Natural Hybridization between Genera That Diverged from Each Other Approximately 60 Million Years Ago

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. abstract: A fern from the French Pyrenees-# Cystocarpium roskamianum-is a recently formed intergeneric hybrid between parental lineages that diverged from each other approximately 60 million years ago (mya; 95% highest posterior density: 40.2-76.2 mya). This is an extraordinarily deep hybridization event, roughly akin to an elephant hybridizing with a manatee or a human with a lemur. In the context of other reported deep hybrids, this finding suggests that populations of ferns, and other plants with abiotically mediated fertilization, may evolve reproductive incompatibilities more slowly, perhaps because they lack many of the premating isolation mechanisms that characterize most other groups of organisms. This conclusion implies that major features of Earth's biodiversity-such as the relatively small number of species of ferns compared to those of angiosperms-may be, in part, an indirect by-product of this slower "speciation clock" rather than a direct consequence of adaptive innovations by the more diverse lineages.

The challenges to inferring the regulators of biodiversity in deep time

“…Mayr [22] argued that species, now considered the canonical unit for macroevolution, obey ecological rules, but conceded that genera also represent a biological reality. The integrated taxonomic evidence that distinct genera occupy morphospace discontinuously and rarely hybridize among themselves due to their long histories of separation [23] support the basic tenet for reproductively isolated taxonomic units. The same could be argued for any higher, clade-based taxa, whatever category names one might wish to apply, provided they reflect fundamentally different organismal constructions.…”

The effect of male-male competition and ornament size on mean and variance of courtship intensity towards heterospecific and conspecific females

“…The length of this process in nature is variable. A single mating between species can lead to origin of a new reproductively isolated species, and sometimes hybridization is observed 60 million years after divergence (Rieseberg 1997, Rothfels et al 2015. These examples are provocatively extreme and come from the plant kingdom.…”

Why are there so few species of ferns?

“…The recent study by Rothfels et al [7] sheds light on this issue. They provided evidence that the progenitors of the naturally occurring intergeneric hybrid 3Cystocarpium roskamianum Fraser-Jenk.…”

“…(Cystopteridaceae) represent lineages that have been diverging for approximately 60 million years, although the particular hybridization event that produced the living, but infertile, individual from the French Pyrenees was apparently quite recent. To put this in perspective, consider the divergence dates of other hybridizations between deeply divergent parental lineages: African cichlids, 8.5 Ma (million years); plethodontid salamanders, 12 Ma; grass genera, 14 Ma; tulip tree species, 10-15 Ma; hylid treefrogs, 34 Ma; and sunfish, 37 Ma (see references in [7]). Thus, the lineages of the parental taxa of 3Cystocarpium have been separated by 1.5 times longer than the deepest known hybridization of animals and about four times longer than those of flowering plants.…”

“…Rothfels et al [7] amplified and cloned the single-copy nuclear marker gapCp from 3Cystocarpium roskamianum and compared the four alleles discovered to 48 gapCp sequences of other taxa in Cystopteridaceae, including species of the inferred parental genera Cystopteris and Gymnocarpium (i.e., based on the morphologically based hypothesis of Fraser-Jenkins [8]). The results of phylogenetic and sequential empirical Bayesian analyses supported (i) a hybrid origin of 3Cystocarpium involving a cross between the cosmopolitan allotetraploid Gymnocarpium dryopteris (L.) Newman and a European member of the Cystopteris fragilis (L.) Bernhardi complex and (ii) a mean age estimate for the most recent common ancestor of the two parental genera of 57.9 Ma (95% highest posterior density interval spanning 40.2-76.2 Ma).…”