attempts in this study–the authors believe that cross-species transplantation will lead to improved understanding of the contributions of the nucleus and egg cytoplasm to the growth and development of vertebrates.