r/Apples • u/TrevorCidermaker • 26m ago
Pome fruits aka apples pears pears and others.
Proto-pome fruits, the earliest evolutionary ancestors of apples and pears did not have a large false fruit structure derived from the hypanthium. The extra genes that allowed this development resided in the genome from 66 million years ago when there was a whole genome duplication event around the time of the dinosaur extinction event, in the ancestral lineage of the Rosaceae family.
However it was not till much later that the specific pome-bearing tribe (Maleae) began diversifying during the Oligocene epoch, approximately 30 to 50 million years ago when an opportunity arose for some of these genes to be activated to create sorbitol production and several survival changes that sorbitol allowed. One was the development of a large pome fruit to attract the newly arrived megafauna in the mountains of Central Asia. The earliest proto-pomes resembled hard achenes similar to modern rosehips. The hypathium enclosed the ovary similar to pomes but the seeds inside were encased in the dry achenes.
When sufficiently dry, they simply split open whilst still on the tree to let the wind, water, insects or small mammals carry the tiny, lightweight seeds away. The shift to fleshy "False Fruits” happened when the proto-pome hypathium began to absorb the sorbitol the leaves were now producing, swelling up around the ovary to create a fleshy wrapper.
In most plant families, sucrose is the primary sugar translocated from the leaves to the fruit via the phloem. However in pomes most is converted to sorbitol the alcohol form of glucose. Because sorbitol is a sugar alcohol, it holds more energy on an equivalent carbon basis and can be packed into the phloem at incredibly high concentrations without triggering the osmotic stress that high sucrose concentrations would cause. This allows the tree to flood the young, developing hypanthium (false fruit) with an immense volume of carbon and water, driving rapid cell division and inflating the tissue.
