The other day there was an interesting discussion about watermelon genetics that started on the Alan Bishop Homegrown Goodness plant breeding forum from a fellow who lives in Australia. Turns out Watermelon genetics are sort-of complicated, but interesting.
The discussion started by asking about which traits in watermelon were dominant, mostly referring to flesh color but also open to other traits as well. The original poster mentioned that he started his own mass cross of over 30 watermelon varieties together (a grex) in preparation to developing his own landrace adapted watermelon to his Australian climate. He said this past season he planted only the seed for any F1 hybrids from any yellow fleshed watermelons he had but got about 90% red fleshed watermelons and concluded that obviously red-fleshed watermelons were dominant. The interesting thing is they are BOTH dominant AND recessive at the same time! Yes, watermelon genetics is a little complicated to say the least, lol.
Wait… what??!… haha yes, you did read that last sentence correctly. Red-fleshed watermelons are both dominant to yellow-fleshed watermelons AND recessive to yellow-fleshed watermelons. Turns out there are actually TWO different kinds of yellow-fleshed watermelons.
Watermelon Flesh colors range from various forms of red, pink, yellow, orange, and white. So how does one figure out what is recessive and/or dominant over what? Turns out most of these have already been studied and we can interpret that data. I’ve recently resurrected my old website domain and turned it into a plant breeding wiki of sorts. Feel free to check it out @ www.biolumo.com. The main resource i am using is the wonderful watermelon genetics info posted online by the hosted by North Carolina State University and in particular Todd C. Wehner part of the Department of Horticultural Science at North Carolina State University. http://cuke.hort.ncsu.edu/cgc/cgcgenes/wmgenes/gene12wmelon.html
From the data available we can come up with a rough basic pictorial based diagram. I like pictures; they help me understand things better. Basically there are at least two types of red-fleshed known as “Scarlet Red” and “Coral Red” in addition to two forms of yellow-fleshed known as “Canary Yellow” and “Salmon Yellow”. Turns out Canary Yellow is dominant to all other forms of color. Scarlet Red is dominant to Coral Red, Orange, and Salmon yellow. Coral Red is dominant to Orange and Salmon Yellow. Orange is dominant to Salmon Yellow. You get the point. And basically seems to work in a cascading effect of “more color” to “less color”.
I personally prefer the taste of the Canary Yellows over most red/pink, though there are still some good red ones out there! What i don’t like are the Salmon Yellows (and maybe orange). To me and in my climate the Salmon-Yellow watermelons have a weird mealy and/or mushy texture and have a muted / poor flavor. By contrast the Canary Yellows seem to be really sweet and might even get sweeter more easily / earlier in a northern and colder climate like mine. That is just my personal preference, your taste buds and soil conditions may differ.
Now this is a general simplified version. There are a few caveats however. Such as the fact that there is a Canary Yellow inhibitor gene that when present will turn a Canary Yellow back into a red that is hiding underneath. Also the fact that there may be a few other minor colors that have not been studied yet such as “dark red“, “rose” , and “pink“. It is possible that these colors are just minor variations of the former reds and function the same way from “more color” to “less color” in terms of dominance. It is also possible that if these are indeed separate shades of color that they may buck this trend and function in completely different ways from different biochemical pathways. Hard to say at this point. But i will leave the possibility open either way in case new studies in the future address these watermelon flesh colors.
Oh, and what about white-flesh?! Yes that’s right, we have completely forgotten to talk about white fleshed watermelons. Oh, you didn’t know there were white-fleshed watermelons? Yeah there are. They are not generally as common but there are white fleshed watermelons out there. Turns out white-fleshed is a little more weird. Let me explain.
White-fleshed watermelons are currently being studied more in depth in China and a new paper is due any time in the near future. But until then all we have is the data gathered already from a past study on it. According to that study: white-flesh were found to be dominant over all color. In an F2 (Second Filial) Generation the ratio is: (12 white : 3 canary-yellow : 1 red).
Pretty interesting huh? Yeah, basically if i interpret this information correctly is that for whatever reason white flesh overrides color. In the wild, watermelons were originally thought to be white fleshed and low in sweetness. This is certainly the case in the wild citron melon (Citrullus lanatus var. citroides) which has hard white flesh and bland flavor. The bitter apple melon too (Citrullus colocynthis) but it obviously is very bitter.
The genetics for watermelon at this point captured my interest so i decided to find out what i could about seed coat colors. If you thought watermelon flesh genetics was complicated, you’ll find the genetics for watermelon seeds is a nightmare. Nevertheless i waded knee deep into the confusing data and came up with some generic info that i think can give us a basic trend that we can use.
The genetics for watermelon seed colors and patterns is a nightmare. Truly it is. Partly because the studies we have don’t all agree and we don’t have examples of what these old researchers were really studying. One person’s “tan” might be another persons “light brown”, etc. You get the point. Very subjective. But based on the studies we have it basically looks like in general there are three genes working together and we can come up with a basic trend that we can follow.
Basically black seeds are dominant to other colors. Brownish or greyish seeds with a particular black mottling striping with black dots is next in line. Tan or brown seeds are probably next in line. Green seeds (not pictured here and rare) are dominant over red. Red seeds are the most recessive except for white. White seeds are the most recessive and recessive for all three gene combinations. This is a very simplified interpretation and there are probably actually more than three genes. In my population i have grey seeds which is not a color that has been studied. Also i have no idea what “tan” actually is so i lumped it in with brown. Brown too has not been studied, nor has “reddish-brown” among others.
Watermelon Fruit shape is relatively simple however. Yay! Simple co-dominance at work. Two long genes (OO) give you long fruit. One long gene and one round gene (Oo) or heterozygous gene pairs give you medium oval shaped fruit. And two copies of the other round gene (oo) gives you round spherical fruit. Easy peasy!
Golden-rind fruit are easy genetics too. Simple recessive (go). This is a trait more common now as it helps people identify when a watermelon is ripe. They turn bright yellow when ripe.
And the last trait i will mention is the “explosive rind” trait.
Haha, it’s not as scary as it sounds, but it’s not particularly a trait you want in your watermelons. Fortunately it is recessive and hopefully you wont encounter it in many varieties. I’ve seen it in the unusual striped variety but fantastic tasting ‘Osh Kirgizia’ watermelon, but otherwise not that much. Officially explosive rind (e) causes the fruit rind to burst or split when cut. This is true, but i also find that often when this trait is present the fruits themselves have a higher rate of splitting open while ripening on the ground and even when you lightly grab one to harvest. Not a trait that a market grower would want. For a small backyard gardener it’s not a huge deal as you can eat them right away, but still a slight inconvenience, especially if they split in the field and ants get to them. Black ants really do love sweet watermelon flesh.