Just like an elephant and a manatee …

© evolgen at scienceblogs.com

© evolgen at scienceblogs.com

There is a positive correlation between the time since two lineages have diverged and the strength of the reproductive barriers between them.
Rothfels et al. (2015) have described a natural hybridization event between two fern genera that diverged from one another approximately 60 million years ago. This is an incredibly deep hybridization event,

roughly akin to an elephant hybridizing with a manatee or a human with a lemur.

In animals and flowering plants, reproductive barriers evolve quickly, such that species are incompatible after several millions of years. Yet, a fern found in the French Pyrenees was morphologically intermediate between two genera that were even placed in different families (Rothfels et al. 2012, 2015).
The fern, now called xCystocarpium roskamianum is infertile, but grows

vigorously via rhizome growth and does well in cultivation.

This new study

provides a new upper limit for the length of time it may take before reproductive barriers are complete, in this case, a cumulative total of approximately 120 million years of independent evolution (60 million years for each parent lineage).

This is the deepest natural hybridization event yet documented in plants or animals. In other words, both pre-zygotic and post-zygotic barriers have remained incomplete for a really long time!
Ferns have free-living haploid and diploid stages, both of which are multicellular. They may have

greater developmental robustness to variations of dosage and gene-gene interactions. … Particularly useful would be a series of laboratory crosses of ferns and other nonflowering plants to investigate the strength of viability isolation across a range of evolutionary depths in these groups.

What is really interesting is that the pattern of slower evolution of reproductive isolation in groups that are characterized by abiotic gamete dispersal (e.g., wind or water) implicates the operation of selection at levels other than the individual.

One reason we live in a world with more than 250,000 species of flowering plants but only around 10,000 fern species (and approximately 1,000 gymnosperms, 1,200 lycophytes, 12,000 mosses, 9,000 liverworts, and 100 hornworts) may just be that populations of non-flowering lineages take longer to achieve complete genetic separation from one another because the have fewer mechanisms to prevent the sperm of one species from encountering the egg of another.

Rothfels et al. (2012) A revised family-level classification for eupolypod II ferns (Polypodiidae: Polypodiales). Taxon 61:515–533.
Rothfels et al. (2015) Natural Hybridization between Genera That Diverged from Each Other Approximately 60 Million Years Ago. Am Nat 185: 433-442.

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