If you Google search Guppylog, you will see that that question has been raised here quite often. So you are in good company. :)

However you will also also find a lot of reference to that question. In 100 years in the hobby, to my knowledge (obviously limited), that cross hasn't happened. Guppies and platys are pretty good tank mates (so are Corys and guppies) but that doesn't mean a cross will happen. :)

Most of the livebearers we keep in the aquarium hobby are members of the family Poecilidae. The Poeciliid species are distinguished partly on the shape of the male organ, the gonopodium. (DNA analysis is also a hot topic now. We should hear about the Poeciliids soon.)

The Guppy gonopodium is designed to fit only the genital pore of the female guppies. Their opening is designed to fit the gonopodiums of male guppies. Platys are designed to fit platys.

Though it is not the whole story, one significant factor separating Poeciliid species is that they are "reproductively isolated." They can not interbreed with each other. In aquariums, there are a disconcerting number of crosses of closely related fish species which would not otherwise encounter each other in the wild (and therefore never needed to be reproductively isolated). But usually these are between fairly closely related species.

Even the closely relate swordtails and platys rarely if ever interbreed in the wild. Even in the same river systems, adults of their species live in different habitats and females respond to different courting behaviors.

Going back to the 1930s, researchers like Dr. Myron Gordon, tried to cross different members of the genus Xiphophorus and although some of the crosses were difficult, in time the platy (X. maculatus) was crossed with the swordtail (X. variatus) and the variatus (X. variatus).

I was surprised to read that not all crossings were successful. Initially they were very difficult. Sometimes the eggs were not fertilized. Sometimes the embryos died - often of cancer. Indeed scientists learned that if certain strains were crossed, adult fish would be the result, but they would quite consistently develop cancers. And that was what that whole operation was about, trying to understand about various cancers so that some day cures could be developed.

Not surprisingly, researchers have suggested that the gonopodiums (gonopodia?) of the different (30 or so) Xiphophorus species are different one from another, but more alike than they are to other 270 Poeciliids.

Guppies and mollies are more closely related (though how close is being challenged). Once in a great while (every 100,000? million? billion?) matings a cross is produced.

That also can happen with a horse and donkey. A mule, which is sterile, is produced.

In laboratory controlled experiments with killifishes, many crosses don't take place or die in the embryo. Among the crosses that have been realized, again many are sterile or mules.

Josh (see his Gambusia and guppies down the page) has been considering crosses from about a year. Recently he considered crossing hobby strains of mollies, something which is quite possible. Then he drops the do-able crosses and keeps wanting to cross much less closely relate fish. I know that I must sound like a grinch, suggesting that he might be more successful crossing a cat and a horse.

The idea of developing a new or better strain of a fish is very attractive to many aquarists. In controlled experiments, I have no problem with attempting to cross species to see if it can be done. For most of us around home it is a huge waste of time and money.

If we want to improve upon certain fish, see how much bigger your deltatail guppies can be. See how straight the lines of a deltatail can be. See how bright and vibrant the colors of a platy strain can be. There are literally hundreds of hobby strains of guppies and other hobby livebearers. Some are difficult enough to reproduce and grow up as spectacular as their parents. Do a Google image search for platys and guppies. Look for what you might like to keep and breed. Look to see what over generations you could even improve.

There are so many thing you can do with fish. Who knows, maybe in a laboratory an artificial insemination of a cross will work. Maybe not. But those are sometimes nearly impossible to do at home. :)