Botany online 1996-2004. No further update, only historical document of botanical science!
Several possibilities getting a look-in in different stages of development or in different generations exist.
Mortality of hybrids, weakness of hybrids. A mortality or weakness of hybrids can, for example, be based on either disharmonies of the nuclear and the plastid genome or on disharmonies between two nuclear genomes. Embryogeny can also be prevented, if the endosperm does not develop and the zygote is thus not supplied with nutriments, even though the genomes of pollen and egg may seem compatible at first.
Developmental sterility of the hybrids. The plants develop normally, produce flowers, but meiosis ceases before completion. Sterile pollen and /or sterile eggs result.
Segregation sterility of the hybrids. The hybrids are sterile, because whole chromosomes, chromosomal segments or gene complexes distribute unevenly onto the gametes.
Sterility or weakness of the F2-generation. The F1-hybrids are normal, healthy, and fertile, but the F2-generation contains many weak or sterile individuals.