Ferns

Many species of ferns form hybrids in nature and hybridization is believed to have had a major role in fern evolution. A hybrid species is the offspring of a sexual union between two different species. Most hybrids cannot engage in sexual reproduction because they lack homologous (corresponding) chromosomes, which typically pair off during production of sperm and egg cells. However, since many fern species can engage in apogamy and apospory, fern hybrids can often reproduce and proliferate.

A hybrid species is often identified by the number of chromosomes in its cells and by the presence of aborted spores. Chromosome number is often used to infer evolutionary relationships of hybrid ferns. The ferns, as a group, tend to have very high chromosome numbers due to the widespread occurrence of polyploidy. One fern species, Ophioglossum reticulatum, has 631 chromosomes, the largest number of any organism.