Graft Chimera

On a small traffic island on Rodney Road, in Backwell, in the English county of North Somerset, stands a horticultural curiosity—a cherry tree producing two distinct colors of blossom: pink on one side and white on the other. The dual-color tree is thought to have been planted in the late 1950s. Its name, Strawberries and Cream Tree, was given by the town’s children.

The Strawberries and Cream Tree in Backwell, North Somerset, England. Photo: Mojo0306/Wikimedia Commons

The Strawberries and Cream Tree is a graft chimera of a native wild cherry tree (Prunus avium) and a Prunus 'Kanzan', an ornamental cherry variety from Japan. The wild cherry produces the white blossom, while the 'Kanzan' produces the pink blossom. A graft chimera arises when tissues of two genetically different plant partially fuses following grafting to form a single growing organism that preserves both types of tissue in a single shoot. A well-known example of a graft-chimera is the Laburnocytisus 'Adamii', caused by the fusion of a Laburnum and a broom. The resulting tree bears the typical yellow flowers of a Laburnum and purple flowers of a broom.

Laburnocytisus 'Adamii', also known as Adam's laburnum or broom laburnum. Photo: Simon Garbutt/Wikimedia Commons

Chimerism or chimera is an unusual phenomenon. In animals, a chimera forms by the fusion of two or more fertilized eggs, which can result in an individual who possess genetic materials of more than one type. This gets expressed in the individual through subtle variation in form, such as different eye color in both eyes, or the possession of both female and male sex organs. In plants, chimeras is often due to mutation during ordinary cell division. But sometimes a chimera can result from grafting.

Probably the world’s first graft chimera was the Bizzaria, a graft between the Florentine citron and sour orange. The tree produces both regular Florentine citron and sour oranges, as well as a bizarre ugly, lumpy, yellow and green fruit with the morphological features of both species.

Bizzaria. Photo: Labrina/Wikimedia Commons

Description of Bizzaria first appeared in horticultural literature in 1674, when the Florentine gardener Pietro Nati discovered the chimera growing from a graft junction. The repeated emergence of such unusual specimens initially puzzled the scientific community leading to extensive speculation about the nature of genetic inheritance and plant hybridization.

Charles Darwin proposed that these new hybrids are the result of the rootstock and scion fusing with each other at the graft-junction site to asexually generate a new plant. He called them “graft-hybrids”. But German botanist Hans Winkler in 1907 reinvestigated this phenomenon by grafting black nightshade (Solanum nigrum) into tomato (Solanum lycopersicum), and discovered that the shoots arising at the junction of the two tissues had the characters of both the nightshade on one side and tomato on the other. Winkler called this shoot a chimera, inspired by the fire breathing Greek monster composed of a lion's head, a goat's body, and a serpent's tail.

Chimera is still a rare phenomenon, although many have been created through grafting. Today there are chimeras of roses and apples and pears.

A chimera apple. Photo: flyingbrickciderco.com.au

References:
# Margaret H.Frank, Daniel H.Chitwood, Plant chimeras: The good, the bad, and the ‘Bizzaria’, https://www.sciencedirect.com/science/article/pii/S0012160616300902
# Britannica, https://www.britannica.com/science/chimera-plant-anatomy
# Wikipedia, https://en.wikipedia.org/wiki/Strawberries_and_Cream_Tree