Scientists report an extreme case of two very unrelated ferns species capable of interbreeding.
Recent research into the fern ×Cystocarpium roskamianum conducted by scientists from Duke and the University of British Colombia has big implications for the study of how species form.
A species is commonly defined as a group of organisms which can interbreed and produce fertile offspring. Hybridizations are the exception that proves this rule, rare cases in which two species successfully breed and produce a viable, albeit usually infertile, offspring. You’ve almost certainly heard of a few, including the liger (hybrid of a lion and a tiger) and mule (hybrid of a horse and a donkey). Hybrids are rare for a number of reasons. Most species are genetically incompatible. Imagine trying to merge the genomes of a house-cat and an oak tree. Even for species which are compatible, other factors interfere. Two species may not live in the same habitat, or may have different mating behaviors, or may mate at different times of the year. Because of these barriers, hybrids are almost exclusively from species that share a recent common ancestor. Horses and donkeys split less than 5 million years ago. Lions and tigers have a common ancestor less than 10 million years ago. Even the most extreme hybrids (bred under artificial laboratory conditions) diverge less that 40 million years ago.
The fern ×Cystocarpium roskamianum shatters these hybridization records. Discovered in the French Pyrenees, it shares the physical characteristics of ferns from the genera Cystopteris and Gymnocarpium, which diverged around 60 million years ago. In terms of time, this is like a hybrid between an elephant and a manatee, or a human and a lemur.
The researchers used genetic sequencing to confirm ×Cystocarpium’s heritage. It is a hybrid of Gymnocarpium dryopteris, the common oak fern and Cystopteris fragilis, the common fragile fern.
The confirmation of ×Cystocarpium’s origins has several important consequences. First, it sets a new upper boundary for the time taken to form natural reproductive boundaries between two species. There are several other possible contenders for the title, mostly plants- four ferns, a conifer, and a clubmoss, along with a game bird. While future research may find a more extreme natural hybrid than ×Cystocarpium, it has demonstrated that the possible range of times to form reproductive boundaries is much larger than previously believed.
Secondly, ×Cystocarpium suggests a reason for patterns in plant biodiversity. While there are over 250,000 species of flowering plants, there are only about 10,000 fern species, a pattern matched by other nonflowering plants (for example, the 12,000 species of moss). It is possible that ferns speciate so slowly because they naturally hybridize frequently and well. This makes them slower to genetically separate.