Biology of Cloning: History and Rationale
Frank B. Salisbury · BioScience · 2000
Robert G. McKinnell and Marie A. Di Berardino's article, “The Biology of Cloning: History and Rationale” (BioScience 49: 875–885), was certainly an excellent summary of one important aspect of the cloning field. I learned much from reading it. The use of the term biology in the title of the article, however, upset me. Biology is the study of life in all its forms, including plants and even viruses (even if they are alive only when they are reproducing in a host), but the authors limit their fine article exclusively to members of the Kingdom Animalia. Cloning of plants goes back to biblical times—check Romans 11:17, 24. Half a century ago I was taught in a genetics class at the University of Utah that a clone consisted of all the genetically identical offspring of a single parent, produced by asexual means. All the navel orange trees of southern California, for example, form a clone that came from the so-called Parent Navel Orange Tree, a tree that is still protected and kept alive in the safety island of a street in San Bernardino. Probably because of the significance of animal cloning, the media have picked up on the term clone, but they seem to assume that a clone is always an individual. Of course a clone can be an individual if there is only one asexual offspring, as with Dolly, the cloned sheep; plant clones, however, often consist of whole populations, and this could quite possibly be the case with animals as well. And in molecular biology, a gene (or even a genome) is said to be cloned when millions of replications are made. In about 1953 in a plant physiology class at Caltech, we spent a few weeks talking about the question of totipotency in plants and, by extension, in animals. It had been clear for centuries that many plant tissues were totipotent (that is, could reproduce an entire organism), but were individual cells also totipotent? Because each nucleus contained a copy of the entire genome, it certainly seemed logical that individual cells could be totipotent. We needed a way to induce single, somatic plant cells to regenerate an entire plant. This was achieved in the late 1950s in the laboratory of Frederick C. Steward at Cornell University. In rotated, liquid tissue cultures, slices of carrot roots broke down into single cells, and each of these cells could produce a so-called somatic embryo when they were plated out on auger that contained the proper mixture of nutrients and plant hormones. That is, new perfectly normal carrot plants could be produced from nonreproductive cells (somatic cells) from the root rather than only from the egg and pollen cells produced by carrot flowers. Single-cell cloning had been achieved! (Yes, there was some question for a few years about whether the embryos were coming from single cells or small clumps of cells, but this debate was eventually settled in favor of the single cells.) How could cloning be achieved in animals, especially in mammals? Could somatic cells be used to produce offspring that are genetically identical to a parent? It took more than the proper mixture of nutrients and hormones. It took a nuclear transplant from a somatic cell into an enucleated egg and then, for mammals, maturation in a surrogate uterus. Some animal physiologists who work in this field seem to think that this process is the only way in which cloning can be achieved. McKinnell and Di Berardino even define cloning as involving a nuclear transplant! We should stick to the original definition, which applies to all organisms. My Random House dictionary gives a version of the broad definition given above and notes that the term clone was coined in the early 1900s from the Greek klon, meaning a slip or twig. Of course, rooting twigs or slips (or grafting them onto a more hardy root stock) is how plants were cloned for centuries before Steward and others managed to produce clones from single cells or zoologists learned to transplant nuclei. Some simple animals (e.g., planarians, various species of flatworms) undergo cloning by asexually “budding” offspring; others can be cloned without nuclear transplant by cutting them into pieces, each of which regenerates a new organism. It occurs to me that a report of cloning of animals—that is, humans!—goes back even to the beginning of the Bible: the creation of Eve from Adam's rib (Genesis 2:21–22). Of course, there had to be an additional manipulation to get rid of that Y chromosome!