To the editor:

According to reports early this year1, the cloned sheep “Dolly,” born five years ago at the Roslin Institute (Edinburgh, UK), has begun to show untimely symptoms of osteoarthritis. This has prompted suggestions that inoculation of somatic nuclei into enucleated oocytes and subsequent in vitro and in vivo manipulation of the embryos might predispose cloned animals to precocious senescence.

Cloning is a very old, well-tested process in propagation of arboreal plants. Apomixis (vegetative propagation and asexual seed formation) is very common in certain plants (such as angiosperms and pteridophytes). For example, some trees can grow both from cross-fertilized and from naturally occurring apomictic seeds (such as nucellar embryogenesis in some citrus), the latter of which carry only the complete maternal genotype. For centuries, a great many cultivated trees have been vegetatively propagated by cuttings, grafting, and in the past 20 years also by in vitro culture of somatic organs, such as shoots, apical meristems, and protoplasts (micropropagation).

Asexually propagated trees show ontogenetic patterns that differ from those of the corresponding sexually propagated seedlings. For example, compared with cross-fertilized pears, grafted pears (Pirus communis L.) have a shorter lifespan (20–30 years versus 100 years) and become sexually mature sooner (3–4 years instead of 20–30 years). In addition, grafted seedlings can still exhibit morphological characters typical of juvenility (such as thorny shoots and smaller leaves), which usually disappear from the canopy top at the same time as onset of flowering and fruiting.

Modern horticulturists take the earlier sexual maturity of vegetatively propagated trees as one of the most important economic benefits of this procedure, whereas they consider the shorter lifespan of these plants to be less important as trees are reared intensively in orchards and are normally renewed long before their physiological end.

Animals and plants follow similar ontogenetic models: after birth, they grow, reach sexual maturity, become old, and die. Despite the obviously different physiological and biochemical backgrounds of these patterns, in both animals and plants apoptosis depends on the coordinate expression of genes regulating divisional cycles and apoptotic pathways. Therefore, it seems reasonable that the behavior of asexually propagated trees could mirror the precocious senescence now being witnessed in cloned animals.