“Geographic range dynamics drove ancient hybridization in a lineage of angiosperms”

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Ryan A. Folk, Clayton J. Visger, Pamela S. Soltis, Douglas E. Soltis, and Robert P. Guralnick (Aug 2018)

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Phyloclimatic modeling shows past climatic cooling may explain movement of genetic material across thousands of miles

Heuchera abramsii (San Gabriel alum-root), a high alpine species found only in a very small part of the Transverse Ranges of southern California. This is a member of the group that received genetic material from Mitella species through hybridization.
(Credit: R. A. Folk)

In response to past climate change, many species have dramatically shifted in geographic range, resulting in novel plant and animal communities and species interactions. This is predicted to continue in the face of human-caused climate change. The biology of present-day species leaves historical footprints of these past geographic shifts; for species that do not now co-occur, evidence of past biological interactions has been found throughout the Tree of Life. Among these is hybridization—sexual reproduction across species boundaries, which can result in the flow of genetic material across species boundaries and even across long distances. But how could individuals of different species have interbred if they currently lack the opportunity to do so? Folk and coauthors built a new set of computational tools to infer how past climate change would have created new plant communities in the past where hybridization was possible.

Mitella diphylla (two-leaf mitrewort), a woodland species found broadly in eastern North America. The ancestor of this and its sister species (Mitella nuda) experienced hybridization with California Heuchera and transferred genetic material from its chloroplast and mitochondrion.
(Credit: R. A. Folk)

Folk and coauthors focus on the flowering plant genus Heuchera—common garden plants known also as coral bells. A group of species found in southern California experienced ancient hybridization dating to the Ice Age. This ancient hybridization event resulted in genetic content originating from closely related Mitella (bishop’s cap) plants, today found more than 1000 km to the north of California Heuchera. Resolving this remarkable biological scenario helps provide insights on the movement of the North American flora and why hybridization has been so prevalent in its history.

Folk and coauthors provide a new framework to test hypotheses about past species contact—even for those totally lacking a fossil record. Even beyond their focus on hybridization, the ability to reconstruct a common geographic history opens new research opportunities—moving from understanding single groups of species in isolation to testing new ideas about past communities of plants and animals and their interactions.

Habitat of Heuchera elegans (urn-flowered alum-root), in crevices on loose stony slopes, as with other Heuchera species in southern California.
(Credit: R. A. Folk)

Abstract

Elucidating the dynamic distribution of organismal lineages has been central to biology since the 19th century, yet the difficulty of combining biogeographic methods with shifts in habitat suitability remains a limitation. This integration, however, is critical to understanding geographic distributions, present and past, as well as the time-extended trajectories of lineages. Here we link previous advances in phyloclimatic modeling to develop a framework that overcomes existing methodological gaps by predicting potential ecological and geographic overlap directly from estimated ancestral trait distributions. We show the utility of this framework by focusing on a clade in the montane angiosperm genus Heuchera, noteworthy in that it experienced ancient introgression from circumboreally distributed species of Mitella, lineages now ~1300 km disjunct. Using this system, we demonstrate an application of ancestral state reconstruction to assess geographic range dynamics in a lineage lacking a fossil record. We test hypotheses regarding inferred past geographic distributions and examine the potential for ancient geographic contact. Application of this multifaceted approach suggest potential past contact between species of Heuchera and Mitella in western North America during cooler periods of the Pleistocene. Integration of niche models and phylogenetic estimates suggests that climatic cooling may have promoted range contact and gene flow between currently highly disjunct species. Our approach has wide applicability for testing hypotheses concerning organismal co-occurrences in deep time.