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The self-pollination of tomatoes and its impact


We are confronted with questions about pollination very often. How does pollination exactly work for this or that plant or crop? Are there male and female plants? Does this plant need a pollinator? Of course, God forbid, the background to these favourite questions is not the genuine interest in the sex of the plants, but the concern for the fruit yield. Mostly, the answers to these questions are fortunately less complicated than the questions are intended. This is also the case with the tomato: the cultivated tomato, as we know it, is self-fertile. That’s a fact. Now we should all be relieved of our worries. But it's not so easy with the self-fertility of tomatoes...

Tomatoes are self-fertile

The flowers of the tomato are self-fertile, they satisfy themselves, they have a female receiving organ (pistil) and pollen sacs as male organs (stamen).




And the pollen of the same variety, even the same plant, more precisely the same flower, can easily pollinate this flower. For the tomatoes, a ban on inbreeding is missing. That what is forbidden with us humans and what does not work at all with many plants and is not allowed by nature, is what the tomatoes are continuously doing: they pollinate themselves and form fruits and offspring (seeds). The self-fertility of tomatoes has reasons and of course a story...

The architecture of the tomato flower and the impacts of pollination

The self-fertility of the tomato is due to the structure of the tomato flower. The pistil is initially hidden in the wreath of the anthers, and when it finally outgrows its prison and looks out of the stamen, it has usually already been pollinated by the pollen of the same flower.


Picture: A closed tomato flower


Picture: A tomato flower with removed petals and anthers and visible pistil


Picture: An overripe tomato flower with the pistil growing out of the stamen

It can be assumed that far more than 90% of the flowers of a tomato plant pollinate the plant itself, even if other tomato varieties are planted right next to it. Frequently one reads in the extensive and numerous amounts of articles about open-pollinated varieties and heirloom tomatoes (meaning old, traditional varieties) that the seeds produced by the plant will “breed true”. Well, this actually only means that no one intervened in the pollination process; but the plants most likely have been pollinated by themselves (i.e. not open and free). The pollen of other varieties – transported by the wind or insects – is (almost) always too late, unless you intervene in breeding and remove the anthers early, in order to artificially apply the pollen of a different variety.

The benefits of self-pollination

How can there be benefits you may ask? This is unnatural, which ultimately leads to inbreeding and less diversity…And yet the domesticated, cultivated tomato works exactly the same way: it pollinates itself, but fortunately has the property to show no or almost no inbreeding depression despite the inbreeding.

With each generation of selfing, the two chromosome strands of the tomato carry more and more identical information, and after about eight generations, one can then quite rightly speak of open-pollinated varieties. Open-pollinated means the following: the seeds of a tomato plant are (almost) identical and they are also comparable to the mother. The self-pollination of the tomato has immense advantages as a crop: once a beautiful and fertile plant with delicious fruits has been found and it is increasingly reproduced by selecting the most beautiful and best individuals for sowing, the variety will become more stable and therefore readily available.

The emergence of self-pollination

Of course, the questioners from above are also correct: self-pollination is rather a degeneration phenomenon in many crop plants or partly due to the intervention and the advantage of humans. In nature, self-pollination has its drawbacks: diversity gets smaller and there are fewer variations for adapting to changing environmental conditions...

One can perhaps imagine the rise of self-pollination, e.g. in the domestication of the tomato as follows: people always choose the most fertile plants to propagate further; and a spontaneous mutation that makes an originally self-infertile plant self-fertile, of course, leads to more fertile plants because they no longer rely on cross-pollination and the corresponding vectors (i.e. insects). And it is precisely these plants that are then selected and propagated by humans. Of course, in many cases, other (animal) fruit lovers follow the same logic: they prefer large and fertile individuals and spread their seeds.

Are vibrators needed for tomato sex?

Does tomato sex really work by itself? Well, at some point the stamens burst and the pollen spills over the pistil – and the matter is complete. Of course, natural movement (wind) helps, too. When growers then started to cultivate tomatoes in windless greenhouses, they no longer wanted to trust the natural tomato sex and used vibrators throughout the rows of tomatoes to make the stamens burst.

The most efficient vibrators, however, are the bumblebees that are now commonly used in tomato crops: when they cling to the tomato blossoms and suck the nectar, they vibrate so intensely that the stamens burst earlier and more intense than when there is just a light breeze or when gardeners stroke their hands over the plants. When growing tomatoes in your garden, however, you can look forward to visiting insects and bumblebees, but never forget to touch, caress and slightly move your tomato plants when walking by. Gardener love can help with tomato sex!

The impact of self-pollination in tomatoes

The self-pollination of tomatoes has very extensive impacts, which we briefly list here:

1. A reliable yield, as the tomato does not rely on pollinators and vectors and insects.
2. A variety can be grown authentically, as it does not need any pollinators. The diversity of the variety is desirable in the tomato garden and pleases the gardener, but is not necessary for pollination.
3. Conversely, you do not have to be afraid of outcrossing and mixing even in mixed plantings.
4. The seeds of an open-pollinated tomato variety (note: not the seeds of F1 hybrid varieties) can be sown again and yield identical progeny. Thus old tomato varieties, heirloom tomatoes, local varieties, even family varieties can be passed on from generation to generation and maintained.
5. Against the widespread rumour (old tomato varieties are good because it gives them more diversity) the self-fertility of the tomato rather leads to genetic impoverishment: if one were to leave the cultivars, especially the open-pollinated varieties, to themselves, nothing new would arise. They would always reproduce themselves, ending up at a genetic dead end. Only chance, which sometimes allows cross-pollination (remember the self-pollination rate above of more than 90%), leads to more diversity. And of course the human being, the ultimate sex worker with his/her breeder hands, who emasculates (fertilises) the flowers carefully and then pollinates a very different variety with the pollen.

Note: tomatillos are self-infertile!

The fact that it can also be quite different, that there are also crops with self-infertility, can be seen with the name related Tomatillos. They are largely self-infertile. So if you cultivate only one plant in your garden, you will not be able to harvest tomatillos.

Picture: Tomatillo 'Aurora' - the Tomatillo with the large, sweet and sour fruits

Here you must always plant at least two tomatillos next to each other, so that the pollination works and you can enjoy a reasonable harvest. Luckily, the tomatillos' self-fertilisation barrier is not quite as strict and rigidly regulated, as they accept the pollen of another plant of the same variety.
Is self-pollination good or bad?

Well, that depends on the perspective. In an evolutionary view of nature, this is rather bad because the chance of change, and thus the long-term adaptability gets smaller and smaller and more reliant on chance (and humans). From a human perspective, self-pollination is of course positive because it has made intuitive breeding and then the preservation of family and local varieties possible.

Oh, and we are only human.


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