Cloned plant sporting

November 23, 2011 1:46 PM

What's the genetic mechanism that produces a 'sport' in a cloned plant? I'm thinking specifically of apples.

How can there be a genetic mutation in a large functioning piece of plant, like a grafted branch?

posted by schrodycat to science & nature (2 answers total)

A "sport" is a change in the leafbud (mitose), as opposed to a mutation in the flower (meiose), so that it can generally only be propagated vegetatively. Often the mutation is from somatic recombination or chimaeral rearrangement.
posted by exogenous at 2:19 PM on November 23, 2011

Genetic mutations can occur anywhere and everywhere. They primarily occur during the synthesis, or copying of the DNA, and subsequent mitotic division. The plant cells make a new copy of their entire genome every time a cell divides. This is so that each daughter cell will have a complete copy of the genome. However, for every base of DNA copied, there is a baseline error rate, or as suggested by exogenous' link, there can be re-arrangements of the DNA that lead to the daughter cells having a mutation not seen in the mother cell. So, for instance, if you looked very very closely at your cloned plant, and sequenced all its individual cells, they would all be slightly different from each other on a genetic level. We're a bit like this too, in that each cell division produces ~3 novel basepair substitutions. Most of these little mutations are meaningless and don't affect us or the cell at all, but others are of larger effect, and they can lead to cancer, or bitter apples.

BUT, here's were plants are weird and not like us. They can grow organs, like flowers or leaves, or limbs, from essentially any cell in the plant. So if a shoot buds off from a cell that has a long history of mutations, it may have accumulated enough re-arrangements to affect the taste or character of an apple which also buds off from this new shoot. Therefore, each apple's genetic pedigree is a bit different. If the mutations are dominant, or the flower selfs. (which apples don't seem to) you will see a different character to the fruit from others on the same tree.

This is an extremely non-conservative form of reproduction, and rather appalling to those of us that always think of mutations as deleterious or bad. But variety, or variation, to the apple can actually be a relatively good thing. Exploring all the variations available in your genome may actually help your offspring survive, as you, the parent tree, have no control over the environment your baby apples will sprout in. The mutations may be okay, or even better, in some other context.

Finally, just because I've gotten wound up on this, plants are the exception for allowing mutations to accumulate in the cells that will form their offspring. Most animals have something called "germline set aside" such that very early in the embryo (like 8 cell divisions, or synthesis rounds of the genome) the cells that will become the sperm or egg in the adult stop dividing and just go along for the ride of the rest of embryogenesis. This limits the number of errors that would manifest in the progeny of that animal.

And a more appleo-centric explanation here:
Sports do not arise that often, but are not difficult to find if you know what you are looking for. A sport usually occurs because of a genetic mutation or fault in a new shoot. As the shoot grows into a branch, all the leaves or fruit on that one branch will be noticeably different from the rest of the tree. The difference could be a more interesting colouration of the fruit, or the fruit on the sport branch may appear to be less affected by disease. If the sport characteristics are desireable the grower can propagate new trees from the sport branch, and these new trees will then be identical to the sport branch rather than the original tree. It is sometimes said that sports are genetically identical to the original variety, but this is debateable since the sport could be the result of a mutation or it could be the result of a different expression of the same genetic structure. Either way, a sport is far closer genetically to its original variety than a normal variety bred from parent varieties would be. It is probably best thought of as a failed clone rather than an offspring in the conventional sense.
posted by Cold Lurkey at 2:51 PM on November 23, 2011 [3 favorites]