I have a good understanding of how seedless watermelons are made. Theoreticaly it is impossible for black viable seeds to form in a seedless watermelon, yet on occasion they do form and they are viable. It irritated me not knowing how this is possible.
I searched the internet to find out how it was possible for a seedless watermelon to produce black viable seeds. Strangely enough I couldn't find any explanation.
I spoke to/emailed some plant nerds and a certain professor at a university who asked I didn't name him (and I won't - wink) and I now understand how seedless watermelons can produce black viable seeds.
If you would like to read a simplified version of how seedless watermelons can produce viable seeds then keep reading.
If you are a plant geneticist and would like to elaborate on any points that I have simplified too much, or anywhere that I have strayed a bit too far and confused things, then feel free to leave a comment!
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| Watermelon grown from a viable seed from a seedless watermelon |
First you need a basic understanding of how seedless watermelons were made. To understand how seedless watermelons are made you need to understand a little about ploidy.
Haploid
Haploid refers to the number of chromosomes in a gamete. This is usually half the number of chromosomes that are carried in a normal cell. Pollen, for example, is haploid.
Diploid
People (and most animals) are diploid, meaning we have two sets of chromosomes in each cell. We get one set from our mother, and one set from our father. This is simple. This is how people can carry recessive genes for things and pass them onto their children.
Many watermelons are also diploid, meaning they have two sets of chromosomes in each cell. In watermelons n = 11, in other words a diploid watermelon will have 22 chromosomes in each cell (two sets of 11 chromosomes).
When haploid pollen is produced in a diploid the chromosomes need to pair up, in other words we need to halve the number of chromosomes. 1/2 of 22 = 11, this is a whole number so the pollen is viable.
If a diploid watermelon gets pollinated by a diploid watermelon then the resultant seeds will grow into diploid watermelons. They will get one set of chromosomes from the mother and one set of chromosomes from the father.
Tetraploid
Years ago people discovered how to induce polyploidy in plants. Polyploidy means they are carrying more than two homologous sets of chromosomes. After we learned to induce polyploidy in plants some tetraploid watermelons were produced.
Tetraploids carry four sets of chromosomes, they get two sets from the mother and two sets from the father. Tetraploid watermelons have 44 chromosomes in each cell (four sets of 11 chromosomes).
When haploid pollen is produced in a tetraploid the chromosomes need to pair up, in other words we need to halve the number of chromosomes. 1/2 of 44 = 22, this is a whole number so the pollen is viable.
If a tetraploid watermelon gets pollinated by a tetraploid watermelon then the resultant seeds will grow into tetraploid watermelons. They will get two sets of chromosomes from the mother and two sets of chromosomes from the father.
Triploid
This is where things start to get tricky. Triploid watermelons have three sets of chromosomes. Triploid watermelons are created by crossing a diploid with a tetraploid.
Triploids get one set of chromosomes from the diploid father and two sets of chromosomes from the tetraploid mother. Triploid watermelons have 33 chromosomes (three sets of 11 chromosomes).
In animals this is often lethal, but in plants it usually isn't. If you plant these triploid seeds they grow and flower but can not normally produce viable pollen.
When haploid pollen is produced in a triploid the chromosomes need to pair up, in other words we need to halve the number of chromosomes. 1/2 of 33 is not an even number, so the chromosomes can't pair, which usually renders the pollen infertile.
Watermelon fruit will abort if the flower is not pollinated. For this reason a triploid watermelon needs to be pollinated by a diploid watermelon to produce fruit. As the triploid has an uneven number of chromosomes the seeds usually abort early, leaving you with those white empty non-viable seeds. Thus a seedless watermelon. This was an amazing breakthrough when it was first achieved by the University of Kyoto.
If you ever grow seedless watermelons at home (why would you bother, they are more bland than seeded varieties) you will have two packets of seeds. You will have seeds of the triploid watermelons that will bear seedless fruit, and a pollinator variety which will be a diploid.
How does a seedless watermelon ever produce viable black seeds
Now that you have a basic understanding of how a seedless watermelon is produced we can get back to the initial question of how do seedless watermelons produce black seeds. There are a few scenarios that can happen, one is far more common than the others.
The most common situation is where during meiosis (specifically the prophase 1’s third ‘pachytene’ pairing stage of cell division) a triploid cell does not have an even number of chromosomes to pair properly. Remember the 1/2 of 33 thing from earlier, it just doesn't work.
During that stage, under some conditions, the triploid cell will double making it hexaploid with six sets of chromosomes instead of the three a triploid would normally have had. Hexaploid have 66 chromosomes (six sets of 11 chromosomes).
When haploid pollen is produced in a hexaploid the chromosomes need to pair up, 1/2 of 66 = 33, this is a whole number so the gamete is viable.
This hexaploid embryo carries three sets of chromosomes in its haploid state. If it is pollinated by diploid pollen (which in its haploid state carries one set of chromosomes) the seed produced is viable! In this case the seed would receive one set of chromosomes from the (diploid) father, and three sets of chromosomes from the (~partially hexaploid) mother.
If you were to grow this seed it would be tetraploid plant, carrying four sets of chromosomes. This tetraploid then grows just like any other tetraploid.
If I grow out this tetraploid seed and it self-pollinates (or is otherwise pollinated with tetraploid pollen) the seeds will grow into tetraploid plants with seeded fruit. Just like any other tetraploid, seeds would gain two sets of chromosomes from the mother and two sets of chromosomes from the father.
If I grow this tetraploid and it is pollinated with a regular diploid then all the seedlings will be triploids which would then be able to be seedless.
Some other scenarios
There is a far less common scenario where during meiosis a set of chromosomes is mysteriously lost. This happens, but not very often. When this happens the cell essentially becomes diploid which can then produce a haploid gamete. If self-pollinated, or if pollinated by another diploid, the resultant fruit will have seeds and the plants will remain diploid. This is extremely rare, so you would need to grow out a lot of seeds in order to have a chance of seeing this happen.
Another thing I have been told of is spontaneous mutations. There are various mutations that could cause a triploid to produce seed, these would be very rare and are not really discussed in literature so anything I could say on them would be largely guess work on my part. Based on their rarity, I don't expect to ever see one of these in my life.
Seedless watermelons - why do people sell them
While they don't bother me, many people dislike watermelon seeds. For some reason there is an old myth that you can't eat watermelon seeds or they will hurt your stomach. I just eat the seeds along with the flesh. They are small so easy to swallow, they have never hurt my stomach. Strangely enough customers disliking the seeds is not the main reason that you can only find seedless in the stores.
Triploid (seedless) watermelons are not all that round, they are more angular with flatter sides so tend not to roll as easily. Diploid watermelons are often rather round (or oblong or whatever other shape you want to call them, the point I am trying to make is that they roll easily) and have a bad habit of rolling. Losses due to rolling off a counter and smashing have been reduced now that triploid watermelons are sold. This is good for profits.
More importantly than that, triploid watermelons can be stored for many months more than diploid watermelons. They can be sent across the country and stored in a warehouse for months before needing to be sold. This is good for business and great for profits. It also means that customers can get 'fresh' watermelon out of season.
What now
I am now collecting viable seeds from seedless watermelon that in the hopes of growing out a tetraploid watermelon. I have never eaten a tetraploid watermelon before so am curious to see if it is as bland as the triploid ones or if it tastes as good as an old fashioned diploid.
I am also curious to see if tetraploid fruit is round like a diploid or more angular like a triploid. Or if it is as simple to grow as a diploid, or if it has issues like triploids tend to have.
While I am curious, I won’t be testing the keeping ability to see if it lasts for many months like a triploid.
If I grow one out to maturity and get to eat the fruit I will try to write a post comparing its growth, yield and taste to a diploid watermelons.
Do you have any viable seeds? You could also use colchine, or something similar.
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