gender placement in flowering plants

" What could be the reason for this variation?"

Better ask the maker.........

tom 🕊 , I think you could probably write a book on that topic, I'm sure there are a few out there. Search "Breeding strategies in flowering plants"

My take on it is that optimally a plant wants to have sex with a different individual in a population in the same way we don't generally want to have children with our siblings. There's a reason for that, we all know that if you do that its possible that any faulty genes we carry will be uncovered as we share those genes with our family. Close family members are likely to have the same or similar sets of genes. We carry two copies of a gene for a reason to compensate for any defective ones we might have. Mate with your sister and those will be exposed and can de deleterious. One individual is going to have exactly the same genes.

However plants are sort of stuck, literally stuck in the ground so they can't go running around and choose a mate. So flowering plants have come up with a neat strategy to get around that problem.

Flying insects.

The trouble with that is that now they have to rely on another organism and there are some large problems associated with that. For a start those insects might not be too interested in doing that, answer nectar and flowers. On top of that they might not be so keen on neatly transferring pollen from one sex of flower to another in the correct way.

So there are a few strategies that they have come up to deal with that. One is to have separate individuals of each sex (like humans). Sounds good but remember they are plants so are stuck so what happens when there are no other stuck plants of a different sex nearby. So that's a potential problem but maybe more of a problem for certain species in certain areas more of the time than others. So definitely a solution.

If that's a problem what about having male and female on the same plant, that way you avoid the having to find a partner but you still face the mating with yourself problem but its sort of dual purpose. Many plants have what's called self incompatibility so that the pollen from the same individual can't fertilise the same individual. In this system you can try and get insects to transfer pollen and get a nice out crossing but if things don't work out maybe you can get by with mating with yourself and so produce seeds to carry on but still retain the possibility to cross fertilise in the next generation. So you can use it to get past a difficult year or difficult few years. Perhaps a local volcano erupted and made the local environment unusually cold for the time your flowers were open so no insects but you might still produce seeds or just a very cold spring.

Having both sexes on the same plant you can go either seperate flowers or both sexes in the same flower. Separate flowers probably increases your chances of out crossing but you can't self pollinate. Both sexes same flower you can.

Bottom line as a plant or as a human you want to have sex involving exchanging genes so that you can effectively adapt for the future. The environment is always going to change and if there's no variation there's no way that you can adapt to changing circumstances. We sort of have a perfect example right now, the virus we have affects people differently, many people have no symptoms, a few bad symptoms and a few die. You can be assured that's partly at least down to genetic differences in the individuals. If we all were the same genetically we might have the misfortune to have a set of genes that were susceptible and everyone dies.

So I think there are multiple strategies that have evolved. We play a role as well. I know most tomatoes now are self pollinating but wild types aren't. The reason is we've selected plants that are self pollinating as we tend to want the same thing as we planted the previous year so we are always going to select the same as we expect and in the long run that favours self pollination.

Then there are the parthenocarpic plants, like Diva cucumber, which produce seedless fruits without pollination.

Gender is nothing but a social construct, Tom. Get with the program.