Jenny Harris of Balclutha asks :-

A friend's orchard in Clyde produced a pear that was exactly half red and half green. Why was this?

Vincent Bus, an apple and pear breeder with HortResearch, responded.

This phenomenon is called a chimaera. The name is derived from a mythological creature first described by Homer in the Iliad: “She was of divine race, not of men, in the fore part a lion, in the hinder a serpent, and in the middle a goat, breathing forth in terrible manner the force of blazing fire. And Bellerophon slew her, trusting the signs of the gods.” One can imagine the different parts of the creature’s body having sharp demarcation lines very similar to the sharp distinction of the affected and unaffected areas of this pear as if they were glued together.

Chimaeras are the result of genetic mutations that happen in one cell very early in the development of plant tissues and is passed on to all cells that develop from this one cell. In the example of this pear fruit that is half red/half green, it suggests that the mutation happened at the two-cell stage of the fruit.

Some chimaeras only show a band of affected skin, i.e. the mutation happened at a somewhat later stage, while others will result in completely affected tissues or whole plants. Examples of these are the many completely red-skinned sports that have been found for many apple cultivars, such as ‘Royal Gala’ and one well-known pear cultivar, ‘Max Red Bartlett’.

Another mutant in pear has been ‘Taylor’s Gold’, which is a completely russeted variation of the European pear cultivar ‘Doyenné du Comice’. Such mutations are very sought after because of their novelty as they provide a point of differentiation in the market place. These mutants are often identified as a branch with fruit that is distinctively different from the rest of the tree. By taking buds from these trees and grafting them onto rootstocks to produce new trees, the mutations are tested for their stability as some mutations will revert back to their original state. This largely depends on the tissue layer in which the mutation occurred.

Tissues, such as the fruit skin, develop from the outer layer of the meristem, which is the region of active cell division in the formation of plant organs. They are the least permanent mutations, while those that occur in the core of the meristem usually are permanent and often can be used in breeding as they are passed on to their progeny. For example, ‘Max Red Bartlett’, is the progenitor of many breeding lines for red skin colour in the HortResearch pear breeding programme, while an apple selection nicknamed ‘Big Red’ is the main source of red flesh in the apple breeding programme. In the latter case, all the tissues of the trees, from the wood to the leaves to the fruit, are red.