Attack of the clones

You, my friend, have food déjà vu. That Granny Smith apple you are eating? You’ve eaten it before.

In fact, genetically, it’s the same exact fruit.

Each variety of apple you are familiar with, your Pink Ladies, your Jonagolds, is actually a single individual, cloned many many many times over. Domesticated apples are propagated through a technique called grafting, where a stem from one individual, which the farmer selects for fruit type, is attached to a rootstock from another individual, selected for being adapted to the local climate, pest resistant, or for compact size.  This mode of reproduction, where genetically identical copies of individuals are grown from fragments of the original, is more generally know as vegetative reproduction. Lots of species are able to do this trick, including lots of invasive species – one reason why mowing or mechanically chopping up your weed clogged lake can be a very bad idea.

If you were to grow an apple tree from the seeds of a fruit you enjoyed, assuming it grew at all, it wouldn’t produce similar fruit for you. This is because apples are obligate out-crossers, which means they can only sexually produce with individuals that are different than they are. (Many plant species are self fertile – that is, an individual’s pollen can fertilize their own ovules, producing offspring where a single plant is the seed mother and father.) Because they must out-cross, they won’t “breed true,” and the differences between generations and between siblings is more dramatic than most plants. Also, many varieties are triploid, that is, a hiccup during sexual reproduction leaves them with an extra copy of genes from their mother, making it harder for them to reproduce from seed. For these individuals/varieties, grafting is pretty much the only way they can reproduce. Additionally, they are largely insect-pollinated, so it’s very hard to control where the pollen came from.

New individuals, with tasty new fruits or other useful properties, historically arose by accident, and were often named after the farmer who found them. You just never know what you’re going to get with sexual reproduction.

5 thoughts on “Attack of the clones

  1. Are all triploid plants that require outcrossing a reproductive dead end? How often would this mutation arise in nature?

    • Triploid individuals should be able to cross with other triploid individuals without a problem. But in a population full of diploids how likely are our two chromosome-crossed lovers to meet? If they do meet, however, they can found a whole new species. Since they are not able to breed with the diploids of the parental population, they are instantly reproductively isolated; by the biological species concept, they are separate species.

      This seems to happen in nature more than you might think: in plants 30 – 80% of plant species are considered polyploids. Meiosis is a messy business.

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