What are the benefits of self-pollination over cross-pollination and vice versa? I'd think that cross-pollination creates more genetic diversity, but then self-pollination must have some benefits too, or it wouldn't be around.

Thank you.

You're right about cross-pollination, which exists for the same reasons that sexual reproduction exists in most multicellular organisms. Mixing genes with another individual is a good idea to make sure your offspring are well adapted to deal with the environment and to cope with new parasites or diseases.

Given this, self-pollination seems like a backwards step for plants. There are however advantages to being able to reproduce asexually (i.e. to make clones), especially for an organism that can't move. Even if they never attract a pollinator (such as an insect), they can still produce offspring, and their offspring should be well-adapted to the local environment because the mother plant is growing fine by itself.

Lots of plants reproduce asexually without flowers - by vegetative splitting, often from the roots, but sometimes from leaves or branches, such as spider plants. Plants like lilies produce multiple bulbs that turn into new plants the next year. The trouble is that these clones can't get very far. Self-pollination can be seen as a bit of reverse engineering, adding a short-cut to the flowering cycle. It seems like a waste of time to split the genome then stick it all back together again, but they end up producing seeds that can disperse long distances or might survive in the soil long after the mother plant has died. They get the advantages of flowering without needing pollen from elsewhere.

So which is better? The answer is that it depends. Take, for example, the humble dandelion. In the British Isles (and all of North America, where it isn't native), dandelions are mostly asexual - they pollinate themselves. This means that they can spread quickly without wasting time and they become excellent weeds, taking advantage of the short growing season. Further south in the Mediterranean, they reproduce mostly sexually. There the growing season is longer, there are many more species, and the level of competition they face is much greater.  There it's an advantage to keep mixing genes and evolving to stay ahead of the intense competition.

I'm sorry for the long answer, but this is a tricky question, and not one to which we have all the answers! Reproduction in plants is a lot more complicated than in animals, which is why I think it's so much more interesting.

Last edited by Markus Eichhorn (7th Sep 2007 13:51:39)