Abstract Detail

Systematics

Engle-Wrye, Nicholas [1], Folk, Ryan [2].

Using Heuchera to infer historical patterns of gene flow driven by Pleistocene glaciation, and why extensive population level sampling is crucial for understanding evolutionary processes.

Hybridization is thought to be a major driver of novel variation and diversification in many plant groups, and the advent of genome-scale datasets has led to renewed interest in its detection and characterization in numerous taxa. Nevertheless, understanding of the major drivers of hybridization in plants remains poor compared to polyploidy and other evolutionary mechanisms. We know little about why certain taxonomic groups and geographic areas are particularly prone to hybridization, and little about the long-term implications of hybrid history for diversification. Here we expand population level sampling and re-evaluate our previous investigation into climatic drivers and patterns of hybridization in a hybrid zone of Heuchera (Saxifragaceae), one of the most prolific examples of chloroplast capture in plants.
We hypothesized that: 1) the proportions of the parental nuclear genome inherited within the hybrid zone was proximity-based with respect to either parent’s range, 2) that the chloroplast genome would show distinct patterns of gene flow from the nucleus, that 3) Pleistocene glaciation is implicated in the original formation of the hybrid zone by pushing the previously allopatric Heuchera americana and Heuchera richardsonii into contact, and 4) post-Pleistocene population expansion led to the geographic success of their hybrid Heuchera americana var. hirsuticaulis.
To test this, we assembled a deeply sampled population phylogenomic dataset comprising 277 low-copy loci and nearly 600 accessions, analyzed with coalescent phylogenetic methods. We then used phyloclimatic modeling with paleoclimatic data covering the Pleistocene to infer potential refugial areas and periods of range contact. Ancestral niche reconstructions indicate 2 refugia in the southeastern US that would have resulted in range overlap, while the strong molecular framework reveals evidence for proximity based inheritance of nuclear DNA. By contrast, chloroplast genomes show completely distinct patterns, demonstrating an initial capture event that started in the eastern US and completely overtook the entire H. americana clade with the exception of western-most populations of H. americana and H. richardsonii that escaped gene flow completely.
Doubling our population sampling has falsified the previous scenario of a single chloroplast capture event in the H. americana clade by revealing that H. americana populations in the western refugial area (west of the Mississippi river) still have the ancestral chloroplast. Incomplete sampling has therefore remarkably underestimated divergence among chloroplast haplotypes within H. americana by several million years, as well as underestimating the number of gene flow events, while a population-level perspective enables us to independently verify that hybridization happened in the refugia east of the Mississippi river but not in the western refugium. We conclude that an expanded population-level view, contrary to the typical macroevolutionary sampling practices, was essential to uncovering our best-evidence scenario. As a population-level process occurring over extended time scales, studying ancient hybridization requires strong sampling integrating over temporal scales to prevent inferring overly simplistic evolutionary scenarios.

1 - Mississippi State University, 295 Lee Blvd, Mississippi State, MS, 39762, United States
2 - Mississippi State University, Biological Sciences, 295 Lee Blvd., P.O. Box GY, Mississippi State, MS, 39762, United States

Keywords:
hybridization
Heuchera
Pleistocene glaciation
population level sampling.

Presentation Type: Oral Paper
Number: SYST I006
Abstract ID:609
Candidate for Awards:None