In fruit trees, sports are naturally-occurring mutations that can give rise to a new variety. Two examples are Suffolk Pink and Winter Wonder. Here is some background information on sports in apple varieties.
Nature loves biodiversity
We are all familiar with the fact that if you plant the seed of an apple, the tree that grows and the fruit that it produces will be totally different with respect to the parent tree. This is because the cross-fertilization that occurs in pollination ensures that all the seeds are genetically different. This is the way in which nature generates diversity, which in the long terms helps trees to survive by ensuring that, whatever the climatic conditions, sooner or later there will be a tree suited to the local environment.
So the trees sold by a nursery have been created by grafting part of the original tree onto a rootstock, to ensure that the fruit will be the same as the parent variety. One of the new trees can then be used to obtain more buds and twigs that can be grafted, and so on.
Mutation creates new varieties
But nature has another card up its sleeve. As a newly-grafted bud grows on the rootstock, the cells multiplying and diversifying to form the new trunk, branches and bud, there is always the possibility that a chance mutation occurs in a single cell, which then multiplies to form a bud or shoot that is genetically different to the rest of the tree. If this shoot is then taken by the nurseryman and propagated, the entire new tree will no longer correspond to the original variety.
For any organism, mutations are nearly always negative occurrences, producing tissue that is not viable or that doesn’t provide any benefit, and in fact both plants and animals have complex defence mechanisms enabling mutant cells to be destroyed. But sometimes a mutation retains nearly all the characteristics of the parent tissue, and adds some positive benefits. In a plant, when there is enough genetic similarity, the new tissue is accepted by the original tree and can in turn be successfully propagated.
Charles Darwin loved “sports”
Mankind has observed and used these chance mutations or “sports” for centuries. Charles Darwin wrote, in his book The Variation of Plants and Animals Under Domestication, “Many cases have been recorded of a whole plant, or a single branch, or bud suddenly producing flowers different from the proper type in colour, form, size, doubleness, or other character. Half the flower, or a smaller segment sometimes changes colour”.
Examples of “good sports”
An example of a bud sport that has been appreciated by mankind is the development of flowers, such as roses, which have a single whorl of five petals in the wild, but can develop multiple whorls in the case of a chance mutation.
Another example is the overriding of the self-incompatibility mechanism in pollination. In most varieties, the pistil rejects pollen with the same genetic make-up, and a fruit thus lacking seeds usually doesn’t develop.
Citrus sports lead to modifications in the process, permitting the development of seedless fruit. Another notable citrus sport gave rise to the blood orange, which develops a dark red anthocyanin colour when the developing fruit is exposed to cold.
Mutations enabled the selection of white-skinned grapes instead of the usual dark skins.
In apples, sports that have been studied include mutations that increase or decrease fruit size, change fruit shape, or that change the architecture of the tree. An example is the Wijcik sport, first noticed by American fruit grower Anthony Wijcik, a single shoot on a McIntosh tree. When propagated, this produced a columnar tree with a thick leader and short lateral spurs instead of branches. This has been used commercially for high-density planting of trees that need less pruning and could be used for mechanical harvesting. In this case, the mutation presumably changes the expression of growth hormones such as gibberellins.