Although it is not intrinsic in the formal definition it is a matter of common observation that members of different cytodemes are essentially cross-incompatible and any hybrids that do arise are usually highly infertile.
[3] Perhaps the most extreme case on record concerns the wild grass, teosinte, Euchlaena mexicana and the strikingly different Maize, or Indian Corn, Zea mays, both 2n=20, fully interfertile and yielding fertile hybrids.
[4] There is perhaps a remote possibility that two such very different species could have evolved independently from distinct sources and converged in their chromosomal ideotype until they became members of the same cytodeme (including the capacity to cross-breed).
It is more likely, though, that the cytodeme arose first complete with its suite of chromosomes and breeding patterns all intact and then, remaining constant in its fundamentals, it diversified into species sometimes so different as to merit generic distinction.
Doubling the chromosome number of the sterile hybrid between two diploid cytodemes yields an allotetraploid which is both fertile within its bounds and essentially incompatible with all previous life forms.