Reproductive isolation between taxa is typically thought to increase with genetic divergence, until a point at which complete reproductive isolation is achieved. The time until complete reproductive isolation is reached varies among taxa, with values ranging from 4 to 34 million years ago.

Recently, Rothfel et. al. found a fern species in the French Pyrenees that was intermediate in morphology to two distantly related fern genera (Cystoperis and Gymnocarpium). When they sequenced the single-copy nuclear marker GapCp “short” in the putative hybrid (Cystocarpium roskamianum) they found four distinct alleles. Two were closely related to sequences from taxa in the Gymnocarpium: Gymnocarpium appalachianum and Gymnocarpium disjunctum, implicating the allotetraploid Gymnocarpium dryopteris as one parent of Cystocarpium roskamianum. The other two alleles were closely related to sequences from the Cystopteris fragilis species complex. Using plastid sequences the Rothfel et. al. then determined that Gymnocarpium and Cystopteris diverged from one another approximately 60 million years ago.

The formation of Cystocarpium is therefore the deepest natural hybridization yet documented. One potential reason the authors give for incomplete prezygotic isolation, despite such deep divergence, is that ferns have abiotically mediated fertilization. This finding could help to explain the low diversity of non-flowering land plants relative to angiosperms.

--Karen Barnard-Kubow

Rothfel et al. Natural hybridization between genera that diverged from each other approximately 60 million years ago. Am. Nat. 185, 433–442. (2015)

Photo Credit: Harry C. Roskam