>>20102725I believe they've just been cultivated for different traits and are a distinct cultivar. The Japanese watermelons I've had were a bit smaller, and their rinds and pith were thinner than I've experienced in the US. I wouldn't be surprised if they just selected for all of those features over time together. Commercial fruit development is an ongoing and serious industry, with a famous Japanese grape cultivar being pretty new, developed only in the 80's or 90's iirc. There's a new apple cultivar that's being heavily marketed in North America called Cosmic Crisp.
Related, "aesthetically perfect" fruit in Japan are a whole category of product. A decade ago, I passed though an area in Hokkaido whose regional claim to fame seemed to be cantaloupes. The luxury, perfectly round, perfect smelling, perfect-whatever-aspect omiyage cantaloupes were being sold for 5000 yen each.
