Is the flesh of a fruit selected for the benefit of the sprouting seed or to attract the frugivorous seed disperser animal?

Question

In a question about the low protein content in fruits, one answer suggests the energy, carbohydrate, and protein content in a fruit evolved to entice animal seed dispersers, not foster the sprouting seed.

Another answer suggests plants are made mostly of cellulose, hence why the fruit has more carbohydrates than protein. The implication here is that the content of the fruit is optimized for the benefit of the sprouting seed, which is constructing a plant, not an animal.

Both of the answers to this other question: Plant a whole fruit or only seeds? suggest fruit is all about dispersal.

Which is the correct explanation?

It seems like it would be an easy answer to prove experimentally: plant two seeds in nutrient-poor soil, one with its fruit whole, another with its fruit mostly eaten (or even entirely passed through an animal's digestive tract). Then see which seeds thrive better.

Obviously fruits cultivated for human consumption have had their evolutionary course commandeered for benefit of the frugivore. And to a lesser extent probably many plants have a symbiotic relationship with their seed dispersers. But is that relationship an enhancement of the primary evolutionary purpose, or an suppression?

Answer 1

While I know you didn't mean this, it's worth clarifying there is no purpose in evolution, the fruit is what it is because that's how the various evolutionary processes such as natural selection and random drift have shaped it.

That said there isn't a known evolutionary mechanism for a fruit evolving in any other way than to benefit itself. The only way I can think of in which the second option you suggest, that the plant does it explicitly to help the animal survive could be true, is if the animal surviving helps the plant in some way. For example it is well known that some trees produce sugar water for ants to eat, and in exchange, the ants will attack herbivores that try to eat the plant.

Otherwise, the sugar is there to help the tree's offspring, but not how you think. The plant's baby doesn't literally use the sugar in the fruit for itself. Instead the plant offers the sugar to entice an animal to eat the seed. The seed will then be deposited by the animal in pile of undigested nutrients in a place far away from the parent plant. If the plant just dropped the seed below itself, it would have to compete with it's own child for sunlight, water, etc. So instead the fruit offers the sugar to an animal to get the seed dispersed. In this way, the plant must offer a benefit to an animal for it's own selfish interest. But the seed itself will only use the nutrients on the inside of the shell casing, even if the fruit is never eaten and still surrounds the seed.